Plants in aquatic ecosystems: current trends and future directions

  • PLANTS IN AQUATIC SYSTEMS
  • Review Paper
  • Published: 20 April 2017
  • Volume 812 , pages 1–11, ( 2018 )

Cite this article

  • Matthew T. O’Hare 1 ,
  • Francisca C. Aguiar 2 ,
  • Takashi Asaeda 3 ,
  • Elisabeth S. Bakker 4 ,
  • Patricia A. Chambers 5 ,
  • John S. Clayton 6 ,
  • Arnaud Elger 7 ,
  • Teresa M. Ferreira 2 ,
  • Elisabeth M. Gross 8 ,
  • Iain D. M. Gunn 1 ,
  • Angela M. Gurnell 9 ,
  • Seppo Hellsten 10 ,
  • Deborah E. Hofstra 6 ,
  • Wei Li 11 ,
  • Silvia Mohr 12 ,
  • Sara Puijalon 13 ,
  • Krzysztof Szoszkiewicz 14 ,
  • Nigel J. Willby 15 &
  • Kevin A. Wood 16  

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Aquatic plants fulfil a wide range of ecological roles, and make a substantial contribution to the structure, function and service provision of aquatic ecosystems. Given their well-documented importance in aquatic ecosystems, research into aquatic plants continues to blossom. The 14th International Symposium on Aquatic Plants, held in Edinburgh in September 2015, brought together 120 delegates from 28 countries and six continents. This special issue of Hydrobiologia includes a select number of papers on aspects of aquatic plants, covering a wide range of species, systems and issues. In this paper, we present an overview of current trends and future directions in aquatic plant research in the early twenty first century. Our understanding of aquatic plant biology, the range of scientific issues being addressed and the range of techniques available to researchers have all arguably never been greater; however, substantial challenges exist to the conservation and management of both aquatic plants and the ecosystems in which they are found. The range of countries and continents represented by conference delegates and authors of papers in the special issue illustrates the global relevance of aquatic plant research in the early twenty first century but also the many challenges that this burgeoning scientific discipline must address.

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Acknowledgements

We are grateful to André Padial, Baz Hughes and two anonymous reviewers for their helpful comments on earlier drafts of this manuscript.

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Matthew T. O’Hare & Iain D. M. Gunn

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Francisca C. Aguiar & Teresa M. Ferreira

Department of Environmental Science, Saitama University, 255 Shimo-okubo, Sakura, Saitama, 338-8570, Japan

Takashi Asaeda

Department of Aquatic Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Droevendaalsesteeg 10, 6708 PB, Wageningen, The Netherlands

Elisabeth S. Bakker

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School of Geography, Queen Mary University of London, London, E1 4NS, UK

Angela M. Gurnell

Finnish Environment Institute SYKE, Freshwater Centre, Paavo Havaksen tie 3, 90570, Oulu, Finland

Seppo Hellsten

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Silvia Mohr

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Sara Puijalon

Faculty of Environmental Engineering and Spatial Management, Poznan University of Life Sciences, Wojska Polskiego 28, 60-637, Poznan, Poland

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Biological & Environmental Science, Faculty of Natural Science, University of Stirling, Stirling, FK9 4LA, UK

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O’Hare, M.T., Aguiar, F.C., Asaeda, T. et al. Plants in aquatic ecosystems: current trends and future directions. Hydrobiologia 812 , 1–11 (2018). https://doi.org/10.1007/s10750-017-3190-7

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Received : 20 October 2016

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Accepted : 08 April 2017

Published : 20 April 2017

Issue Date : May 2018

DOI : https://doi.org/10.1007/s10750-017-3190-7

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  • Published: 28 April 2021

The relationship between plant growth and water consumption: a history from the classical four elements to modern stable isotopes

  • Oliver Brendel   ORCID: orcid.org/0000-0003-3252-0273 1  

Annals of Forest Science volume  78 , Article number:  47 ( 2021 ) Cite this article

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Key message

The history of the relationship between plant growth and water consumption is retraced by following the progression of scientific thought through the centuries: from a purely philosophical question, to conceptual and methodological developments, towards a research interest in plant functioning and the interaction with the environment.

The relationship between plant growth and water consumption has for a long time occupied the minds of philosophers and natural scientists. The ratio between biomass accumulation and water consumption is known as water use efficiency and is widely relevant today in fields as diverse as plant improvement, forest ecology and climate change. Defined at scales varying from single leaf physiology to whole plants, it shows how botanical investigations changed through time, generally in tandem with developing disciplines and improving methods. The history started as a purely philosophical question by Greek philosophers of how plants grow, progressed through thought and actual experiments, towards an interest in the functioning of plants and the relationship to the environment.

This article retraces this history by following the progression of scientific questions posed through the centuries, and presents not only the main methodological and conceptual developments on biomass growth and transpiration but also the development of the carbon isotopic method of estimation. The history of research on photosynthesis is only touched briefly, but the development of research on transpiration and stomatal conductance is presented with more detail.

Research on water use efficiency, following a path from the whole plant to leaf-level functioning, was strongly involved in the historical development of the discipline of plant ecophysiology and is still a very active research field across nearly all levels of botanical research.

1 Introduction

The ratio of biomass accumulation per unit water consumption is known today as water use efficiency (WUE) and is widely relevant to agriculture (e.g. Blum 2009 ; Tallec et al. 2013 ; Vadez et al. 2014 ), to forest ecology (e.g. Linares and Camarero 2012 ; Lévesque et al. 2014 ) and in the context of global climate change (Cernusak et al. 2019 ). This ratio can be defined at various levels, from the physiological functioning of a leaf to the whole plant and at the ecosystem level. This historical review starts at the whole plant level, where WUE can be simply measured by quantifying the amount of water given to a plant and the plant’s increase in biomass during the experiment. The ratio of biomass produced divided by the cumulative water lost during growth is termed whole plant transpiration efficiency (TE= biomass produced/water lost). Historically, the ratio has also been calculated in its inverted form (water lost/biomass produced) and various terms have been used to denote these ratios (see Box 1). As knowledge, concepts and technology advanced, it became desirable to measure TE also at the leaf level, where it is defined either as the ratio of net CO 2 assimilation rate to transpiration (or to the stomatal conductance for water vapour). Therefore, some history of the two leaf-level components of WUE is included here. Numerous articles have been published on the history of the development of research on photosynthesis, and other than the reviews cited in this article, the publications by Govindjee are notable, especially Govindjee and Krogmann ( 2004 ), as they include a long list of other writings on the history of photosynthesis. On the other hand, little has been written about the history of research on transpiration and stomatal conductance. Notable is Brown ( 2013 ), who wrote specifically on the cohesion-tension theory of the rise of sap in trees, including many writings from the late nineteenth century. Consequently, here, photosynthesis research is only broached briefly, whereas transpiration research is more detailed.

As the development of the research on WUE spans a very long period, starting with Greek philosophers, publications are in several languages. Classical writings were in Greek or in Latin, and for these translations are available. However, from the mid-seventeenth century onwards, national languages were more and more used, which can be seen in the number of French- and German-language publications. This review is also a tribute to these nowadays less known seventeenth, eighteenth and nineteenth century French and German natural philosophers and their contribution to the development of the science of plant ecophysiology. Also, towards the beginning of the twentieth century, publications became too numerous to allow a comprehensive review; thus, the author focussed on the use of the carbon stable isotopes methodology and on tree ecology.

Box 1 Short history of names for whole plant transpiration efficiency (TE)

2 what is plant matter made of.

Various Greek philosophers were interested in how substances can change from one thing into another. Thales (624–c. 546 BC) thought that all things come from water, whereas Anaximenes argued that “pneuma” (air) should be the basis of all things (Egerton 2001a ). These assertions were the basis of more than 2000 years of philosophical dispute.

In “De Causis Plantarum”, Theophrastos (371–287 BC) assumed that plants draw nutrition, which consisted of varying amounts of the four elementary humours, from the earth through their roots (Morton 1981 ). Some centuries later, in a Christian work translated in 400 AD from Greek into Latin and known as “Pseudo-Clement’s Recognitions”, an apparent thought experiment was described to “prove that nothing is supplied to seeds from the substance of the earth, but that they are entirely derived from the element of water and the spirit (spiritus) that is in it” (Egerton 2004c ). The author of this thought experiment suggested putting earth into big barrels, growing herbaceous plants in it for several years, then harvesting them and weighing them. His hypothesis was that the weight of the earth would not have changed, and the author used this as an argument that the vegetation biomass could have come only from water. This thought experiment revealed a progress in scientific thinking because the question was posed more precisely than before. It stood out at a time when botany mainly consisted of naming plants and “theoretical botany effectually went out of existence” (Morton 1981 ).

It appears that the question of how plant matter is produced was not pursued in Roman or Arabic writings, which were more concerned with agricultural (the former) and medical (the latter) aspects of plant sciences (Egerton 2001b , 2002 ). Not until the High Middle Ages was a renewed interest shown in plant growth. Adelard of Bath, a twelfth century English natural philosopher, devoted the first four chapters of “Questiones Naturales” (c. 1130–1140; Morton 1981 ) to the question of what plant matter is made of. He argued, within the concepts of the four elements theory, “by just as much as water differs from earth, by so much does it afford less nourishment to roots, I mean than earth does”, clearly being in favour of earth as the source for plant nourishment. His arguments were only theoretical and speculative.

A major step occurred in botanical sciences between the fifteenth and sixteenth centuries; scholars began making experiments to test antique and medieval hypotheses against observations in nature (Egerton 2003 ). In the mid-fifteenth century, and probably related to the translation and printing of the botanical books by Theophrastus (Morton 1981 ), the thought experiment from “Recognitions...” was taken up by Nicholas of Cusa in the fourth part of his “Idiota de mente”, “De staticis experiments”. At a time when the naming of plants for pharmacology was the major interest of savants, he proposed experimental investigations. Nicholas of Cusa described the same thought experiment as did Pseudo-Clement’s Recognitions ; he concluded similarly that “the collected herbs have weight mainly from water” (1450; translation into English by Hopkins 1996 ). Cusa additionally suggested that the plants should be burned at the end of the experiment and the ash weight be taken into account. It is not clear whether the thought experiment was ever physically done.

In the sixteenth century, botanical science began to separate from medical sciences, with the establishment of lectureships in universities (e.g. Padua in 1533) and the establishment of botanical gardens (Egerton 2003 ). The bases existed for advancing science in the seventeenth century of Enlightenment. Francis Bacon, an influential philosopher of his time, conducted a series of plant growth experiments which are reported in his “de Augmentis Scientiarum” (1623; Spedding et al. 1900 ). Bacon discovered that some plants sprouted more quickly in water than in soil (Egerton 2004b ). He concluded that “for nourishment the water is almost all in all, and that the earth doth but keep the plant upright, and save it from over-heat and over-cold” (Hershey 2003 ), thus still upholding the theory proposed by Thales and Nicholas of Cusa. In “The History of the Propagation and Improvement of Vegetable”, Robert Sharrock ( 1660 ) reported that some plants both rooted and grew entirely in water. Although he noted different amounts of transpiration over time, he did not discuss this in relation to plant growth.

In 1662, Johannes Baptista van Helmont published his now-famous willow experiments (van Helmont 1662 ). This may be the first report of an experiment that was based on the thought experiment of Nicholas of Cusa (Hershey 2003 ) with the minor differences of beginning with dried soil and not using herbaceous plants, but rather a willow tree. After weighing the soil, he irrigated it with rain water and planted the weighed stem of a willow tree. The experiment ran for 5 years. At the end, the tree was weighed again, as was the dried soil. He found the soil weighed about 2 ounces less than at the beginning of the experiment, whereas 164 pounds of wood, bark and roots was produced. He concluded that the organic matter could only have come out of the water. Helmont was unaware of the existence of carbon dioxide, but he did know of “gas sylvestre”. He also knew that burning oak charcoal would produce nearly the same amount of gas sylvestre and ash. However, he did not connect this information with the plant growth he had observed (Hershey 2003 ). Robert Boyle published similar experiments in “The sceptical Chymist” (Boyle 1661 ). Boyle claimed that he had done his experiments before he knew of Helmont’s (Egerton 2004c ), although he discussed Helmont’s results and arguments in detail in his book. Boyle doubted the direct transformation of water into plant matter. He admitted, however, that it might be possible that other substances contained in the water could generate new matter (Boyle 1661 ). In the 1660’s, Edme Mariotte also criticised van Helmont’s theory that water alone constituted the only element to produce plant matter. He thought similarly to Boyle that elements in the water could contribute to the plant matter. He also showed that nitrogen compounds were important for plant growth (Bugler 1950 ).

John Woodward, in his “Some Thoughts and Experiments Concerning Vegetation” (Woodward 1699 ), took up again the question of what comprised the source of plant growth. Woodward criticised Helmont’s and Boyle’s experiments, mainly on the precision of weighing the dry soil before and after the experiment, but also the contamination of the irrigation water by terrestrial vegetable or mineral matter. Consequently, he developed a series of hydroponics experiments, where by growing plants in sealed vials, in different types of water and weighing them regularly over the same time period, he could calculate how much biomass was gained over a set time period. He was able to draw a series of conclusions from these experiments by calculating the ratio of water lost to plant mass gained in the same period of time, thereby calculating the inverse of transpiration efficiency. This was probably the first time that the inverse of transpiration efficiency was calculated using experimental data. He showed that 50 to 700 times as much water was lost than biomass gained. He also reported that plants grown in water containing more terrestrial matter grew more and with less water consumed. From these observations, he concluded that water serves only as a vehicle for the terrestrial matter that forms vegetables and that vegetable matter is not formed out of water. He is still remembered more for his geological publications (Porter 1979 ) than for his contributions to botany (Stanhill 1986 ).

In his “history of ecology” series, Egerton ( 2004c ) nicely sums this period thusly: “each of these authors (Bacon, Boyle, Helmont, Sharrock) built upon the work of his predecessors and improved somewhat the understanding of plant growth and how to study it. However, they still fell short of a basic understanding of plant growth. Before that could be achieved, chemists would have to identify the gases in the air”. This series of studies shows that from the end of the seventeenth century onwards, experiments replaced speculation (Morton 1981 ), in botany as well as in many other areas of science.

From the end of the seventeenth century, the question of how plants grow was still unresolved, although it was known that nutrients were conducted from the roots in the ascending sap to the leaves. A major improvement in the understanding of how transpiration and its variations work was the discovery of cells by Robert Hooke towards the middle of the seventeenth century (Egerton 2005 ) and subsequently the discovery of stomata on leaf surfaces. One of the first to describe stomata may have been Malpighi in “Anatomy of Plants” (Malpighi 1675 in Möbius 1901 ). Based on Malpighi’s and Grew’s ( 1682 ) studies, John Ray suggested in “Historia Plantarum” (Ray 1686 in Lazenby 1995 ) that the apertures in the leaves, when open, would give off either breath or liquid. Ray may have been the first to have connected stomata with transpiration. He also suggested that the loss of water by evaporation is compensated constantly by water from the stem, and thus transpiration results from a constant water flux. He also observed that sap ascends the stems of trees in sap-bearing vessels which do not contain valves. He did, however, admit that it cannot be capillary forces that make water go up tall trees.

Ideas on photosynthesis developed slowly from the middle of the seventeenth century onwards. Malpighi ( 1675 ) suggested that leaves produce (“concoct and prepare”) the food of plants and from leaves this food passes to all parts of the plant. Similarly, Claude Perrault in “Essais de Physique” (Perrault 1680 ) defended the hypothesis that the root acts as the mouth of the plant and that the leaves serve to prepare the food arriving with the sap from the root so that it can be used in the rest of the plant. John Ray in “History Plantarum” (Ray 1686 in Lazenby 1995 ) concurs with this, however adding in “The wisdom of God” (Ray 1691 in Lazenby 1995 ) that “not only that which ascends from the Root, but that which they take in from without, from the Dew, moist Air, and Rain”. He also thought that light could play a role in this preparation of the plant sap. At this time, most authors (Malpighi, Perrault, Mariotte, Ray) knew about the circulation of sap, up as well as down, and that leaves served somehow to transform the upcoming sap into food for the plant.

In 1770 , Lavoisier published “Sur la nature de l’eau” (“On the nature of water”, translation by the author) and reviewed the literature on the possibility of water changing into earth to nourish plants. Lavoisier cited the Van Helmont experiment and later works which tested Van Helmont’s idea by growing plants in water (e.g. Boyle, however he did not cite Woodward). He was critical of the idea that it could be a transformation of water that would constitute plant material. This was based mainly on experiments by himself and others, showing even distilled water would contain traces of “soil”. However, he also defended the idea, based mainly on Charles Bonnet’s observations, that leaves absorb vapours from the atmosphere that contribute to plant growth.

Helmont had coined the term “gaz” in the mid-seventeenth century and had been able to distinguish different gazes from air (Egerton 2004a ). It was only in the middle of the eighteenth century that gases were studied in the laboratory and several observations by different researchers would finally lead to an understanding of respiration and photosynthesis (Tomic et al. 2005 ; Nickelsen 2007 ). Richard Bradley seems to be one of the first to clearly state (in letters from 1721 to 1724) that plant nourishment can be drawn from the air. Hales ( 1727 ) agreed with this theory, which was not yet widely accepted (Morton 1981 ), and suggested that light might be involved, which helped to pave the way for the discovery of photosynthesis. Black ( 1756 ) was able to identify carbon dioxide (which he called fixed air) using a lime water precipitation test. He demonstrated that this “fixed air” did not support animal life or a candle flame (Egerton 2008 ). Charles Bonnet ( 1754 ) made an important observation, i.e. branches with leaves that were submerged under water would produce air bubbles on their surfaces when sunlight shone on them, but not after sunset. Senebier refined these experiments in 1781 (Morton 1981 ), by showing that the leaves produced no oxygen in the sunlight when the surrounding water was free of carbon dioxide and that the rate of oxygen production was higher with carbon dioxide-saturated water. Tomic et al. ( 2005 ) present nicely the steps leading up to the term photosynthesis. This began with Priestley ( 1775 ) demonstrating that the air given off by animals and by plants was not the same, Ingen-Housz ( 1779 ) observed the important role of light, and the dispute between Senebier and Ingen-Housz from 1783 to 1789 resolved more clearly the functions of carbon dioxide emission (respiration) and absorption (photosynthesis). Based on these results and his own very detailed observations, de Saussure reported in 1804 that the carbon necessary for plant growth is absorbed mainly by green leaves from atmospheric carbon dioxide and he estimated that the largest part of the accumulated dry matter of plants is made of this carbon. Thus, the dispute of what the plant matter is made of that began in antique Greece was resolved at the end of the eighteenth century.

3 How much water do plants need to grow?

The late eighteenth century marked the beginning of applied agricultural science and the rise of plant physiology (Morton 1981 ). Work continued on transpiration and stomata, with a large number of experiments. Burgerstein ( 1887 , 1889 ) managed to assemble 236 publications on transpiration of plants from 1672 to 1886, citing short abstracts of each and comparing them critically. Also, Unger published in 1862 a major review article covering such subjects as the relationship of transpiration to temperature and humidity; daily cycles, including night; differences in adaxial and abaxial leaf surfaces; the impact on transpiration of type, number, size and distribution of stomata; the structure of the epidermis (cell layers, cuticle, hairs and wax); development of the mesophyll; size of intercellular spaces and cell turgor; and the impact of plant transpiration on the atmosphere (Unger 1862 in Burgerstein 1887 ). Scientists started to reflect on the interaction of plants, or more specifically their leaves, with their environment, and experimentation included the responses of stomata to light quantity (Möldenhawer 1812 ) and quality (Daubeny 1836 in Burgerstein 1887 ). Based on inconsistent observations by e.g. Banks, Möldenhawer and Amici, advances were also made on the functioning of stomata (Mohl 1856 ). However, progress was mainly based on a comment in von Schleiden ( 1849 ) that the state of the stomata would be the result of the water in- or outflow of the pore cells (called “Schliesszellen”) and he showed experimentally that stomata close when the pore cells lose water. As knowledge of transpiration, stomatal opening and their dependence on environmental variables increased, new questions arose about the water consumption of plants.

Another milestone along the way to understanding the transpiration of plants in the nineteenth century was the publication by Sir John Bennet Lawes ( 1850 ), “Experimental investigation into the amount of water given off by plants during their growth; especially in relation to the fixation and source of their various constituents”. He described experiments on wheat, barley, beans, peas and clover using differently fertilised soils. He was using plants in closed containers and an especially designed balance to “estimate the amounts of water given off” (Fig. 1 ). He observed increased evapotranspiration with higher temperatures during the growing season, and asked whether “this increased passage of water through the plants, carrying with it in its course many important materials of growth from the soil, and probably also influencing the changes in the leaves of these, as well as of those derived from the atmosphere, will not be accompanied with an equivalently increased growth and development of the substance of the plant”. This was followed by an important discussion of the influence of temperature on evaporation and growth as well as the resultant ratio. He discussed in the introduction “the relationship of the water given off to the matter fixed in the plants”; he gave his results in this ratio and in the inverse ratio, and applied these ratios to different scientific questions. The first ratio (transpired water divided by plant matter, the inverse of today’s TE) was used to interpret his results in terms of water use compared to field available water, and the latter’s ratio (plant matter divided by transpired water, equivalent to today’s TE) was used to discuss his results in terms of functional differences among species. From the observed functional differences, he concluded that there was “some definite relationship between the passage of water through the plants and the fixation in it of some of its constituents”. He was, thereby, introducing a new question about the link between dry matter accumulation and transpiration, which will be treated in the next chapter.

figure 1

Illustration from Lawes ( 1850 , p. 43) of the special balance constructed for weighing plants in their “jars” to estimate the amounts of water given off and also the “truck” on which a series of jars was moved to the balance

Towards the end of the nineteenth century, research interest started to include agricultural questions of water use. Marié-Davy ( 1869 ) measured transpiration (standardised by leaf surface) of over 30 plant species, including eight tree or shrub species as well as herbaceous and agricultural plants. He estimated transpiration per soil area, thereby establishing that a prairie would transpire more than trees. von Höhnel ( 1879 ) estimated long-term transpiration of branches of 15 tree species (standardised on leaf surface or leaf dry weight). He used these data of branch transpiration to upscale to whole trees and concluded that compared to agricultural plants, the amount of rain seemed sufficient for tree growth. Hellriegel ( 1871 ) had already similarly concluded for cereals in the Mark Brandenburg (Germany) region that rainfall would not be sufficient, as had Marié-Davy ( 1874 ) for wheat in the Paris (France) region. In parallel with these more quantitative interrogations about water use, from the mid-nineteenth century, scientists started to ask more functional questions about the relationship between transpiration and dry matter accumulation, in a context of vigorous growth of botanical sciences and the complex relation between organisms and their environment (Morton 1981 ).

4 Are transpiration and dry matter accumulation linked?

Lawes ( 1850 ) had already reflected on a functional relationship between water flux and plant matter accumulation. In the following years, there were several publications on the transpiration of trees, and although no transpiration efficiency was estimated, the understanding of tree transpiration advanced. Many comparative studies were published. Lawes ( 1851 ) on “Comparative evaporating properties of evergreen and deciduous trees” considered twelve different tree species. He provided measurements of the variation in transpiration with temperature and hygrometry data. With these, he concluded that “evaporation is not a mere index of temperature but that it depends on vitality influenced by heat, light and other causes”. In the late nineteenth century, several researchers estimated and compared values of the ratio of transpiration and dry matter accumulation for different plants (Burgerstein 1887 ). With the growing evidence of variation in this ratio, scientists started to reflect on the relationship between transpiration and dry matter accumulation, aided by the development of new measurement techniques. A major question was if there would be a tight coupling between transpiration and dry matter accumulation, resulting in a constant transpiration efficiency, or if variation could be observed.

Dehérain ( 1869 ) studied evaporation and the decomposition of carbonic acid in leaves of wheat and barley. Using an ingenious apparatus, he was probably the first to directly measure evaporation of water in parallel with carbonic acid decomposition. He studied the effect of variously coloured light, and although he did not calculate the ratio between evaporation and carbonic acid decomposition, he did conclude that light of different colours had a similar effect on carbonic acid decomposition and on water evaporation from the leaves. His final conclusion was that “it is likely that there is existing between the two main functions of plants, evaporation and carbonic acid decomposition, a link, of which we need to determine its nature” (translation from the original French by the author). Several other scientists also commented on the relationship between transpiration and dry matter production. Fittbogen ( 1871 ) supposed, similarly to Lawes ( 1850 ) before him, but with more experimental evidence, that there should be a positive relationship between transpiration and production of dry matter. Dietrich ( 1872 in Burgerstein 1887 ) supposed that this relationship would be linear, whereas Tschaplowitz ( 1878 in Burgerstein 1887 ) introduced the idea that there should be an optimum transpiration at the maximum production of matter. Therefore, when the transpiration would increase over this optimum, this would lead to a decrease in assimilation rate. He was one of the first to suggest a non-linear relationship between transpiration and assimilation. Sorauer in “Studies on evaporation” ( 1880 ) defended the hypothesis that transpiration was not only a physical phenomenon but was also physiological. He stated that “It is not possible as yet to study the plant internal processes which regulate the transpiration, however it is possible to quantify the relationship between dry-matter and transpiration” (translation from German by the author), suggesting thereby TE as a means to advance the understanding of plant internal processes. Sorauer was probably at the cutting edge of science of his time. He pointed out specifically that variability among plants of one species was due to genetics (German, “erbliche Anlagen”), a startling and even daring assertion for his time. He asserted that for comparative studies, genetic variability needed to be minimised. To achieve this, he used, when possible, seeds from the same mother plant, grown in the same environmental conditions and a large number of repetitions. Using these protocols, he was probably one of the first to estimate TE on tree seedlings, showing that there was within species diversity in transpiration and growth, but that their ratio was more constant. He concluded from experiments on pear and apple trees that the pear trees used less water for the same biomass growth. He was able to go one step further and demonstrate that this difference was due to less transpiration per leaf area. By comparing different woody and herbaceous plants with different growth types, he postulated that when plants had a small leaf area combined with high transpiration, they had either a very strong growth increment, a high dry matter percentage, or a large root system. Overall, he observed relationships between dry matter production and transpiration; he concluded that there must be some regulation of the transpiration per unit leaf area by the co-occurring dry matter production.

Hellriegel ( 1883 ) argued that one cannot estimate a constant ratio between transpiration and production as there were factors which influence each independently. He also commented that it might make sense to estimate mean values of transpiration for various agricultural plants, as this would be for practical and scientific value. He thought that the most logical standardisation would be by the mass of the dry matter produced during the same time period. He called this “relative Verdunstungsgrösse” which can be translated into English as “relative transpiration”. He was probably one of the first to give a name to the ratio between whole plant transpiration and dry matter production. He proposed a theory that for a long-term drought, plants would acclimate their morphology to decrease their “relative transpiration”. He provided additional experimental evidence that barley had decreased in relative transpiration over as many as seven levels of soil water deficit, relative to field capacity. Using his own observations, he proposed that when calculating a mean “relative transpiration” for a single species, variation of transpiration should be minimised and that plants should be tested together only under optimal conditions. Given the relatively small differences in relative transpiration that he observed among different crops, Hellriegel suggested that these differences would not explain why some crops grow better in wet locations and others on dry locations. Hellriegel was thus probably one of the first scientists to point out that the relationship between drought adaptation and “relative transpiration” might not be straightforward.

Understanding how biomass and water loss were connected was studied by Iljin ( 1916 ) on a newly detailed level. He measured simultaneously water loss and carbon dioxide decomposition and reported his data as grammes of water lost per cubic centimetre of carbon dioxide decomposed. He concluded from studying more than 20 plant species that “...it is generally agreed that the rates of water loss and of CO 2 assimilation are directly proportionate to stomatal aperture, and that consequently there exists a close connection between these two processes”.

At the end of the nineteenth century, the ratio of transpiration versus dry matter accumulation was recognised as an important plant trait, which varies among and within species in a complex interaction of each component with the other and with environmental factors.

5 How do plants differ in water requirement and how do they respond to variations in environmental factors?

In the late nineteenth century, several researchers estimated and compared values of the ratio of transpiration and dry matter accumulation for a range of cultivated plants (Fittbogen 1871 ; Dietrich 1872 ; Farsky 1877 , cited in Burgerstein 1887 ), giving evidence of the growing interest of agricultural scientists. The number of studies of transpiration efficiency greatly increased, thereby driving a new standardisation in terminology. King ( 1889 ) studied the inverse of transpiration efficiency and described it as “the amount of water required for a ton of dry matter”, and promulgated this terminology by using it in the titles of his publications between 1892 and 1895. Similarly, Leather ( 1910 ) published “Water requirements of the crops of India”, in which he defined the “transpiration ratio” as “the water transpired to the weight of dry plant produced”. The shift from a purely descriptive use of “water requirement” to a clearly defined one was provided by Kearney and Shantz ( 1911 ) as “… the degree to which a plant is economical in its use of water is expressed in its water requirement, or the total quantity of water which it expends in producing a pound of dry matter”. The term “water requirement” is the inverse of the modern transpiration efficiency, and was used by a rapidly increasing number of publications which were published on the water use of crops in the early twentieth century. Montgomery ( 1911 ) may have been the first to use the term for a plant trait in “Methods of determining the water requirements of crops”.

At the beginning of the twentieth century, the importance of gaining knowledge on the water requirements of plants can be seen in the technical effort that went into the measuring equipment. von Seelhorst ( 1902 ) presented a system of growing boxes on rails, placed belowground, including the balance, so that the top of the growing boxes was at the same level as the surrounding soil (Fig. 2 ). In the now well-known studies on “The water requirement of plants. I. Investigations in the Great Plains in 1910 and 1911”, Briggs and Shantz ( 1913a ) measured the water requirement for 21 crop and weed species, sometimes for different varieties of the same crop and under controlled and field conditions. In the same year, they reviewed the available literature on water requirement (Briggs and Shantz 1913b ), increasing their dataset to 31 different crop species. They discussed in detail studies from 29 different authors, many of which had only published once or twice on this subject. A few researchers were notable for their number of publications on the water requirement of crop plants: King with 6 publications between 1889 and 1905, and von Seelhorst with 9 publications between 1899 and 1907. The largest contributions came from Hellriegel ( 1883 ; 10 species) and Leather ( 1911 ; 15 species). Kiesselbach ( 1916 ) also reviewed 59 publications from 1850 to 1915 “which had studied transpiration in relation to crop yield, based upon plants grown beyond the seedling stage”. There were regular publications of original work from 1870s onwards, with more than one publication per year from 1890 onwards. The difference among species and the impact of environmental factors on water requirement was one of the main questions raised. These reviews and the increasing amount of newly published work per year are evidence of the growing interest in the “water requirement” of plants as a trait of increasing importance in agricultural sciences.

figure 2

Illustration from von Seelhorst ( 1902 ), showing the quite sophisticated outdoor installation “Vegetationskasten” (growing boxes, translations by the author) to weigh plants in small waggons, with a “Kastenwagen” (boxwaggon), b “Waagebalken” (scale beam), c “Deckbretter” (cover board) and d “Waagentisch” (weighing table)

With regard to species differences in water requirement among crops, Schröder ( 1895 , cited in Maximov 1929 ) found two groups, among seven cereals, which differed in water requirement by a factor of 2. Millet, sorghum and maize were known to be drought resistant, and showed a lower water requirement than the remaining plants. These differences were confirmed by Kolkunov ( 1905 , cited in Maximov 1929 ), Briggs and Shantz ( 1914 ), Briggs and Shantz ( 1917 ) and Shantz ( 1927 ). Millet, sorghum and maize are now known to use the C4 carbon pathway of photosynthesis.

With regard to external environmental influences on plants, Briggs and Shantz ( 1913b ) distinguished between soil, atmosphere and plant factors. Soil factors which were investigated were soil moisture content, soil type, cultivation, soil volume, soil temperature, effect of fertilisers in soil or water cultures and effect of previous crops. Weather factors considered were air temperature and humidity, shade and carbon dioxide content. Other factors studied in direct relationship to the plants were parasite attacks, relative leaf area, cutting frequency, defoliation, planting density and the age of plants.

A critique of the term “water requirement” was not long in coming. Dachnowski ( 1914 ) wrote, “It is assumed by many writers that a definite and quantitative relation exists between transpiration and growth, and that hence the ratio of the weight of water absorbed and transpired by a plant during its growth to the green or dry substance produced is an adequate and simple measure of growth.”, followed by an argument why this was not the case.

6 Why do plants differ in transpiration efficiency?

The adaptations of plants to dry environments were an important ecological topic at the beginning of the twentieth century, as the discipline of “physiological ecology” (Iljin 1916 ; Moore 1924 ) began to develop. Iljin ( 1916 ) studied more than 20 different plant species in situ from different ecological locations, e.g. wet bottom soils and variously facing slopes of ravines with different aspects. Iljin proposed that “the water requirements of the different species should be very different, and consequently the amounts of water available should differently affect their processes of life”. Using his observations, he was able to show that “… in no case was the water loss per unit of decomposed CO 2 found to be equal to or more in xerophytes than in mesophytes”, thus suggesting a higher transpiration efficiency. He argued that mesophytes would have to close stomata “… in dry places in order to reduce evaporation, thus diminishing the rate of assimilation as well, whereas in the case of xerophytes, which are adapted to extreme conditions of existence, assimilation in similar circumstances proceeds actively”. He then tried to confirm his hypothesis by transplanting mesophytes from wetter sites to the drier environment of xerophytes. Iljin showed experimentally that in all cases, a higher water requirement was measured for mesophytes transferred to a drier site compared to their original site and compared to xerophytes at the dry site. He interpreted his observations as “plants growing in dry places are adapted to a more economical consumption of water”. He held this to be true for among- and within-species variation.

A milestone in forest “physiological ecology” was Bates’ ( 1923 ) study of the physiological requirements of Rocky Mountain trees. Bates wrote that for foresters, knowledge of demands of tree seedlings for moisture, light, heat and soil fertility was important for planning reforestation. He started a large investigation of six forest tree species, combining field studies to describe ecosystems, with experiments in controlled environments in order to determine species differences in relative transpiration and other water flow-related traits. Bates concluded from the comparison among species that trees of low water requirement would be trees that have a superior control over their water supply. He was however critical of a direct relationship between water requirement and drought resistance in trees. Moore ( 1924 ) commented that in correlating physiological measurements with the habitat characterisation of the species, Bates “... has opened new fields to forest investigations”. He also stressed that the results were counterintuitive in that the most xerophytic species had the highest water requirement, whereas the most mesophytic species had the lowest water requirement.

A similar discrepancy was observed by Maximov ( 1929 ) in the chapter “Efficiency of transpiration” in his book The Plant in relation to water , which was translated from Russian into English rapidly after its publication. Maximov preferred “efficiency of transpiration” to “water requirement”, arguing that the former would be more logically correct, because the determining process (transpiration) should be in the denominator, which also would have the effect that “… an increase in the figure denoting the value of the ratio actually corresponds to an increase of the efficiency per unit of water used”.

In his book, Maximov ( 1929 ) described experiments done at Tiflis Botanic garden (today in Georgia) by Maximov and Alexandrov ( 1917 ), where they studied local xerophytes for 3 years. They found xerophytes with a high efficiency of transpiration, particularly drought-resistant annuals. They also found that plants with a low efficiency of transpiration appeared to be the most typical semi-arid xerophytes. The mesophytes all displayed a medium efficiency. Maximov noted from other observations on the same plants that the “… majority of xerophytes with a low efficiency of water expenditure possess very extensive root systems, far exceeding in length the sub-aerial portions of the plant”. He also observed that these plants showed a strong transpiration and that this transpiration might constitute the “pump” which could draw water through such an extensive root system. He also observed that “members of the group of annual xerophytes with a high efficiency of transpiration are characterised by a relatively large leaf surface, which develops very rapidly”. He argued that this would confer a high intensity of assimilation. From these observations, he concluded a “lack of direct proportionality between efficiency of transpiration and the degree of drought resistance”, but also that “the magnitude of the efficiency of transpiration affords one of the most satisfactory tests of the ecological status of a plant”. Maximov applied the ecological classification developed by Kearney and Shantz ( 1911 ), which they had based on plants of the arid and semi-arid regions of North America: (1) drought-escaping with an annual growth cycle restricted to favourable conditions; (2) drought-evading, delay by various means the exhaustion of soil moisture; (3) drought-enduring, can wilt or dry but remains alive; and (4) drought-resisting, can store a water supply. It should be noted that the ecological definitions behind these concepts have changed with time and are used slightly differently today. Shantz ( 1927 ) argued that many of the drought-evading plants had a low water requirement and Maximov noted that this group included the highly efficient xerophytes with a large leaf area. Maximov also observed that xerophytes from the third group (drought-enduring) could show a very low efficiency of transpiration and belonged to the group of xerophytes with large root systems. Without concluding directly, he suggested a relationship between the transpiration efficiency of a xerophyte and its ecological strategy when facing limited soil water content. These studies by Maximov are among the most complete concerning the relationship between a plants’ resistance to drought and their transpiration efficiency, reflecting the interest of scientists in ecological questions of plant functioning, especially in relation to drought.

Although work on crop plants advanced greatly in the early twentieth century, results were scarcer for tree species. Raber ( 1937 ) concluded his book on “Water utilization by trees, with special reference to the economic forest species of the north temperate zone” with detailed discussions of available data for forest trees. He commented that “much more work on the water requirements of trees of all ages and under varying site conditions is needed”. And he continued that “In view of the importance of planting drought-resistant species in regions where the water supply is below the optimum for most tree species, it is extremely urgent to know more about what qualities make for drought resistance and what species possess these qualities to the greater degree.” These conclusions by Raber show that from the beginning of the twentieth century, the estimation of transpiration efficiency had taken an important place in ecological studies on forest tree species, however not without some critical thoughts on the subject.

7 What is the functional importance of transpiration?

Already in the 1870s and 1880s, the role of stomata in the diffusion of carbon dioxide into the leaf (during the day) and out of the leaf (during the night) was discussed in the scientific literature, as shown by the extensive literature review by Blackman ( 1895 ) (see also section 4 above). Especially the functional importance of transpiration was an open question. There were two opposing lines of thought. As summarised by Iljin ( 1916 ), one defended the line of inquiry that transpiration was important only in the process of transporting mineral salts from roots to leaves; the other held that the opening of stomata was necessary for absorbing the carbonic acid from the atmosphere, which leads to a loss of water and is described as an “inevitable evil”. Iljin ( 1916 ) preferred the second line of investigation and attributed a major role to the stomatal aperture, which controlled both the absorption of carbonic acid from the atmosphere and the loss of water. He concluded that in “physiologico-ecological” investigations, assimilation should be studied together with transpiration. Maskell published a series of papers in 1928, where especially “XVIII.—The relation between stomatal opening and assimilation.” (Maskell and Blackman 1928 ) used an apparatus to estimate apparent CO 2 assimilation and transpiration rate in parallel (Fig. 3 ), and was therefore able to study in detail their interdependence, developing the first mathematical descriptions, based on the development of the theories about the diffusion of gases (Brown and Escombe 1900 ). Methodological advances intensified research on the leaf-level relationship between assimilation and transpiration and allowed the study of plant functioning in more detail. The major step forward was the construction of an infrared gas analyser (URAS: in German “Ultrarotabsorptionsschreiber”, IRGA, infrared gas analyser) by Lehrer and Luft in 1938 (Luft 1943 ) at a laboratory of BASF, IG Farbenindustrie. Normally used in industry and mining, Egle and Ernst ( 1949 ) may have been the first to describe the use of the URAS for plant physiological measurements. By 1959, the URAS was routinely used for measuring stomatal resistance or transpiration in parallel and simultaneously with CO 2 assimilation, on the same leaf (Rüsch 1959 ). This was a great improvement on previous methods and led rapidly to a set of equations for calculating assimilation and stomatal conductance (Gaastra 1959 ).

figure 3

Two figures taken from Maskell and Blackman ( 1928 ): on the top, Figure 1 (p. 489) showing a “Combined assimilation chamber and porometer for simultaneous investigation of apparent assimilation and stomatal behaviour. A. Section of leaf chamber passing through porometer chamber. B. Back view of leaf chamber showing also air-flow meter attached by pressure tubing to porometer and to leaf chamber”. On the bottom, Figure 5 (p. 497) “Relation between porometer rate and apparent assimilation at ‘high’ light, December 1920.” Exp t LI and LII correspond to 2 days of continuous measurements to what Maskell called “diurnal march”

Scarth ( 1927 ) argued that there would be little advantage for a plant to have a high rate of transpiration, but stressed the “... advantage of maintaining the fullest diffusive capacity of the stomata and the highest possible pressure of CO 2 in the intercellular spaces”. He concluded that the principal function of stomata “... is to regulate that very factor which is presumed to regulate them, viz. the concentration of CO 2 in the leaf or, respectively, in the guard cells”. Maskell and Blackman ( 1928 ) tested this hypothesis experimentally and concluded that the rate of uptake of carbon dioxide was determined by variations in stomatal resistance and by resistances within the leaf, thereby introducing the importance of the CO 2 concentrations in the chloroplasts. The suggestion of a strong link between the leaf internal carbon dioxide concentration and leaf-level WUE represented a large advance in the theoretical understanding of WUE.

Penman and Schofield ( 1951 ) proposed, perhaps, the first theoretical link between the leaf-level transpiration ratio (leaf transpiration divided by assimilation) and the ratio of the coefficients of diffusion of water vapour and carbon dioxide in air, and the water vapour and carbon dioxide air-to-leaf pressure gradients. Gaastra ( 1959 ) suggested that the leaf internal conductance to carbon dioxide is a pivotal point of the ratio of assimilation to transpiration and of the water economy of crop plants. Bierhuizen and Slatyer ( 1965 ) showed that the transpiration ratio could be predicted from water vapour and carbon dioxide gradients over a range of light intensities, temperatures and relative humidities. These studies were the first to suggest that whole plant transpiration efficiency might be regulated directly by leaf functioning and would be therefore a trait in its own right and not only the ratio of two plant traits.

8 How can the transpiration ratio be improved?

Because water is increasingly scarce in a warming world, Rüsch ( 1959 ) queried whether the luxury of highly transpiring tree species could be justified. He argued for selective breeding of tree species varieties with low transpiration-to-assimilation ratio T/A by means of minimising transpiration while maximising assimilation. Also Polster et al. ( 1960 ) assessed the potential suitability of tree species to sites by their dry matter production and transpiration ratio. Troughton ( 1969 ) and Cowan and Troughton ( 1971 ) suggested that genetic selection of plant varieties could be used to improve the transpiration ratio by decreasing leaf internal resistance to carbon dioxide diffusion. Cowan and Farquhar ( 1977 ) built on this theme by proposing that stomata might optimise carbon gain to water lost by varying the conductances to diffusion and thereby maximising the ratio of the mean assimilation rate to mean rate of evaporation in a fluctuating environment. Approaches which target photosynthesis, stomatal opening, leaf internal resistance to carbon dioxide diffusion or stomatal optimisation in order to improve plants performance have since been followed in plant breeding and have largely been reviewed elsewhere (e.g. Condon et al. 2004 ; Cregg 2004 ; Vadez et al. 2014 ).

9 Intrinsic water use efficiency and carbon stable isotopes

Another milestone towards contemporary research on water use efficiency was the use of stomatal conductance to water vapour rather than transpiration by Farquhar and Rashke ( 1978 ) and to calculate water use efficiency as assimilation divided by stomatal conductance. This definition allowed an estimation of water use efficiency resulting only from plant functioning, without a direct impact from leaf-to-air vapour pressure difference and was named by Meinzer et al. ( 1991 ) “intrinsic water use efficiency” (W i ). Knowledge of W i facilitated the search for a genetic basis of within species variation, e.g. Brendel et al. ( 2002 ), Condon et al. ( 2002 ) and Chen et al. ( 2011 ).

Development of the stable carbon isotope method for estimating W i resulted in a widely applicable screening method, and a large increase of publications around plant water use efficiency. Based on the two-step fractionation model (atmospheric CO 2 – leaf internal CO 2 – plant carbon) proposed by Park and Epstein ( 1960 ), various models explaining the difference in carbon isotope composition between atmospheric CO 2 and plant carbon were developed in the late 1970s and early 1980s, e.g. Grinsted ( 1977 ), Schmidt and Winkler ( 1979 ) and Vogel ( 1980 ). Vogel’s model contained many theoretical aspects which, however, lacked experimental understanding. In parallel, Farquhar ( 1980 ) developed a similar model, but which resulted in a simple, elegant mathematical equation relating plant natural abundance carbon isotope discrimination, relative to atmosphere, to the ratio of leaf internal to atmospheric CO 2 concentration. This was, in turn, related to W i . Experimental evidence showed that carbon isotope measurements, in wheat, reflected long-term water use efficiency (Farquhar et al. 1982 ) as well as whole plant transpiration efficiency (Farquhar and Richards 1984 ). They concluded that carbon isotope discrimination may provide an effective means to assess and improve WUE of water-limited crops. Strong correlations between whole plant TE and stable carbon isotope measurements of plant organic material were shown in a host of papers to be. Some of these papers were (1) for crops and other annuals (Hubick et al. 1986 ; Ehleringer et al. 1990 ; Virgona et al. 1990 ) and (2) for trees (Zhang and Marshall 1994 ; Picon et al. 1996 ; Roupsard et al. 1998 ). The isotopic method has spread rapidly as a general estimator of WUE and continues to be used widely in screening programmes for plant improvement as well as in ecological research, e.g. Rundel et al. ( 1989 ) and notably used in tree rings (McCarroll and Loader 2004 ).

10 Closing remarks

Water use efficiency is probably one of the oldest of plant traits to stimulate across the centuries the interest of philosophers, theologians, Middle Age savants, natural philosophers and modern plant scientists across different disciplines (plant physiology, ecophysiology, ecology, genetics, agronomy). The interest began as a purely philosophical one, progressed to thought experiments, towards an interest in plant functioning and its relationship to the environment.

Already in the early Renaissance (mid-fifteenth century), an experimentation was proposed, in a time when botany consisted mainly of naming plants (Morton 1981 ). It is then also an early example of an actually performed experimentation, the famous willow experiment by Van Helmont ( 1662 ) as well as of early “in laboratory” experimentation on plants (hydroponics experiments by Woodward 1699 ). The question of what makes plants grow, between water and soil, kept natural philosophers busy up to the end of the eighteenth century, when the assimilation of CO 2 was discovered and the question finally solved.

Early in the nineteenth century, the interest and experimentation turned to the amount of water that plants would need to grow, in the context of a developing research on agricultural practices (Morton 1981 ). Biomass was used to standardise the water losses which allowed comparisons among species (crops as well as trees) and a beginning study of the impact of different environmental variables.

At the end of the nineteenth century, knowledge on the physiological aspects of CO 2 assimilation and the control of transpiration by stomata had sufficiently advanced, so that scientists started to reflect on their inter-dependency. Was transpiration only a physical process or was there a physiological control? Was transpiration regulated by the dry matter production? Or does the stomatal opening determine the rate of CO 2 assimilation?

At the turn of the twentieth century, the study of species differences led to questioning why these differences did exist. As the discipline of “physiological ecology” developed, “water requirement” was inverted into an “efficiency”, reflecting an evolution from standardising transpiration to a trait in its own right. This introduced ecological questions about the adaptation of plants to dry environments and the relation to transpiration efficiency. Counterintuitive results stimulated the discussion and linked differences in WUE to different ecological strategies.

Methodological and theoretical advances in the description of leaf gas exchange in the mid-twentieth century showed the central role of stomata in the control of transpiration and CO 2 assimilation, leading to the idea that stomata might optimise water losses versus carbon gain. The development of carbon stable isotopes as an estimator of leaf-level WUE was an important step not only to further develop these theoretical considerations, but also towards large-scale studies. In parallel, modelling approaches were developed to scale from leaf-level WUE to whole plant TE, e.g. Cernusak et al. ( 2007 ), and to the field or canopy, e.g. Tanner and Sinclair ( 1983 ).

At least from the beginning of the twentieth century onwards, also critical views on the relationship between water requirement and its relation to growth mostly in terms of yield were published (Dachnowski 1914 ). Viets ( 1962 ) asked “Is maximum water use efficiency desirable?”, especially in terms of crop production. Sinclair et al. ( 1984 ) considered different options for improving water use efficiency, however concluding that most of these have important limitations or drawbacks. This discussion is ongoing, as can be seen by the article published by Blum ( 2009 ): “Effective use of water (EUW) and not water-use efficiency (WUE) is the target of crop yield improvement under drought stress”.

Exploration and application of transpiration efficiency at the whole plant level, and its derivatives at other levels, are still a very active research field across nearly all levels of forest science: concerning very rapid processes at the leaf level (Vialet-Chabrand et al. 2016 ), up-to-date genetic and genomic approaches for breeding (Plomion et al. 2016 ; De La Torre et al. 2019 ; Vivas et al. 2019 ), studying local adaptation of trees to their environment in a population genetic context (Eckert et al. 2015 ) or an ecological context (Pellizzari et al. 2016 ), water use efficiency from the plant to the ecosystem (Medlyn et al. 2017 ), estimated at the population level (Rötzer et al. 2013 ; Dekker et al. 2016 ) or modelling up to the global earth level (Cernusak et al. 2019 ), just to name a few. Thus, the first curiosity of Greek philosophers has motivated scientists through history, with many exciting discoveries still to come.

Change history

17 june 2021.

A Correction to this paper has been published: https://doi.org/10.1007/s13595-021-01073-0

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Acknowledgements

Much of the historical background is based on A.G. Morton’s “History of Botanical Sciences” as well as to Frank N. Egerton’s “A History of the Ecological Sciences” series in the “Bulletin of the Ecological Society of America”. The author is also largely indebted to C. Schuchardt from the Library of the Staatsbetrieb Sachsenforst for help with the quest for rare German publications. The author would also like to thank E. Dreyer and J.M. Guehl (both from the SILVA Unit at INRAE Nancy, France) who commented extensively on an earlier version of the draft and J. Williams (University of Sussex), L. Handley and J. Raven (University of Dundee) who made many valuable suggestions and improved language.

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Brendel, O. The relationship between plant growth and water consumption: a history from the classical four elements to modern stable isotopes. Annals of Forest Science 78 , 47 (2021). https://doi.org/10.1007/s13595-021-01063-2

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  • Published: 28 March 2024

New water accounting reveals why the Colorado River no longer reaches the sea

  • Brian D. Richter   ORCID: orcid.org/0000-0001-7216-1397 1 , 2 ,
  • Gambhir Lamsal   ORCID: orcid.org/0000-0002-2593-8949 3 ,
  • Landon Marston   ORCID: orcid.org/0000-0001-9116-1691 3 ,
  • Sameer Dhakal   ORCID: orcid.org/0000-0003-4941-1559 3 ,
  • Laljeet Singh Sangha   ORCID: orcid.org/0000-0002-0986-1785 4 ,
  • Richard R. Rushforth 4 ,
  • Dongyang Wei   ORCID: orcid.org/0000-0003-0384-4340 5 ,
  • Benjamin L. Ruddell 4 ,
  • Kyle Frankel Davis   ORCID: orcid.org/0000-0003-4504-1407 5 , 6 ,
  • Astrid Hernandez-Cruz   ORCID: orcid.org/0000-0003-0776-5105 7 ,
  • Samuel Sandoval-Solis 8 &
  • John C. Schmidt 9  

Communications Earth & Environment volume  5 , Article number:  134 ( 2024 ) Cite this article

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Persistent overuse of water supplies from the Colorado River during recent decades has substantially depleted large storage reservoirs and triggered mandatory cutbacks in water use. The river holds critical importance to more than 40 million people and more than two million hectares of cropland. Therefore, a full accounting of where the river’s water goes en route to its delta is necessary. Detailed knowledge of how and where the river’s water is used can aid design of strategies and plans for bringing water use into balance with available supplies. Here we apply authoritative primary data sources and modeled crop and riparian/wetland evapotranspiration estimates to compile a water budget based on average consumptive water use during 2000–2019. Overall water consumption includes both direct human uses in the municipal, commercial, industrial, and agricultural sectors, as well as indirect water losses to reservoir evaporation and water consumed through riparian/wetland evapotranspiration. Irrigated agriculture is responsible for 74% of direct human uses and 52% of overall water consumption. Water consumed for agriculture amounts to three times all other direct uses combined. Cattle feed crops including alfalfa and other grass hays account for 46% of all direct water consumption.

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Introduction

Barely a trickle of water is left of the iconic Colorado River of the American Southwest as it approaches its outlet in the Gulf of California in Mexico after watering many cities and farms along its 2330-kilometer course. There were a few years in the 1980s in which enormous snowfall in the Rocky Mountains produced a deluge of spring snowmelt runoff capable of escaping full capture for human uses, but for most of the past 60 years the river’s water has been fully consumed before reaching its delta 1 , 2 . In fact, the river was overconsumed (i.e., total annual water consumption exceeding runoff supplies) in 16 of 21 years during 2000–2020 3 , requiring large withdrawals of water stored in Lake Mead and Lake Powell to accommodate the deficits. An average annual overdraft of 10% during this period 2 caused these reservoirs– the two largest in the US – to drop to three-quarters empty by the end of 2022 4 , triggering urgent policy decisions on where to cut consumption.

Despite the river’s importance to more than 40 million people and more than two million hectares (>5 million acres) of cropland—producing most of the vegetable produce for American and Canadian plates in wintertime and also feeding many additional people worldwide via exports—a full sectoral and crop-specific accounting of where all that water goes en route to its delta has never been attempted, until now. Detailed knowledge of how and where the river’s water is used can aid design of strategies and plans for bringing water use into balance with available supplies.

There are interesting historical reasons to explain why this full water budget accounting has not been accomplished previously, beginning a full century ago when the apportionment of rights to use the river’s water within the United States was inscribed into the Colorado River Compact of 1922 5 . That Compact was ambiguous and confusing in its allocation of water inflowing to the Colorado River from the Gila River basin in New Mexico and Arizona 6 , even though it accounts for 24% of the drainage area of the Colorado River Basin (Fig.  1 ). Because of intense disagreements over the rights to the Gila and other tributaries entering the Colorado River downstream of the Grand Canyon, the Compact negotiators decided to leave the allocation of those waters rights to a later time so that the Compact could proceed 6 . Arizona’s formal rights to the Gila and other Arizona tributaries were finally affirmed in a US Supreme Court decision in 1963 that also specified the volumes of Colorado River water allocated to California, Arizona, and Nevada 7 . Because the rights to the Gila’s waters lie outside of the Compact allocations, the Gila has not been included in formal accounting of the Colorado River Basin water budget to date 8 . Additionally, the Compact did not specify how much water Mexico—at the river’s downstream end—should receive. Mexico’s share of the river was not formalized until 22 years later, in the 1944 international treaty on “Utilization of the Waters of the Colorado and Tijuana Rivers and of the Rio Grande” (1944 Water Treaty) 9 . As a result of these political circumstances, full accounting for direct water consumption at the sectoral level—in which water use is accounted according to categories such as municipal, industrial, commercial, or agricultural uses—has not previously been compiled for the Gila River basin’s water, and sectoral accounting for Mexico was not published until 2023 10 .

figure 1

The physical boundary of the Colorado River Basin is outlined in black. Hatched areas outside of the basin boundary receive Colorado River water via inter-basin transfers (also known as ‘exports’). The Gila River basin is situated in the far southern portion of the CRB in Arizona, New Mexico, and Mexico. Map courtesy of Center for Colorado River Studies, Utah State University.

The US Bureau of Reclamation (“Reclamation”)—which owns and operates massive water infrastructure in the Colorado River Basin—has served as the primary accountant of Colorado River water. In 2012, the agency produced a “Colorado River Basin Water Supply and Demand Study” 8 that accounted for both the sectoral uses of water within the basin’s physical boundaries within the US as well as river water exported outside of the basin (Fig.  1 ). But Reclamation did not attempt to account for water generated from the Gila River basin because of that sub-basin’s exclusion from the Colorado River Compact, and it did not attempt to explain how water crossing the border into Mexico is used. The agency estimated riparian vegetation evapotranspiration for the lower Colorado River but not the remainder of the extensive river system. Richter et al. 11 published a water budget for the Colorado River that included sectoral and crop-specific water consumption but it too did not include water used in Mexico, nor reservoir evaporation or riparian evapotranspiration, and it did not account for water exported outside of the Colorado River Basin’s physical boundary as illustrated in Fig.  1 . Given that nearly one-fifth (19%) of the river’s water is exported from the basin or used in Mexico, and that the Gila is a major tributary to the Colorado, this incomplete accounting has led to inaccuracies and misinterpretations of “where the Colorado River’s water goes” and has created uncertainty in discussions based on the numbers. This paper provides fuller accounting of the fate of all river water during 2000–2019, including averaged annual consumption in each of the sub-basins including exports, consumption in major sectors of the economy, consumption in the production of specific types of crops, and water consumed by reservoir evaporation and riparian/wetland evapotranspiration.

Rising awareness of water overuse and prolonged drought has driven intensifying dialog among the seven US states sharing the basin’s waters as well as between the United States, Mexico, and 30 tribal nations within the US. Since 2000, six legal agreements affecting the US states and two international agreements with Mexico have had the effect of reducing water use from the Colorado River 7 :

In 2001, the US Secretary of the Interior issued a set of “Interim Surplus Guidelines” to reduce California’s water use by 14% to bring the state within its allocation as determined in the 1963 US Supreme Court case mentioned previously. A subsequent “Quantification Settlement Agreement” executed in 2003 spelled out details about how California was going to achieve the targeted reduction.

In 2007, the US Secretary of the Interior adopted a set of “Colorado River Interim Guidelines for Lower Basin Shortages and the Coordinated Operations for Lake Powell and Lake Mead” that reduced water deliveries to Arizona and Nevada when Lake Mead drops to specified levels, with increasing cutbacks as levels decline.

In 2012, the US and Mexican federal governments signed an addendum to the 1944 Water Treaty known as Minute 319 that reduced deliveries to Mexico as Lake Mead elevations fall.

In 2017, the US and Mexican federal governments established a “Binational Water Scarcity Contingency Plan” as part of Minute 323 that provides for deeper cuts in deliveries to Mexico under specified low reservoir elevations in Lake Mead.i

In 2019, the three Lower Basin states and the US Secretary of the Interior agreed to commitments under the “Lower Basin Drought Contingency Plan” that further reduced water deliveries beyond the levels set in 2007 and added specifications for deeper cuts as Lake Mead drops to levels lower than anticipated in the 2007 Guidelines.

In 2023, the states of California, Arizona and Nevada committed to further reductions in water use through the year 2026 12 .

With each of the above agreements, overall water consumption has been reduced but many scientists assert that these reductions still fall substantially short of balancing consumptive use with 21st century water supplies 2 , 13 . With all of these agreements—excepting the Interim Surplus Guidelines of 2001—set to expire in 2026, management of the Colorado River’s binational water supply is now at a crucial point, emphasizing the need for comprehensive water budget accounting.

Our tabulation of the Colorado River’s full water consumption budget (Table  1 ) provides accounting for all direct human uses of water as either agricultural or MCI (municipal, commercial, industrial), as well as indirect losses of water to reservoir evaporation and evapotranspiration from riparian or wetland vegetation including in the Salton Sea and in a wetland in Mexico (Cienega de Santa Clara) that receives agricultural return flows from irrigated areas in Arizona. We explicitly note that all estimates represent consumptive use , resulting from the subtraction of return flows from total water withdrawals. Table  2 provides a summary based only on direct human uses and does not include indirect consumption of water. We have provided Tables  1 and 2 in English units in our Supplementary Information as Tables SI-1 and SI-2 . We have lumped municipal, commercial, and industrial (MCI) uses together because these sub-categories of consumption are not consistently differentiated within official water delivery data for cities utilizing Colorado River water. More detail on urban water use by cities dependent on the river is available in Richter 14 , among other studies.

We differentiated water consumption geographically using the ‘accounting units’ mapped in Fig.  2 , which are based on the Colorado River Basin map as revised by Schmidt 15 ; importantly, these accounting units align spatially with Reclamation’s accounting systems for the Upper Basin and Lower Basin as described in our Methods, thereby enabling readers accustomed to Reclamation’s water-use reports to easily comprehend our accounting. We have also accounted for all water consumed within the Colorado River Basin boundaries as well as water exported via inter-basin transfers. Water exported outside of the basin includes 47 individual inter-basin transfer systems (i.e., canals, pipelines, pumps) that in aggregate export ~12% of the river’s water. We note that the Imperial Irrigation District of southern California is often counted as a recipient of exported water, but we have followed the rationale of Schmidt 15 by including it as an interior part of the Lower Basin even though it receives its Colorado River water via the All American Canal (Fig.  2 ).

figure 2

The water budget estimates presented in Tables  1 and 2 are summarized for each of the seven “accounting units” displayed here.

These results confirm previous findings that irrigated agriculture is the dominant consumer of Colorado River water. Irrigated agriculture accounts for 52% of overall consumption (Table  1 ; Figs.  3 and 4 ) and 74% of direct human consumption (Table  2 ) of water from the Colorado River Basin. As highlighted in Richter et al. 11 , cattle-feed crops (alfalfa and other hay) are the dominant water-consuming crops dependent upon irrigation water from the basin (Tables  1 and 2 ; Figs.  3 and 4 ). Those crops account for 32% of all water consumed from the basin, 46% of all direct water consumption, and 62% of all agricultural water consumed (Table  1 ; Fig.  3 ). The percentage of water consumed by irrigated crops is greatest in Mexico, where they account for 86% of all direct human uses (Table  2 ) and 80% of total water consumed (Table  1 ). Cattle-feed crops consume 90% of all water used by irrigated agriculture within the Upper Basin, where the consumed volume associated with these cattle-feed crops amounts to more than three times what is consumed for municipal, commercial, or industrial uses combined.

figure 3

All estimates based on 2000–2019 averages. Both agriculture and MCI (municipal, commercial, and industrial) uses are herein referred to as “direct human uses.” “Indirect uses” include both reservoir evaporation as well as evapotranspiration by riparian/wetland vegetation.

figure 4

Water consumed by each sector in the Colorado River Basin and sub-basins (including exports), based on 2000–2019 averages.

Another important finding is that a substantial volume of water (19%) is consumed in supporting the natural environment through riparian and wetland vegetation evapotranspiration along river courses. This analysis—made possible because of recent mapping of riparian vegetation in the Colorado River Basin 16 —is an important addition to the water budget of the Colorado River Basin, given that the only previous accounting for riparian vegetation consumption has limited to the mainstem of the Colorado River below Hoover Dam and does not include vegetation upstream of Hoover Dam nor vegetation along tributary rivers 17 . Given that many of these habitats and associated species have been lost or became imperiled due to river flow depletion 18 —including the river’s vast delta ecosystem in Mexico—an ecologically sustainable approach to water management would need to allow more water to remain in the river system to support riparian and aquatic ecosystems. Additionally, 11% of all water consumed in the Colorado River Basin is lost through evaporation from reservoirs.

It is also important to note a fairly high degree of inter-annual variability in each sector of water use; for example, the range of values portrayed for the four water budget sectors shown in Fig.  5 equates to 24–47% of their 20-year averages. Also notable is a decrease in water consumed in the Lower Basin between the years 2000 and 2019 for both the MCI (−38%) and agricultural sectors (−15%), which can in part be attributed to the policy agreements summarized previously that have mandated water-use reductions.

figure 5

Inter-annual variability of water consumption within the Lower and Upper Basins, including water exported from these basins. The average (AVG) values shown are used in the water budgets detailed in Tables  1 and 2 .

The water accounting in Richter et al. 11 received a great deal of media attention including a front-page story in the New York Times 19 . These stories focused primarily on our conclusion that more than half (53%) of water consumed in the Colorado River Basin was attributable to cattle-feed crops (alfalfa and other hays) supporting beef and dairy production. However, that tabulation of the river’s water budget had notable shortcomings, as discussed previously. In this more complete accounting that includes Colorado River water exported outside of the basin’s physical boundary as well as indirect water consumption, we find that irrigated agriculture consumes half (52%) of all Colorado River Basin water, and the portion of direct consumption going to cattle-feed crops dropped from 53% as reported in Richter et al. 11 to 46% in this revised analysis.

These differences are explained by the fact that we now account for all exported water and also include indirect losses of water to reservoir evaporation and riparian/wetland evapotranspiration in our revised accounting, as well as improvements in our estimation of crop-water consumption. However, the punch line of our 2020 paper does not change fundamentally. Irrigated agriculture is the dominant consumer of water from the Colorado River, and 62% of agricultural water consumption goes to alfalfa and grass hay production.

Richter et al. 20 found that alfalfa and grass hay were the largest water consumers in 57% of all sub-basins across the western US, and their production is increasing in many western regions. Alfalfa is favored for its ability to tolerate variable climate conditions, especially its ability to persist under greatly reduced irrigation during droughts and its ability to recover production quickly after full irrigation is resumed, acting as a “shock absorber” for agricultural production under unpredictable drought conditions. The plant is also valued for fixing nitrogen in soils, reducing fertilizer costs. Perhaps most importantly, labor costs are comparatively low because alfalfa is mechanically harvested. Alfalfa is increasing in demand and price as a feed crop in the growing dairy industry of the region 21 . Any efforts to reduce water consumed by alfalfa—either through shifting to alternative lower-water crops or through compensated fallowing 20 —will need to compete with these attributes.

This new accounting provides a more comprehensive and complete understanding of how the Colorado River Basin’s water is consumed. During our study period of 2000–2019, an estimated average of 23.7 billion cubic meters (19.3 million acre-feet) of water was consumed each year before reaching its now-dry delta in Mexico. Schmidt et al. 2 have estimated that a reduction in consumptive use in the Upper and Lower Basins of 3–4 billion cubic meters (2.4–3.2 million acre-feet) per year—equivalent to 22–29% of direct use in those basins—will be necessary to stabilize reservoir levels, and an additional reduction of 1–3 billion cubic meters (~811,000–2.4 million acre-feet) per year will likely be needed by 2050 as climate warming continues to reduce runoff in the Colorado River Basin.

We hope that this new accounting will add clarity and a useful informational foundation to the public dialog and political negotiations over Colorado River Basin water allocations and cutbacks that are presently underway 2 . Because a persistent drought and intensifying aridification in the region has placed both people and river ecosystems in danger of water shortages in recent decades, knowledge of where the water goes will be essential in the design of policies for bringing the basin into a sustainable water supply-demand balance.

The data sources and analytical approaches used in this study are summarized below. Unless otherwise noted, all data were assembled for each year from 2000–2019 and then averaged. We acknowledge some inconsistency in the manner in which water consumption is measured or estimated across the various data sources and sectors used in this study, as discussed below, and each of these different approaches entail some degree of inaccuracy or uncertainty. We also note that technical measurement or estimation approaches change over time, and new approaches can yield differing results. For instance, the Upper Colorado River Commission is exploring new approaches for estimating crop evapotranspiration in the Upper Basin 22 . When new estimates become available we will update our water budget accordingly.

MCI and agricultural water consumption

The primary source of data on aggregate MCI (municipal, commercial, and industrial) and agricultural water consumption from the Upper and Lower Basins was the US Bureau of Reclamation. Water consumed from the Upper Basin is published in Reclamation’s five-year reports entitled “Colorado River—Upper Basin Consumptive Uses and Losses.” 23 These annual data have been compiled into a single spreadsheet used for this study 24 . Because measurements of agricultural diversions and return flows in the Upper Basin are not sufficiently complete to allow direct calculation of consumptive use, theoretical and indirect methods are used as described in the Consumptive Uses and Losses reports 25 . Reclamation performs these estimates for Colorado, Wyoming, and Utah, but the State of New Mexico provides its own estimates that are collaboratively reviewed with Reclamation staff. The consumptive use of water in thermoelectric power generation in the Upper Basin is provided to Reclamation by the power companies managing each generation facility. Reclamation derives estimates of consumptive use for municipal and industrial purposes from the US Geological Survey’s reporting series (published every 5 years) titled “Estimated Use of Water in the United States” at an 8-digit watershed scale 26 .

Use of shallow alluvial groundwater is included in the water accounting compiled by Reclamation but use of deeper groundwater sources—such as in Mexico and the Gila River Basin—is explicitly excluded in their accounting, and in ours. Reclamation staff involved with water accounting for the Upper and Lower Basins assume that groundwater use counted in their data reports is sourced from aquifers that are hydraulically connected to rivers and streams in the CRB (James Prairie, US Bureau of Reclamation, personal communication, 2023); because of this high connectivity, much of the groundwater being consumed is likely being sourced from river capture as discussed in Jasechko et al. 27 and Wiele et al. 28 and is soon recharged during higher river flows.

Water consumed from the Lower Basin (excluding water supplied by the Gila River Basin) is published in Reclamation’s annual reports entitled “Colorado River Accounting and Water Use Report: Arizona, California, and Nevada.” 3 These consumptive use data are based on measured deliveries and return flows for each individual water user. These data are either measured by Reclamation or provided to the agency by individual water users, tribes, states, and federal agencies 29 . When not explicitly stated in Reclamation reports, attribution of water volumes to MCI or agricultural uses was based on information obtained from each water user’s website, information provided directly by the water user, or information on export water use provided in Siddik et al. 30 . Water use by entities using less than 1.23 million cubic meters (1000 acre-feet) per year on average was allocated to MCI and agricultural uses according to the overall MCI-agricultural percentages calculated within each sub-basin indicated in Tables  1 and 2 for users of greater than 1.23 million cubic meters/year.

Disaggregation of water consumption by sector was particularly important and challenging for the Central Arizona Project given that this canal accounts for 21% of all direct water consumption in the Lower Basin. Reclamation accounts for the volumes of annual diversions into the Central Arizona Project canal but the structure serves 1071 water delivery subcontracts. We classified every unique Central Arizona Project subcontract delivery between 2000–2019 by its final water use to derive an estimated split between agricultural and MCI uses. Central Arizona Project subcontract delivery data were obtained from the current and archived versions of the project’s website summaries in addition to being directly obtained from the agency through a public information request. Subcontract deliveries were classified based on the final end use, including long-term and temporary leases of project water. This accounting also includes the storage of water in groundwater basins for later MCI or agricultural use. Additionally, water allocated to Native American agricultural uses that was subsequently leased to cities was classified as an MCI use.

Data for the Gila River basin was obtained from two sources. The Arizona Department of Water Resources has published data for surface water use in five “Active Management Areas” (AMAs) located in the Gila River basin: Prescott AMA, Phoenix AMA, Pinal AMA, Tucson AMA, and Santa Cruz AMA 31 . The water-use data for these AMAs is compiled from annual reports submitted by each water user (contractor) and then reviewed by the Arizona Department of Water Resources. The AMA water-use data are categorized by purpose of use, facilitating our separation into MCI and agricultural uses. These data are additionally categorized by water source; only surface water sourced from the Gila River hydrologic system was counted (deep groundwater use was not). The AMA data were supplemented with data for the upper Gila River basin provided by the University of Arizona 32 . We have assumed that all water supplied by the Gila River Basin is fully consumed, as the river is almost always completely dry in its lower reaches (less than 1% flows out of the basin into the Colorado River, on average 33 ).

Data for Mexico were obtained from Hernandez-Cruz et al. 10 based on estimates for 2008–2015. Agricultural demands were estimated from annual reports of irrigated area and water use published by the Ministry of Agriculture and the evapotranspiration estimates of the principal crops published by the National Institute for Forestry, Animal Husbandry, and Agricultural Research of Mexico 10 . The average annual volume of Colorado River water consumption in Mexico estimated by these researchers is within 1% of the cross-border delivery volume estimated by the Bureau of Reclamation for 2000–2019 in its Colorado River Accounting and Water Use Reports 3 .

Exported water consumption

Annual average inter-basin transfer volumes for each of 46 canals and pipelines exporting water outside of the Upper Basin were obtained from Reclamation’s Consumptive Uses and Losses spreadsheet 34 . Data for the Colorado River Aqueduct in the Lower Basin were obtained from Siddik et al. 30 Data for exported water in Mexico was available from Hernandez-Cruz et al. 10 . We assigned any seepage or evaporation losses from inter-basin transfers to their proportional end uses. All uses of exported water are considered to be consumptive uses with respect to the Colorado River, because none of the water exported out of the basin is returned to the Colorado River Basin.

We relied on data from Siddik et al. (2023) to identify whether the water exported out of the Colorado River Basin was for only MCI or agricultural use. When more than one water use purpose was identified, as well as for all major inter-basin transfers, we used government and inter-basin transfer project websites or information obtained directly from the project operator or water manager to determine the volume of water transferred and the end uses. Major recipients of exported water include the Coachella Valley Water District (California); Metropolitan Water District of Southern California (particularly for San Diego County, California); Northern Colorado Water Conservancy District; City of Denver (Colorado); the Central Utah Project; City of Albuquerque (New Mexico); and the Middle Rio Grande Conservancy District (New Mexico). We did not pursue sectoral water-use information for 17 of the 46 Upper Basin inter-basin transfers due to their relatively low volumes of water transferred by each system (<247,000 cubic meters or 2000 acre-feet), and instead assigned the average MCI or agricultural percentage (72% MCI, 28% agricultural) from all other inter-basin transfers in the Upper Basin. The export volume of these 17 inter-basin transfers sums to 9.76 million cubic meters (7910 acre-feet) per year, equivalent to 1% of the total volume exported from the Upper Basin.

Reservoir evaporation

Evaporation estimates for the Upper Basin and Lower Basin are based upon Reclamation’s HydroData repository 35 . Reclamation’s evaporation estimates are based on the standardized Penman-Monteith equation as described in the “Lower Colorado River Annual Summaries of Evapotranspiration and Evaporation” reports 17 . The Penman-Monteith estimates are based on pan evaporation measurements. Evaporation estimates for the Salt River Project reservoirs in the Gila River basin were provided by the Salt River Project in Arizona (Charlie Ester, personal communication, 2023).

Another consideration with reservoirs is the volume of water that seeps into the banks or sediments surrounding the reservoir when reservoir levels are high, but then drains back into the reservoir as water levels decline 36 . This has the effect of either exacerbating reservoir losses (consumptive use) or offsetting evaporation when bank seepage flows back into a reservoir. The flow of water into and out of reservoir banks is non-trivial; during 1999–2008, an estimated 247 million cubic meters (200,000 acre-feet) of water drained from the canyon walls surrounding Lake Powell into the reservoir each year, providing additional water supply 36 . However, the annual rate of alternating gains or losses has not been sufficiently measured at any of the basin’s reservoirs and therefore is not included in Tables  1 and 2 .

Riparian and wetland vegetation evapotranspiration

We exported the total annual evapotranspiration depth at a 30 meter resolution from OpenET 37 using Google Earth Engine from 2016 to 2019 to align with OpenET’s data availability starting in 2016. Total annual precipitation depths, sourced from gridMET 38 , were resampled to align with the evapotranspiration raster resolution. Subsequently, a conservative estimate of the annual water depth utilized by riparian vegetation from the river was derived by subtracting the annual precipitation raster from the evapotranspiration raster for each year. Positive differentials, indicative of river-derived evapotranspiration, were then multiplied by the riparian vegetation area as identified in the CO-RIP 16 dataset to estimate the total annual volumetric water consumption by riparian vegetation across the Upper, Lower, and Gila River Basins. The annual volumetric water consumption calculated over four years were finally averaged to get riparian vegetation evapotranspiration in the three basins. Because the entire flow of the Colorado River is diverted into the Canal Alimentador Central near the international border, very little riparian evapotranspiration occurs along the river south of the international border in the Mexico basin.

In addition to water consumed by riparian evapotranspiration within the Lower Basin, the Salton Sea receives agricultural drain water from both the Imperial Irrigation District and the Coachella Valley Irrigation District, stormwater drainage from the Coachella Valley, and inflows from the New and Alamo Rivers 39 . Combined inflows to the Sea during 2015–2019 were added to our estimates of riparian/wetland evapotranspiration in the Lower Basin.

Similarly, Mexico receives drainage water from the Wellton–Mohawk bypass drain originating in southern Arizona that empties into the Cienega de Santa Clara (a wetland); this drainage water is included as riparian/wetland evapotranspiration in the Mexico basin.

Crop-specific water consumption

The volumes of total agricultural consumption reported for each sub-basin in Tables  1 and 2 were obtained from the same data sources described above for MCI consumption and exported water. The portion (%) of those agricultural consumption volumes going to each individual crop was then allocated according to percentage estimates of each crop’s water consumption in each accounting unit using methods described in Richter et al. 20 and detailed here.

Monthly crop water requirements during 1981–2019 for 13 individual crops, representing 68.8% of total irrigated area in the US in 2019, were estimated using the AquaCrop-OS model (Table SI- 3 ) 40 . For 17 additional crops representing about 25.4% of the total irrigated area, we used a simple crop growth model following Marston et al. 41 as crop parameters needed to run AquaCrop-OS were not available. A list of the crops included in this study is shown in Table SI- 3 . The crop water requirements used in Richter et al. 11 were based on a simplistic crop growth model, often using seasonal crop coefficients whereas we use AquaCrop-OS 40 , a robust crop growth model, to produce more realistic crop growth and crop water estimates for major crops. AquaCrop-OS is an open-source version of the AquaCrop model 42 , a crop growth model capable of simulating herbaceous crops. Additionally, we leverage detailed local data unique to the US, including planting dates and subcounty irrigated crop areas, to produce estimates at a finer spatial resolution than the previous study. We obtained crop-specific planting dates from USDA 43 progress data at the state level. For crops that did not have USDA crop progress data, we used data from FAO 44 and CUP+ model 45 for planting dates. We used climate data (precipitation, minimum and maximum air temperature, reference ET) from gridMET 38 , soil texture data from ISRIC 46 database and crop parameters from AquaCrop-OS to run the model. The modeled crop water requirement was partitioned into blue and green components following the framework from Hoekestra et al. 47 , assuming that blue and green water consumed on a given day is proportional to the amount of green and blue water soil moisture available on that day. When applying a simple crop growth model, daily gridded (2.5 arc minutes) crop-specific evapotranspiration (ETc) was computed by taking the product of reference evapotranspiration (ETo) and crop coefficient (Kc), where ETo was obtained from gridMET. Crop coefficients were calculated using planting dates and crop coefficient curves from FAO and CUP+ model. Kc was set to zero outside of the growing season. We partitioned the daily ETc into blue and green components by following the methods from ref. 41 It is assumed that the crop water demands are met by irrigation whenever it exceeds effective precipitation (the latter calculated using the USDA Soil Conservation Service method (USDA, 1968 48 ). We obtained county level harvested area from USDA 43 and disaggregated to sub-county level using Cropland Data Layer (CDL) 49 and Landsat-based National Irrigation Dataset (LANID) 50 . The CDL is an annual raster layer that provides crop-specific land cover data, while the LANID provides irrigation status information. The CDL and LANID raster were multiplied and aggregated to 2.5 arc minutes to match the AquaCrop-OS output. We produced a gridded crop area map by using this resulting product as weights to disaggregate county level area. CDL is unavailable before 2008. Therefore, we used land use data from ref. 51 in combination with average CDL map and county level harvested area to produce gridded crop harvested area. We computed volumetric water consumption by multiplying the crop water requirement depth by the corresponding crop harvested area.

Data availability

All data compiled and analyzed in this study are publicly available as cited and linked in our Methods section. Our compilation of these data is also available from Hydroshare at: http://www.hydroshare.org/resource/2098ae29ae704d9aacfd08e030690392 .

Code availability

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Acknowledgements

This paper is dedicated to our colleague Jack Schmidt in recognition of his retirement and enormous contributions to the science and management of the Colorado River. The authors thank James Prairie of the US Bureau of Reclamation, Luke Shawcross of the Northern Colorado Water Conservancy District, Charlie Ester of the Salt River Project, and Brian Woodward of the University of California Cooperative Extension for their assistance in accessing data used in this study. The authors also thank Rhett Larson at the Sandra Day O’Connor School of Law at Arizona State University for their review of Arizona water budget data, and the Central Arizona Project for providing delivery data by each subcontract. G.L., L.M., and K.F.D. acknowledge support by the United States Department of Agriculture National Institute of Food and Agriculture grant 2022-67019-37180. L.T.M. acknowledges the support the National Science Foundation grant CBET-2144169 and the Foundation for Food and Agriculture Research Grant No. FF-NIA19-0000000084. R.R.R. acknowledges the support the National Science Foundation grant CBET-2115169.

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Contributions

B.D.R. designed the study, compiled and analyzed data, wrote the manuscript and supervised co-author contributions. G.L. compiled all crop data, estimated crop evapotranspiration, and prepared figures. S.D. compiled all riparian vegetation data and estimated riparian evapotranspiration. L.S.S. and R.R.R. accessed, compiled, and analyzed data from the Central Arizona Project. D.W. compiled data and prepared figures. A.H.-C. and S.S.-S. compiled and analyzed data for Mexico. J.C.S. compiled and analyzed reservoir evaporation data and edited the manuscript. L.M., B.L.R., and K.F.D. supervised data compilation and analysis and edited the manuscript.

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Correspondence to Brian D. Richter .

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Richter, B.D., Lamsal, G., Marston, L. et al. New water accounting reveals why the Colorado River no longer reaches the sea. Commun Earth Environ 5 , 134 (2024). https://doi.org/10.1038/s43247-024-01291-0

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DOI : https://doi.org/10.1038/s43247-024-01291-0

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MDARD Partners with Alliance for the Great Lakes, MSU Institute of Water Research, and LimnoTech to Deploy Nearly $5 million in Water Quality Monitoring in the Western Lake Erie Basin

April 03, 2024

Water quality research and data are fundamental to achieving environmental outcomes in the WLEB

LANSING Michigan Department of Agriculture and Rural Development (MDARD) Director Dr. Tim Boring today announced plans to partner with the Alliance for the Great Lakes for $4.86 million in funding over the next five years to the Alliance for the Great Lakes to expand water quality monitoring in the Western Lake Erie Basin (WLEB). The research effort utilizes expertise at Michigan State University's Institute of Water Research and LimnoTech , while leveraging $600,000 in funding from the Erb Family Foundation. This significant increase in research and monitoring will aid the state's strategy in developing a plan to combat harmful algal blooms in the WLEB.

"Improving our understanding of nutrient losses and transport in the WLEB is essential to accelerating progress on nutrient loading reductions," said  Boring . "Our department has recognized the need for improved water quality monitoring in WLEB. We know that more holistic farm management focusing on soil health and regenerative agriculture principles can be expected to improve nutrient losses. Through the State of Michigan's Domestic Action Plan adaptive management approach of continuous assessment and improvement, the scientific outcomes of this work improve our ability to make meaningful progress toward water quality improvements."

"We are excited to partner with MDARD on this effort and applaud the leadership of Director Boring who has continually emphasized the importance of expanding monitoring and data collection to help guide conservation decision making," said  Tom Zimnicki, Alliance for the Great Lakes Agriculture and Restoration Policy Director.

Harmful algal blooms occur when colonies of algae grow out of control. Some produce dangerous toxins which can have harmful effects on people and wildlife, but even non-toxic blooms can hurt the environment. The algal blooms need sunlight, slow-moving water, and nutrients to grow. Phosphorus pollution from human activity can make the problem worse, leading to blooms occurring more often.

Monitoring will begin later this spring in five priority HUC-12 sub-watersheds: Lime Creek, Stony Creek (South Branch River Raisin), Headwaters of the Saline River, Nile Ditch, and the S.S. LaPointe Drain. These subwatersheds were selected for more focused and accelerated activities including finer-scale water quality monitoring, completing agricultural inventories, prioritized BMP implementation, and assessing the costs associated with full implementation to achieve a 40 percent total phosphorus reduction goal.  In-stream data collection will include stream flow, total phosphorus and soluble reactive phosphorus, turbidity, and total suspended solids. These gauge stations will be combined with soil moisture, precipitation, and tile outlet sensors deployed through the watershed to better understand the fate and transport of nutrients in the WLEB watershed.

Understanding, tracking, and predicting nutrient loads from the WLEB watershed is difficult due to the complex drivers of nutrient loss within sub-watersheds in the WLEB including variable weather, cropping systems, farm management, nutrient cycling. By increasing monitoring capacity in the WLEB at smaller sub-watershed scales, with an emphasis on deploying higher spatial density monitoring instrumentation, this research will improve the understanding of the impact of various drivers on nutrient transport and enable improved prioritization of conservation and land management practices to meet phosphorus reduction commitments set for Michigan s portion of the WLEB.

Visit the Taking Action on Lake Erie website to learn more about steps you can take to help reduce pollution and protect Michigan s natural resources.

Media Contact:

Mike Philip

Program Contact

[email protected]

517-512-0187

Tom Zimnicki

Alliance for Great Lakes contact

[email protected]

313-969-3499

Jennifer Holton

MDARD Media Contact

[email protected]

517-284-5724

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Concerns and Threats of Contamination on Aquatic Ecosystems

Ishrat bashir.

Sher-e-Kashmir University of Agricultural Sciences and Technology, Jammu, Jammu and Kashmir India

Rouf Ahmad Bhat

Shafat a. mir, zubair a. dar, shakeel ahmad dar.

Aquatic ecosystems are the ultimate sinks for the contaminants. Water contamination is the outcome of human activities such as urbanization, industrialization, and agricultural activities. The overuse of pesticides and fertilizers and sewage from residential and industrial areas ultimately find its way to aquatic environment. Thus results in the degradation of the water quality and leads to the spread of infectious diseases such as dysentery, diarrhea, and jaundice. Contamination in aquatic environs is one of the leading types of pollution which has significant negative health issues and mortality. Water has a natural capacity to neutralize the contamination, but when contamination becomes uncontrolled, water will lose its self-generating capacity. Therefore, there is a need for regular monitoring and controlling of pollutant discharge into the nearby aquatic environs.

Introduction

Anthropogenic activities bring almost contamination and subsequent pollution to our varied ecosystems. “Pollution is defined as the production and or introduction by man, directly or indirectly of substances or energy into the environment, resulting in deleterious effects to living resources, including human beings or interfere with amenities and other uses of the environment (Don-Pedro 1990 ).” Pollution is one of the prime problems that humans face in the whole world particularly in the developing countries. However, produced by humans and their activities, it has harmful effects on man’s environment and resources (Mendil and Uluözlu 2007 ). The discharge of various pollutants into the aquatic environments is the outcome of countless anthropogenic activities, threatening the health of the living beings and damaging the quality of the environment by rendering water bodies unsuitable (Abowel and Sikoki 2005 ; Ekubo and Abowel 2011 ). Aquatic environments are pickers for anthropogenic contamination and industrial wastes and leaks, whether chemicals or solid pollutants (Hampel et al. 2015 ; Bhat et al. 2017 ). These wastes can be “heavy metals, detergents, microfibers, plastic or non-plastic origin,” etc., and contribute to “aquatic pollution problems” (Hampel et al. 2015 ). Aquatic environs are addressee for plenty of pollutants and their outrageous toxic actions (Hampel et al. 2015 ). “Chemicals reaching aquatic ecosystems include radioactive elements” (“strontium, cesium, radon”), metals (“cadmium, mercury, lead”), industrial solvents and “volatile organic compounds” (“tri- and tetrachloroethylene, chlorofluorocarbons, benzene, xylenes, formaldehyde”), “agrochemicals” (“fertilizers and pesticides”), household products (“detergents, cleaners, paints”), “fuel combustion” (“N and sulfur oxides,” “polycyclic aromatic hydrocarbons,” “carbon monoxide,” and “carbon dioxide”), “nanoparticles,” personal care products, “microplastics, antibiotics,” as well as a huge variety of prescription (Hampel et al. 2015 )” and “nonprescription drugs and pharmaceuticals of human and veterinary medicine” (Hughes et al. 2013 ; Larsson 2014 ; Malaj et al. 2014 ; Hampel et al. 2015 ).

“ Aquatic ecosystems, particularly the freshwater ecosystems, are exposed to supplementary contamination than other environs, as water is used in various industrial practices as well as release of discharges commencing from industry” and urban growths (Demirak et al. 2006 ; Fernandesa et al. 2007 ). “ Water pollution is a worldwide task that has augmented in both advanced and emerging nations” (Mateo-Sagasta et al. 2017 ). “Universally, 80% of municipal wastewater is discharged into water bodies untreated, and industry is responsible for dumping millions of tons of heavy metals” (Mateo-Sagasta et al. 2017 ), “solvents, toxic sludge and other wastes into water bodies each year” (WWAP 2017 ; Mateo-Sagasta et al. 2017 ). Agriculture, exploits “70% of water globally and plays a key part in water contamination” (Mateo-Sagasta et al. 2017 ). Huge amounts of “agrochemicals,” “organic matter,” “drug residues,” “sediments,” and “saline drainage” from agricultural lands are released into water bodies (Mateo-Sagasta et al. 2017 ) and hence poses significant threats to “aquatic environments,” “human health,” and “productive activities” (UNEP 2016 ; Mateo-Sagasta et al. 2017 ). Most aquatic ecosystems have a natural tendency to dilute pollution to some extent, but severe contamination of aquatic ecosystems results in alteration in the fauna and flora of the community (Mateo-Sagasta et al. 2017 ). The onset of human civilization in itself discloses the history of aquatic pollution (Mateo-Sagasta et al. 2017 ). Moreover, aquatic pollution did not receive significant consideration until a threshold level was reached with hostile outcomes on the “ecosystems” and “living organisms” including “humans” (Halpern et al. 2008 ; Mateo-Sagasta et al. 2017 ). Therefore, “pollution and its effects are considered as one of man’s greatest crimes against himself. Pollutants may cause primary damage, with direct identifiable impact on the environment, or secondary damage in the form of minor perturbations in the delicate balance of the biological food web that are detectable only over long time periods” (Sharma 2012 ; Al Naggar et al. 2014 ; Ghani 2015 ). Thus, “maintaining the quality of aquatic ecosystems represents one of the most formidable challenges facing global society in the twenty-first century” (Hampel et al. 2015 ).

Aquatic Ecosystems

Aquatic ecosystems are water-based environments in which biotic components interact with abiotic components of the aquatic ecosystem. “Aquatic ecosystems” are usually divided into two types: the “marine ecosystem” and the “freshwater ecosystem” (Barange et al. 2010 ). Marine ecosystem is the largest water ecosystem which covers over 70% of the Earth’s surface. The marine ecosystem is subdivided into “oceans,” “estuaries,” “ coral reefs,” and “ coastal ecosystems.” Freshwater ecosystems cover less than 1% of the Earth’s surface. The various kinds of freshwater ecosystems are lotic ecosystem, lentic ecosystem, and wetland ecosystem.

Human Activities Resulting in Contamination of Aquatic Ecosystems and Their Adverse Impacts

Anthropogenic activities such as “ deforestation,” “filling and construction of canals,” “dams,” “roads and bridges,” “agricultural,” and “industrial and domestic activities” result in contamination of aquatic environments. Human settlements, industries, and agriculture are the main sources of water pollution (Table 1.1 ). In most developed nations, agriculture is the major factor in the degradation of water ecosystems. In the “European Union, 38% of water ecosystems are significantly under agricultural pressure” (WWAP 2015 ; Mateo-Sagasta et al. 2017 ). In the USA, “agriculture is the leading source of pollution in rivers and streams” (Mateo-Sagasta et al. 2017 ), the second main source in wetlands, and the third main source in lakes (USEPA 2016 ; Mateo-Sagasta et al. 2017 ). In China, “ agriculture is accountable for a huge portion of surface-water pollution and is responsible almost entirely for groundwater pollution” (Mateo-Sagasta et al. 2017 ) by nitrogen (FAO 2013 ; Mateo-Sagasta et al. 2017 ). In emerging nations, the unlimited amounts of raw municipal and industrial wastewater are major threats (Mateo-Sagasta et al. 2017 ).

Sources and route of pollutant discharge into aquatic environs (NEST 1991 ; Mateo-Sagasta et al. 2017 )

Agrochemicals

The ever-increasing “demand for food has led to the land clearance and the expansion of agriculture” which have “contributed to the higher pollution loads in the water” (Mateo-Sagasta et al. 2017 ). Increase in the population growth has increased the food demand, which has resulted in the increase in the quantity of agrochemicals used to increase the production (Schwarzenbach et al. 2010 ). The “unsustainable use of agrochemicals” (“fertilizers, pesticides, herbicides and plant hormones”) to rise the production has resulted in “greater pollution masses” in the environment, including “rivers,” “lakes,” “aquifers,” and “coastal waters” (Mateo-Sagasta et al. 2017 ; Bhat et al. 2018 ; Mushtaq et al. 2018 ). More importantly, “agricultural areas gather an extensive variety of agrochemicals from nearby fields” due to “run off,” “direct drift,” and “leaching,” and these areas are “the principal receivers of agrochemicals” (Rathore and Nollet 2016 ).

When “fertilizers are applied at a higher rate than they are fixed by the soil, or taken up by the crops or when they are taken off through surface runoff from the soil surface leads to water pollution.” “Excess nitrogenous fertilizers and phosphate fertilizers can leach into groundwater or reach into surface water bodies through surface runoff” (Mateo-Sagasta et al. 2017 ). If “ organic manure” is applied “in excess in the agricultural fields,” it will lead to “diffuse water pollution.” Mostly, “manure is not stored in confined areas and during heavy rainfall events it can be washed into waterways via surface runoff.” The “high-nutrient concentration together with other substances results in the nutrient enrichment eutrophication” of “lakes,” “reservoirs,” “ponds,” and “coastal waters,” which leads to excessive growth of aquatic plants—“algae blooms” that destroy other aquatic plants and animals. “About 415 coastal areas have been identified worldwide which experience eutrophication” (Mateo-Sagasta et al. 2017 ), of which 169 are hypoxic (WRI 2008 ; Mateo-Sagasta et al. 2017 ). The “excessive buildup of nutrients may also increase the adverse health effects” (Mateo-Sagasta et al. 2017 ), such as “blue-baby syndrome- due to high levels of nitrate in drinking water” (Mateo-Sagasta et al. 2017 ). “Nitrate from agriculture leaches into the groundwater is the most common chemical contaminant in the world’s groundwater aquifers” (Mateo-Sagasta et al. 2017 ).

Pesticides such as “insecticides,” “herbicides, and fungicides” are applied extensively in agriculture fields in several nations (Schreinemachers and Tipraqsa 2012 ; Mateo-Sagasta et al. 2017 ; Bhat et al. 2018 ) and get washed into aquatic ecosystems and pollute the water resources (Mateo-Sagasta et al. 2017 ). They contain “carcinogens and other poisonous substances that may kill aquatic life” or may be absorbed by them (Mateo-Sagasta et al. 2017 ) and pass through the “food chain until they become toxic to humans” (Mateo-Sagasta et al. 2017 ). “Millions of tons of pesticides are used in agriculture fields” (FAO 2016a ; Mateo-Sagasta et al. 2017 ). “Acute pesticide poisoning causes significant human morbidity” (Mateo-Sagasta et al. 2017 ) and “mortality worldwide, especially in low income countries, where poor farmers often use highly hazardous pesticides” (Mateo-Sagasta et al. 2017 ).

Through irrigation, accumulated salts in soils are transported into receiving water bodies by drainage water and cause salinization (Mateo-Sagasta et al. 2017 ). The “intrusion of saline seawater into groundwater aquifers as a result of excessive groundwater extractions for agriculture is another important cause of salinization in coastal areas” (Mateo-Sagasta and Burke 2010 ; Mateo-Sagasta et al. 2017 ). “Major water-salinity problems have been reported in Argentina, Australia, China, India, the Sudan, the United States of America, and many countries in Central Asia” (FAO 2011 ). “Highly saline waters alter the geochemical cycles of major elements such as carbon, iron, nitrogen, phosphorus, silicon and sulphur” (Herbert et al. 2015 ; Mateo-Sagasta et al. 2017 ) with overall impacts on ecosystems (Mateo-Sagasta et al. 2017 ). Salinization can affect freshwater biota (Mateo-Sagasta et al. 2017 ) by “causing changes within species and community composition” (Mateo-Sagasta et al. 2017 ) and results “in decline of the biodiversity of microorganisms, algae, plants and animals” (Lorenz 2014 ; Mateo-Sagasta et al. 2017 ).

Emerging Pollutants

New “agricultural pollutants such as antibiotics, vaccines, growth promoters and hormones have emerged in the last two decades” (Mateo-Sagasta et al. 2017 ). These pollutants can reach water via “leaching and runoff from livestock” (Mateo-Sagasta et al. 2017 ) and “aquaculture farms, as well as through the application of manure and slurries to agricultural land” (OECD 2012 ; Mateo-Sagasta et al. 2017 ). Today, “more than 700 emerging pollutants and their metabolites and transformation products are listed as present in European aquatic environments” (Norman 2016 ; Mateo-Sagasta et al. 2017 ). “Agriculture is not only a source of emerging pollutants, it also contributes to the spread and reintroduction of such pollutants into aquatic environments through wastewater reuse for irrigation and the application of municipal biosolids to land as fertilizers” (Mateo-Sagasta et al. 2017 ). “An estimated 35.9 Mha of agricultural lands are subjected to the indirect use of wastewater” (Thebo et al. 2017 ; Mateo-Sagasta et al. 2017 ). The “potential risks to human health posed by exposure to emerging pollutants via contaminated agricultural products needs attention” (Mateo-Sagasta et al. 2017 ).

The greatest volume of “waste discharged into the aquatic ecosystems is sewage.” Sewage contains “industrial wastes, municipal wastes and domestic wastes which include wastes from baths, washing machines, kitchens and faecal matter.” Fresh water sources “serve as best sinks for the discharge of these wastes” (Das and Acharya 2003 ; Tukura et al. 2009 ). It is estimated that “58% of the wastewater from urban areas and 81% of industrial wastes are discharged directly into water bodies with no or inadequate treatment results in contamination of ~73% of the water bodies” (Vargas-Gonzalez et al. 2014 ). The release of “sewage has led to the increase in water pollution and depletion of clean water resources” (Avalon Global Research 2012 ). “Huge loads of such wastes are generated daily from highly populated cities and are finally washed out by the drainage systems which generally open into nearby rivers or aquatic systems” (Tukura et al. 2009 ). It has resulted in “extensive ecological degradation such as a decline in water quality and availability, intense flooding, loss of species, and changes in the distribution and structure of the aquatic biota” (Oberdorff et al. 2002 ). The “negative impact of sewage is based on the composition and concentration of the contaminants as well as the volume and frequency of wastewater effluents entering water bodies” (Akpor and Muchie 2011 ; Bhat et al. 2017 ). “Sewage is comprised of several microorganisms, heavy metals, nutrients, radionuclides, pharmaceutical, and personal care products.” Sewage is primarily organic in nature; owing to the organic load of sewage, the “oxygen concentration in the receiving waters is reduced, thus sewage is said to have a high BOD.” The “effect of maltreated sewage on surface water is largely determined by the oxygen balance of the aquatic ecosystem, and its presence is essential in maintaining biological life within the system” (DFID 1999 ; Morrison et al. 2001 ; Momba et al. 2006 ). “DO concentrations below 5 mg/L can have a negative effect on the living organisms in the aquatic ecosystem” (Momba et al. 2006 ). Low dissolved oxygen concentration can affect “functioning of some fish species and can eventually lead to the death of fish population” (Igbinosa and Okoh 2009 ; Mehmood et al. 2019 ). Decaying “ organic matter” and “nutrients such as nitrites, nitrates, and phosphorus” in sewage can induce “eutrophication of water courses.” “Eutrophication can lead to growth of plants and algae blooms” in the “aquatic ecosystem” (Bhat et al. 2017 ). “Algal blooms result in toxin production.” Fish species “feeding in water contaminated” by “algal toxins will absorb these toxins and are subject to mass mortality” (Hernandez et al. 1998 ). Due to “eutrophication turbidity of the water increases, plant and animals’ biomass increases, sedimentation rate increases, species diversity decreases, and anoxic conditions may develop, and this could give rise to change in dominant species of the aquatic biota” (Edokpayi 2016 ).

“ Sewage effluent entering into surface waters contains a variety of pathogenic organisms that could result in the transmission of waterborne diseases when such contaminated water is used for domestic and other purposes” (WHO 2006 ; Chigor et al. 2013 ) thus is “detrimental to human health and the society at large” (DWA 1999 ). Some pathogens contaminate water resources (Mateo-Sagasta et al. 2017 ), via runoff (FAO 2006a ; WHO 2012 ; Mateo-Sagasta et al. 2017 ). About “25% of all deaths worldwide are the result of infectious diseases caused by pathogenic microorganisms” (UNEP 2006 ). Scientists have identified about “1400 species of microorganisms that can cause ill health, including bacteria, protozoa, protozoan parasites, parasitic worms, fungi, and viruses” (CSIR 2010 ). Some common “pathogens found in sewage” are presented in Table 1.2 (WHO 2006 ; Christou 2011 ).

Microbial diseases associated with polluted aquatic environs

Heavy Metals

“ Heavy metals enter the aquatic ecosystem from both natural and anthropogenic sources.” Entry may be as a result of “direct discharges into both fresh and marine ecosystems or through indirect routes such as atmospheric deposition and surface run-off” (Biney et al. 1994 ). Important “natural sources are volcanic activity and weathering of rocks.” “Heavy metals are natural constituents of rocks and soils and enter the environment as a consequence of weathering and erosion” (Förstner 1987 ). Heavy metals in “aquatic system can be naturally produced by the slow leaching from soil/rock to water, which are usually at low levels, causing no serious lethal effects on human health” (Chang et al. 2000 ; Rashid et al. 2019 ). The “development of industry and agriculture promotes the rapid increase of heavy metal pollution. Aquatic heavy metal pollution usually represents high levels of Mercury, Chromium, Lead, Cadmium, Copper, Zinc, Nickel etc. in the water system”. “Arsenic, Cadmium, Copper, Mercury and Zinc are the five metals with most potential impact that enter the environment in elevated concentrations through storm water and wastewater discharges as a consequence of agricultural and industrial activity” (Alloway 2013 ). They are important “group of toxic contaminants because of their high toxicity and persistence in all aquatic ecosystems.” Zinc and copper are present in “fertilizers as impurities, while Arsenic, Cadmium and Mercury are constituents of some fungicides and algaecides” (Fifield and Haines 2000 ) (Table 1.3 ).

Different kinds of heavy metal discharge sources in aquatic environs (Fifield and Haines 2000 )

Heavy metals have “high ecological significance because they are not removed from water, but accumulate in the water reservoirs and thus enter the food chain” (Loska and Wiechuła 2003 ). Under “certain environmental conditions,” heavy metals may “accumulate to a highly toxic concentration and cause ecological damage” (Harguinteguy et al. 2014 ). Once released in aquatic environments, they are generally “bound to particulate matter, which eventually settle down and become incorporated into sediments and are released into the water under the suitable conditions such as pH values and Eh, leading to further contamination of aquatic environment” (Xu and Yang 1996 ). Accordingly, sediments represent one of the “ultimate sinks for heavy metals discharged into aquatic environment” (Gibbs 1973 ; Bryan and Langston 1992 ; Harguinteguy et al. 2014 ). “More and more attention has been drawn due to the wide spread occurrence of metal pollution in aquatic system” (Zhou et al. 2008 ). Some “heavy metals” may transform into the “persistent metallic compounds with high toxicity” (Zhou et al. 2008 ), which can be “bioaccumulated in the organisms” (Zhou et al. 2008 ), “magnified in the food chain, thus threatening human health” (Jin 1992 ; Zhou et al. 2008 ). “Various harmful effects including abnormal development of fetus, procreation failure, and immune deficiency has exhibited due to aquatic metal exposure” (Chang et al. 2000 ; Zhou et al. 2008 ). Some heavy metals, including mercury, chromium, cadmium, nickel, copper, and lead, introduced into water systems may pose high toxicities on the aquatic organisms (Wu and Zhao 2006 ). As an example, “cadmium is a priority environmental contaminant with consequences for human health and the maintenance of bio-diversity in affected ecosystems” (Zhou et al. 2008 ) and “the timeliness of a broader, ecosystem-based approach to cadmium research is highlighted based on the overview of recent developments in the field” (Campbell 2006 ; Zhou et al. 2008 ).

Eutrophication

Eutrophication is a leading cause of destruction of many freshwater and marine ecosystems in the world. It is characterized by “excessive plant and algal growth due to the increased availability of one or more limiting growth factors needed for photosynthesis” (Schindler 2006 ), such as sunlight, carbon dioxide, and nutrients. “Eutrophication occurs naturally over centuries as lakes age and are filled in with sediments” (Carpenter 1981 ). However, “human activities have accelerated the rate and extent of eutrophication through both point-source discharges and non-point loadings of limiting nutrients, such as nitrogen and phosphorus, into aquatic ecosystems (i.e. cultural eutrophication), with dramatic consequences for drinking water sources, fisheries, and recreational water bodies” (Carpenter et al. 1998 ; Bhat et al. 2017 ). However, “during 1960s and 1970s, scientists linked algal blooms to nutrient enrichment resulting from anthropogenic activities such as agriculture, industry and sewage disposal” (Schindler 1974 ). The known “consequences of cultural eutrophication include blooms of blue-green algae (cyanobacteria), tainted drinking water supplies, degradation of recreational opportunities and hypoxia” (Bhat et al. 2017 ). The most obvious effect of cultural eutrophication is the creation of dense blooms of noxious, foul “smelling phytoplankton” that reduce water clarity and “harm water quality.” “Algal blooms limit light penetration, reduce growth and cause death of plants in littoral zones and also lower the success of predators that need light to catch prey” (Lehtiniemi et al. 2005 ). Furthermore, high rates of photosynthesis associated with eutrophication can deplete dissolved inorganic carbon and raise pH to extreme levels during the day. “Elevated pH can in turn ‘blind’ organisms that rely on perception of dissolved chemical cues for their survival by impairing their chemosensory abilities” (Turner and Chislock 2010 ). When these “dense algal blooms eventually die, microbial decomposition severely depletes DO, creating a hypoxic dead zone, lacking sufficient oxygen to support most organisms.” Dead zones are found in many “freshwater lakes including the Laurentian Great Lakes” (e.g., central basin of Lake Erie; Arend et al. 2011 ) during the summer. Furthermore, such “hypoxic events are particularly common in marine coastal environments surrounding large, nutrient-rich rivers” (e.g., Mississippi River and the Gulf of Mexico; Susquehanna River and the Chesapeake Bay) and have been shown to affect more than 245,000 square kilometers in over 400 near-shore systems (Diaz and Rosenberg 2008 ). “ Hypoxia and anoxia as a result of eutrophication continue to threaten profitable commercial and recreational fisheries worldwide. Some algal blooms pose an additional threat because they produce noxious toxins” (e.g., microcystin and anatoxin-a) (Chorus and Bartram 1999 ). Over the past century, “harmful algal blooms (HABs) have been linked with (1) degradation of water quality” (Francis 1878 ), (2) “destruction of economically important fisheries” (Burkholder et al. 1992 ), and (3) “public health risks” (Morris 1999 ). Within freshwater ecosystems, “ cyanobacteria are the most important phytoplankton associated with HABs” (Paerl 1988 ). “Toxigenic cyanobacteria, including Anabaena , Cylindrospermopsis, Microcystis, and Oscillatoria (Planktothrix), tend to dominate nutrient-rich, freshwater systems due to their superior competitive abilities under high nutrient concentrations, low nitrogen-to-phosphorus ratios, low light levels, reduced mixing, and high temperatures” (Downing et al. 2001 ; Paerl and Huisman 2009 ; Paerl and Paul 2012 ). Poisonings of “domestic animals, wildlife and even humans by blooms of toxic cyanobacteria” have been recognized throughout the world. Francis ( 1878 ) has first observed dead livestock due to “algal bloom of cyanobacteria” (Bhat et al. 2017 ). Also, cyanobacteria is responsible for several off-flavor compounds (e.g., methylisoborneal and geosmin) found in municipal drinking water systems as well as in aquaculture-raised fishes, resulting in large financial losses for state and regional economies (Crews and Chappell 2007 ). In addition to posing “significant public health risks, cyanobacteria have been shown to be poor quality food for most zooplankton grazers in laboratory studies” (Tillmanns et al. 2008 ; Wilson et al. 2006 ), thus reducing the efficiency of energy transfer in aquatic food webs and potentially preventing zooplankton from controlling algal blooms. Eutrophication is also associated with major changes in aquatic community structure. During “cyanobacterial blooms, small-bodied zooplankton tend to dominate plankton communities, and past observational studies have attributed this pattern to anti-herbivore traits of cyanobacteria” (e.g., toxicity, morphology, and poor food quality) (Porter 1977 ). However, the biomass of planktivorous fish is often positively related to nutrient levels and ecosystem productivity. Piscivorous fishes (e.g., bass, pike) tend to dominate the fish community of nutrient-poor, oligotrophic lakes, while planktivorous fishes (e.g., shad, bream) become increasingly “dominant with nutrient enrichment” (Jeppesen et al. 1997 ). Thus, an alternative explanation for the lack of zooplankton control of cyanobacterial blooms could include consumption of zooplankton by planktivores.

Plastics and Microplastics

Among the several human pressures on aquatic ecosystems, the accumulation of plastic debris is one of the most apparent but least studied. “Plastics generate significant benefits to the human society” (Andrady and Neal 2009 ), but due to its “durability, unsustainable use and inappropriate waste management plastics accumulate extensively in the natural habitats” (Barnes et al. 2009 ). Because of “high mobility, plastic debris has practically permeated the global marine environment” (Cole et al. 2011 ; Ivar do sul and Costa 2014 ), including the “polar region” (Barnes et al. 2009 ), “mid-ocean islands” (Ivar do sul et al. 2013 ), and “the deep sea” (Van Cauwenberghe et al. 2013 ). The sources of marine plastics are not very well characterized. A rough estimation predicts that “70 to 80% of marine litter, most of it is plastics, originate from inland sources and are emitted by rivers to the oceans” (GESAMP 2010 ). Rivers transport considerable amounts of plastics and “thus contribute significantly to the marine plastics pollution” (Moore et al. 2005 ; Lechner et al. 2014 ). “Plastics are dumped in huge volumes in beaches, lakes, navigation channels and other forms of water masses” (Lechner et al. 2014 ). The volume of plastic is even bigger in low-income countries with poor waste disposal regulations. In the marine environment, “plastics of various size classes and origins are omnipresent and affect numerous species that become entangled in or ingest plastics as well as an aesthetic problem” (Gregory 1999 , 2009 ). “Plastics have been reported as a problem in the marine environment since the 1970s, but only recently the issue of plastic pollution in marine and freshwater environments been identified as a global problem” (Carpenter and Smith 1972 ). It has been reported that “single-use plastics (plastic bags and micro beads) are a major source of this pollution” (Desforges et al. 2014 ; Perkins 2015 ).

Under environmental conditions, larger plastic items degrade to so-called microplastics (MPs), typically smaller than 5 mm in diameter. “MPs are considered an emerging global issue by various experts” (Sutherland et al. 2010 ; Depledge et al. 2013 ) and international institutions (GESAMP 2010 ; UNEP 2011 ). Recent studies suggest that “risks of microplastics in the marine environment may pose more threat than macroplastics” (Thompson 2015 ; Diamond et al. 2018 ).

Potential sources of “ MPs include wastewater treatment plants, runoff from urban, agricultural, touristic, and industrial areas, as well as shipping activities, beach litter, fishery and harbors” (Zubris and Richards 2005 ; Norén 2007 ; GESAMP 2010 ; Claessens et al. 2011 ; Dubaish and Liebezeit 2013 ). Another “potential source is sewage sludge that typically contains more MPs than effluents” (Leslie et al. 2012 ). A “broad spectrum of aquatic organisms are prone to MP ingestion ranging from plankton and fish to birds and even mammals, and accumulate throughout the aquatic food web” (Wright et al. 2013 ). Due to their large “surface-to-volume ratio and chemical composition, MPs accumulate environmental chemicals from the surrounding environment including metals” (Ashton et al. 2010 ) and “persistent, bioaccumulative, and toxic compounds” (Koelmans et al. 2013 ) transferring these contaminants from water to biota. “Plastic particles are also dominated by certain human pathogens like specific members of the genus Vibrio ”. Therefore, MPs can act as a vector for waterborne “human pathogens” influencing the water quality. In addition, “plastics contain and release a multitude of chemical additives” (Rochman 2013 ; Dekiff et al. 2014 ) and adsorb organic contaminants from the surrounding media (Bakir et al. 2012 ). Compounds such as MPs can transfered to organisms upon ingestion (Zarfl and Matthies 2010 ), this may increase “the chemical exposure of the ingesting organism and thus, toxicity” (Oehlmann et al. 2009 ; Teuten et al. 2009 ).

An “ oil spill ” is defined as the discharge of “liquid petroleum hydrocarbons” into the environment, mainly in the “ marine ecosystem” caused by human activity. “Environmental pollution caused by an oil spill is detrimental” (Broekema 2016 ). This is because “petroleum hydrocarbons are toxic to all forms of life and harm both aquatic and terrestrial organisms.” The pollution of marine environments has caught the attention of researchers and environmentalists. This is due to the severe impact of oil spills on marine life. “A 1% increase in spill size has been estimated to increase the damage by some US$0.718 million” (Alló and Loureiro 2013 ). “Oil spills, which result from damage, transportation accidents and various other industrial and mining activities, are classified as hazardous waste” (Bartha and Bossert 1984 ). They are considered to be the most “frequent organic pollutants of aquatic ecosystems” (Bossert et al. 1984 ; Margesin and Schinnur 1997 ). Oil spills can occur from multiple sources including “oil tankers” (35.7%), facilities (27.6%), “non-tank vessels” (19.9%), “pipelines” (9.3%), and other sources (7.4%) (Benko and Drewes 2008 ). “ Marine ship-source oil spill can occur as a result of ship accidents or operations, or the intentional discharge of oily wastes into oceans” (Knapp and Van de Velden 2011 ).

Major Oil Spills in the History

It is estimated that “3.2 million tons of oil is released per year from all sources into the environment. The majority of this oil is due to general shipping and industrial activities” (ITOPF 1990 ). During the Iran–Iraq war (1980–1988), approximately 2 million barrels of oil were discharged into the Arabian Gulf sea water. These included “1.5 million barrels from the Nawruz blow-out in 1983” (Watt 1994 ). Following the “Gulf War in 1991, 4 to 8 million barrels of oil were released into the Gulf and the Kuwaiti Desert and, making this the largest oil spill in the history at that time” (Purvis 1999 ). Previous observations indicated that the number of large “oil spills (>700 tons) has decreased significantly over the last 30 years” (ITOPF 1990 ). During the 1990s, the average number of large oil spills per year was about a third of the amount that was witnessed during the 1970s. It should be noted that “1,133,000 tons of oil was lost in the 1990s and 2000s, during 2010–2013, about 22,000 tons of oil was lost” (Levy and Gopalakrishnan 2010 ; Carriger and Barron 2011 ). The BP Deepwater Horizon oil spill on April 20, 2010, caused the discharge of more than 2.6 million gallons of oil into the Gulf of Mexico over just about 3 months. This “oil spill was the second largest in human history” (Levy and Gopalakrishnan 2010 ; Carriger and Barron 2011 ). During the 1991 Gulf War, the deliberate release of “over 6 million barrels of oil” (Randolph et al. 1998 ) into the marine environment was considered as the largest in history.

Impact on Human Health

Oil spills pose a great danger to humans. Direct “contact with crude oil or indirect contact through inhalation of vapors or consumption of contaminated seafood can cause deleterious health effects ranging from dizziness and nausea to certain types of cancers and issues with the central nervous system” (Aguilera et al. 2010 ; Major and Wang 2012 ). Toxic chemicals contained in the oil such as benzenes, toluene, poly-aromatic hydrocarbon, and oxygenated polycyclic aromatic hydrocarbons can harm the air quality (Tidwell et al. 2015 ). As witnessed in the “ Kuwait Oil Fires, between January 16, 1991 and November 6, 1991, produced air pollution which caused respiratory distress” (Petruccelli et al. 1999 ). Oil-related disasters cause water contamination when the oil spillage comes in contact with any drinking water supply, for example, the 2013 incident in Miri, Malaysia, contaminated the water supply for 300,000 natives.

Impact on Coral Reefs

Coral reefs are considered to be important components of marine ecosystems. This is because “coral polyps are important nurseries for shrimp, fish and other animals” (Perkol-Finkel and Benayahu 2007 ). The aquatic organisms that live within and around the coral reefs are at risk of exposure to the toxic substances within oil. They are rapidly degrading because of a variety of environmental and anthropogenic pressures. Thus, they are suffering significant changes in “species diversity,” “species abundance,” “species evenness,” and “habitat structure” worldwide (Hughes et al. 2007 ). “Oil dispersants are potentially harmful to marine life including coral reefs” (Shafir et al. 2007 ). A study using coral nubbins in coral reef eco-toxicology testing (Shafir et al. 2003 ) found that dispersed oil and oil dispersants are harmful to soft and hard coral species at early life stages.

Impact on Marine Mammals

Marine mammals include “bottlenose dolphins, fins, humpbacks, rights, sei whales, sperm whales, manatees, cetaceans, seals, sea otters and pinnipeds.” The physical contact of oil with furred mammals affects these animals because they rely on their outer coats for buoyancy and warmth. Consequently, “these animals often succumb to hypothermia, drowning and smothering when oil flattens and adheres to the outer layer” (Lin and Tjeerdema 2008 ).

Impact on Seabirds

Physical contact is one of the major routes of exposure, and it usually affects seabirds (Table 1.4 ). For example, thousands of African penguins ( Spheniscus demerus ) were oiled following the 2000 treasure oil spill in South Africa.

Mass motility of seabirds collected at “Exxon Valdez and Braer oil spills” (Dauvin 1998 )

Aquaculture Activities

Aquaculture is the farming of aquatic organisms. The “rapidly growing human population is creating an increase in the demand for fish worldwide” (Tidwell and Allan 2001 ). The amount of “fish captured in fisheries is no longer meeting this demand because the annual production of captured fish has not changed significantly since 2011” (FAO 2016b ). “Aquaculture is becoming a more popular fish production method as it has an annual increase of 6% and is projected to produce over half of the fish consumed by 2025” (FAO 2016b ). “Aquaculture has tremendous benefits for the humans like seafood production by fisheries and contributes with 15 to 20% of average animal protein consumption to 2.9 billion people worldwide” (Smith et al. 2010 ). The nutritional quality of aquatic products has “high standard and represents an important source of macro and micronutrients for the people from developing countries” (Roos et al. 2007 ). Despite the undeniable benefits of aquaculture such as the provision of good quality and accessible food for population and the generation of millions of jobs and billion dollars in budget for the developing countries, the activity is one of the most criticized worldwide, mainly because of the environmental impacts (FAO 2016c ). The most common “negative environmental impacts that are associated with aquaculture is water eutrophication, water quality, alteration or destruction of natural habitats, introduction and transmission of diseases” (FAO 2006b ).

Harmful Impacts Related to the Aquaculture Activities Are as Follows

Eutrophication of receiving waters.

Aquaculture can be “a major contributor to eutrophication or organic loads in the receiving waters” (Mateo-Sagasta et al. 2017 ). It is mainly produced by “uneaten feed (especially due to overfeeding), lixiviation of aquaculture feedstuffs” (Focardi et al. 2005 ; Crab et al. 2007 ), “decomposition of died organisms and over fertilization” (Feng et al. 2004 ; Gyllenhammer and Hakanson 2005 ). In Scotland, for example, “the discharge of untreated organic waste from salmon production is equivalent to 75% of the pollution discharged by the human population” (Mateo-Sagasta et al. 2017 ). “ Shrimp aquaculture in Bangladesh generates 600 tons of waste per day” (SACEP 2014 ; Mateo-Sagasta et al. 2017 ). It is well documented that from “the total nitrogen supplemented to the cultured organisms, only 20–50% is retained as biomass by the farmed organisms, while the rest is included into the water column or sediment” (Jackson et al. 2003 ; Schneider et al. 2005 ) and “eventually discharged into the receiving ecosystems, increases the risk of eutrophication and algal blooms (like toxic microalgae-red tides) in lakes” (Mateo-Sagasta et al. 2017 ), reservoirs, and coastal areas (Alonso-Rodriguez and Paez-Osuna 2003 ; Mirto et al. 2009 ; Mateo-Sagasta et al. 2017 ). “Organic pollutants consume dissolved oxygen (DO) in the water as it degrades quality characteristics of fresh water, with the result DO drops, fish and other aquatic life are exposed to extreme conditions or killed due to hypoxia in water bodies” (Mateo-Sagasta et al. 2017 ).

Introduction of Exotic Species

Aquaculture comes in multiple versions, two of which are open systems and closed systems. “Open systems are found offshore in coastal areas, exposed to natural environments” (Lawson 1995 ). These systems are high-risk because they allow unchecked interactions between the farmed fish and surrounding environment, which leads to “free exchange of diseases, parasites and fecal matter” (Ali 2006 ). The recent study has revealed “a parasite transmission of sea lice from captive to wild salmon” (Krkosek et al. 2007 ). The only barrier between the harvested fish and the wild population is a rigid cage or netting system. When these netting systems are damaged during inclement weather such as snowstorms or hurricanes, it allows “fish to escape from the open systems” (Centre for Food Safety 2012 ). There were “25 million reported fish escapes worldwide and the majority occurred when netting was damaged during severe weather conditions” (Centre for Food Safety 2012 ). The escaping of “exotic aquaculture species into the natural ecosystem causes the displacement of native populations, competition for food, space, mates and prey” (Naylor et al. 2005 ).

Destruction of Mangrove Forests

“Aquaculture farms” are constructed in “ mangrove forests ” (Dewalt et al. 2002 ; Stickney and McVey 2002 ; Rajitha et al. 2007 ). “Mangrove forests” are important ecosystems as they act as nurseries for many “aquatic species” as well as nesting areas for “birds, reptiles, crustaceans and other taxonomic groups” (Paez-Osuna 2005 ). The cover of mangrove forest has decreased worldwide from “19.8 million hectares in 1980 to less than 15 millions hectares in 2000.” The annual “deforestation rate was 1.7% from 1980 to 1990 and 1% from 1990 to 2000” (FAO 2007 ), and the “problem of deforestation still continue today.” “Aquaculture has been responsible for the deforestation of millions of hectares of mangrove forest in Thailand, Indonesia, Ecuador, Madagascar and other countries” (Harper et al. 2007 ).

Contamination of Water for Human Consumption

“ Inland aquaculture” has been responsible for the “degradation of water bodies used for human consumption” (Paez-Osuna 2001 ). Aquaculture activities cause death of benthic organisms as well as undesirable odors and the presence of pathogens in the discharge sites (Martinez-Cordova and Enriquez-Ocana 2007 ). The spread and the “outbreaks of diseases are negative consequences of the expansion and diversification of the aquaculture sector” (Crisafi et al. 2011 ; Mancuso 2013 ; Mancuso et al. 2013 ).

Preventive Measures and some Humanistic Solutions

“ Water contamination” can be reduced from a “personal level” to “national and international level.” Every individual has a duty to prevent pollution of water resources. “Water is a basic need for our survival,” and hence it should be our first priority to keep all “water resources” free from contamination. There are various “sources of water contamination.” Thus, the control of water contamination needs a range of preventive measures. “Measures of prevention and control are essential in improving the quality of water” and reducing the “costly treatment measures that are taken to treat water.” Preventive measures and possible solutions to “control water contamination” are given as follows (Xiong et al. 2015 ; Lan et al. 2015 ; Xanthos and Walker 2017 ; Barmentlo et al. 2018 ):

  • “Do not throw rubbish away in places like the beach, riverside and water bodies rather put it in trash can.”
  • “Use water wisely. Do not keep the tap running when not in use.”
  • “Do not throw chemicals, oils, plastics, paints and medicines down the sink drain, or the toilet.”
  • “Buy more environmentally safe cleaning liquids for use at home and other public places.”
  • “Not to overuse pesticides and fertilizers in farms. This will reduce runoffs of the chemical into nearby water sources.”
  • “Natural fertilizers such as peat, compost, manure should be preferred while gardening and farming.”
  • “Implementing water quality laws they can help in protecting aquatic ecosystems by imposing acceptable concentrations of pollutants and prevents the release of pollutants into water resources.”
  • “Proper use and disposeal of chemicals prevent the contamination of aquatic environments.”
  • “Use detergents with low or no phosphate because high phosphate content causes eutrophication of lakes.”
  • “Control storm water runoff. As the storm water runoff flows over impervious surfaces, it collects debris, sediments, chemicals and other pollutants which can have negative effects on the quality of water if the runoff is left untreated.”
  • “Decrease water resistant surfaces such as cement around homes to reduce surface runoff. Vegetation, porous materials, gravel, wood decking etc. can be used instead of cement.”
  • “Avoid throwing garbage into lakes, rivers and streams and help in cleaning litter around water resources.”
  • “Wash your automobiles at carwashes instead of washing it yourself. The wastewater from these carwashes is drained into the sewer and treated which reduce the amount of pollutants in the water.”
  • “Speak up against industries that dump waste into local streams, rivers, and beach fronts to reduce water pollution in your community.”
  • “Implement existing environmental laws. There are very strict laws that help minimize water pollution. These laws are usually directed at industries, hospitals, schools and market areas on how to dispose, treat and manage sewage.”
  • “Do not dispose non-degradable products such as plastic bags or plastic wrappers down the drain.”

The degradation of aquatic ecosystems is largely due to human activities. Increased urbanization and industrialization are greatly responsible for water pollution. Human contribution to water pollution is enormous, such as dumping of solid wastes, industrial wastes, and domestic wastes. Water pollution is a major concern to the world. Environmental education is very important to reduce the pollution of aquatic ecosystems.

Contributor Information

Khalid Rehman Hakeem, Email: [email protected] .

Rouf Ahmad Bhat, Email: [email protected] .

Humaira Qadri, Email: moc.liamg@3ariamuhirdaq .

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No glasses? No problem. Three cool ways to safely view the eclipse.

‘pinhole projectors’ will do the trick. so will plants in your backyard..

The total solar eclipse is just three days away — and it’s sure to amaze and delight the millions who see it.

Everyone in the Lower 48 states will be able to view at least a partial solar eclipse, assuming cloud-free skies .

To savor the eclipse, you don’t need special equipment. Even if you can’t get your hands on eclipse glasses , there are old school, low-tech ways to see it. If you’re crafty and canny, the eclipse can still be a memorable experience.

2024 total solar eclipse

research paper on aquatic plants

Safety first

The first rule of enjoying the eclipse is to avoid looking directly at the sun without eye protection. Even brief glances can cause permanent damage.

The only exception to this rule is for lucky spectators in the path of totality during the few minutes of the total eclipse, when the sun is fully blocked by the moon.

For those witnessing the partial solar eclipse, even when most of the sun’s surface is blocked, the remaining, visible crescent is still intensely bright and cannot be safely viewed without eye protection.

But, if you don’t have eye protection, here are some safe ways to experience the partial eclipse through indirect means:

Make a pinhole projector

A way around looking directly the sun is to make your own eclipse projector using a cereal box. It’s a safe and terrific way to capture the eclipse action.

Clear the kitchen table and find the craft scissors. In addition to the cereal box, you’ll need a piece of aluminum foil, tape and a small nail or pushpin.

First, eat your Froot Loops — or whatever toasted grain you prefer — and keep the box. On a white piece of paper or white cardboard, trace the bottom of the box. Then, clip out the traced rectangle from the paper and put it in the bottom of the opened box. That’s your screen that images of the eclipse will project onto.

Cut out two squares (1.5 inches should suffice) on the lid of the box and then tape the lid back together. For one square, cover the hole in foil and tape it down. Gently put a pushpin or small nail hole through it, as that is the lens that the sun’s light will pass through. The smaller the hole, the sharper the projected image.

When using your personal box theater, turn away from the sun — and let the sun’s rays shine through the tiny pin hole. Look through the other hole in the lid to see the eclipse action — during the eclipse you’ll see the moon biting a chunk from the sun.

Other kinds of small boxes — such as shoe boxes or small package boxes — work well, too. And your kids can decorate them for fun.

Looking to the trees

If you’re not inclined to make a projector box, you can also view the partial phases of the eclipse in the shadows of trees and plants.

The small gaps in between leaves, branches and pine needles act as miniature projectors. When light passes through, a small image of the sun is cast onto the ground. As the partial eclipse progresses, you’ll see the small circles evolve into sickle-shaped crescents, eventually waning to a sliver.

You may consider holding a white piece of paper or poster board beneath a tree or plant to make it easier to spot the shadows.

Gadgets and fingers

Leaves aren’t special — they just happen to be good at producing tiny projections. But realistically, any hole that’s about a quarter inch wide, give or take, will do the trick. That means you could even parade around outside with your pasta colander, cheese grater or serving spoon with holes in it and look at its shadow. Place white paper or poster board on the ground to see the projection more clearly.

You could also just hold your fingers out and crisscross them to make for half a dozen or so small openings between. Just extend your fingers on both hands as if you’re trying to make a W , and then overlap them.

Simple, yet elegant.

A total solar eclipse will pass across the United States on Monday, April 8. See what the eclipse will look like in your city .

Path of totality: Our interactive visual map allows you to traverse the eclipse’s path from Mexico to Maine. If you’re traveling for the eclipse , we rounded up the top things to do in several major cities prime for viewing. In Carbondale, Ill., lucky residents are preparing to experience totality for the second time in seven years .

Preparing for the eclipse: The most important thing you’ll need is eclipse glasses — here’s how to get them and avoid buying fakes . If you want to capture the magic of the moment, check out our guide for photographing the eclipse with your phone. Here’s what to expect in terms of cloud cover and eclipse traffic .

The science: This eclipse may be especially dramatic because the sun is at its most active period in two decades. In the past, solar eclipses have helped scientists learn more about the universe . Here’s everything else you need to know about the solar eclipse.

  • Your ultimate guide to the total solar eclipse, its path and how to watch April 4, 2024 Your ultimate guide to the total solar eclipse, its path and how to watch April 4, 2024
  • Here’s what not to do to safely watch the total solar eclipse April 5, 2024 Here’s what not to do to safely watch the total solar eclipse April 5, 2024
  • Eclipse tourists should plan for overloaded cell networks April 2, 2024 Eclipse tourists should plan for overloaded cell networks April 2, 2024

research paper on aquatic plants

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Thames Water parent ‘owes two Chinese state-owned banks’ as debt downgraded – as it happened

Live, rolling coverage of business, economics and financial markets as Fitch downgrades debt of parent of water company, saying default is likely

  • 4d ago Closing summary: Foreign lenders expected to agree debt extension for Thames Water
  • 4d ago International Paper says it will cut costs but keep DS Smith UK office in bid battle
  • 4d ago Thames Water reportedly owes Chinese banks as Fitch downgrades rating
  • 4d ago Google considering charging for AI-powered search - FT
  • 4d ago UK car sales in strongest March since 2019
  • 4d ago Amazon cuts hundreds of jobs at cloud unit after ditching 'just walk out' stores
  • 4d ago Co-op grocery profits up despite sale of petrol chain
  • 4d ago UK on track to exit recession as services sector output grows
  • 4d ago Eurozone economy returns to growth thanks to strong services output
  • 4d ago Vodafone and Three UK merger faces in-depth competition investigation
  • 4d ago Brexit checks will mean 'higher food prices' - UK businesses

Thames Water vans are parked as repair and maintenance work takes place, in London.

Closing summary: Foreign lenders expected to agree debt extension for Thames Water

The Dutch bank ING and two Chinese state-owned lenders could play a crucial role in deciding the fate of beleaguered Thames Water , it has emerged.

The banks are expected to agree an extension on a £190m loan to the parent company of Britain’s biggest water supplier, which is due to be repaid at the end of this month.

You can read the full story here:

In other finance news today:

Trade groups have warned that consumers could see a rise in food prices after the UK government announced the introduction of post-Brexit charges on imports of EU food and plant products later this month.

The gender pay gap among UK staff at Goldman Sachs has hit its highest level in six years, raising concerns about a lack of women among the bank’s most senior ranks.

Rio Tinto is facing a likely lawsuit in an English court brought by the UK-based law firm Leigh Day on behalf of people living in villages near a mine in Madagascar.

Holidaymakers will continue to face limits on the amount of liquid they can carry on flights out of the UK this summer after the government extended the deadline for airports to install new security scanners by a year.

Despite record levels of shoplifting in its food stores, the Co-op increased profits in its grocery business last year as it signed up 1 million new members and invested more than £90m in cutting prices, including introducing special discounts for members.

UK rail passengers are bracing for travel disruption as train drivers bring some routes on the national network to a halt in a wave of strikes, but two days of similar action on the London Underground have been called off.

You can continue to follow our live coverage from around the world:

In the UK, Rishi Sunak is criticised for saying border security more important than staying in European court of human rights

In the US, Joe Biden is to speak to Netanyahu amid reports US president is furious over Gaza aid convoy strike

In our coverage of the Middle East crisis, Israel says investigation into its airstrike that killed aid workers in Gaza will take weeks

In our coverage of the Russia-Ukraine war, Volodymyr Zelenskiy reiterates calls for stronger air defences as his foreign minister asks Nato for Patriot missiles

Thank you for joining us today. Please click again tomorrow morning to see out this shortened Easter work week. JJ

US stock markets appear to have taken heart from Federal Reserve chair Jerome Powell’s dovish comments last night .

The tech-focused Nasdaq index has gained 1% in the opening minutes on Wall Street, while the S&P 500 gained 0.8% and the Dow Jones industrial average rose 0.7%.

When it rains, it pours ( and flushes faeces into the UK’s rivers ): S&P Global has also cut its rating for Thames Water’s debts.

S&P said that it has a “negative outlook” because of “persistent uncertainties on support from existing or new shareholders”. It added:

Delays in injecting equity during the currently regulatory period bring into question shareholder support and the recovery in the company’s operational performance in the next regulatory period. Without the equity injections needed to execute the business plan, Thames Water could endure longer periods of weak performance.

Thames Water’s bonds are unsurprisingly showing the strains. Here is Reuters on the move in bond prices (which move inversely to yields):

The price of Thames Water’s November 2028 bond fell nearly 1.4 pence to around 95.13, its lowest since October, according to Tradeweb data. Kemble’s May 2026 bonds fell to their lowest on record, down 1.4 pence to 15.127 pence.

The below graph illustrates the struggles of what is meant to be a boring, dependable utility: the yield on its 30-year bonds from 1998 has surged in the last two years.

It is back at six-month highs today. Higher yields generally mean that investors see an increased risk of default.

A graph showing that the yield on Thames Water's 2028 bonds has risen markedly in the last two years.

The Bank of England’s decision maker panel (DMP) survey of about 2,500 businesses from all parts of the economy has eased pressure to keep interest rates high after it found that that firms are finding it easier to recruit staff, wages growth has fallen, and inflation expectations are lower.

Firms reported that prices charged to customers rose by an average annual rate of 5.3% in the three months to March, down from 5.4% in the three months to February. The BoE said businesses expect this still high level of output price inflation to decline over the next year.

Year-ahead own-price inflation was expected to be 4.1% in the three months to March, down from 4.3% in the three months to February. The BoE said:

Output price inflation is, therefore, expected to decline by 1.2 percentage points over the next 12 months based on three-month averages.

Expectations of where wages growth will be over the next three months fell to 4.9% – the lowest since June 2022. The single-month figure at 4.7% was lowest since May 2022.

Illustrating how the demand for staff has eased, firms reported annual employment growth of 2.0% in the three months to March, lower than the 2.3% in February. Looking ahead for a full year, firms said employment growth would be 1.4%.

The DMP came to prominence last year when members of the BoE’s interest rate setting body – the monetary policy committee (MPC) – cited how firms expected to keep prices elevated, despite forecasts that it would fall steeply across the economy.

External MPC member Catherine Mann said responses showing that many firms intended to increase their margins in 2024 showed that the inflation genie was far from being put back in its bottle.

Rob Wood , chief UK economist at Pantheon Macroeconomics , a consultancy, said:

The MPC will take further confidence from the falls in wage growth and price expectations in March’s Decision Maker Panel. It’s still quite a long path back to target consistent services inflation, so rate cuts are likely to be gradual.

The number of Americans claiming unemployment benefits increased by more than expected last week, as the US Federal Reserve decides when to cut interest rates.

Initial jobless claims rose to a seasonally adjusted 221,000 for the week ending on 30 March, according to government figures. A Reuters poll of economists showed an average forecast of 214,000 claims.

Reuters reported:

Labour market resilience is anchoring the economy, with gross domestic product increasing at a brisk 3.4% annualized rate in the fourth quarter. Growth estimates for the first quarter are as high as a 2.8% pace. That strength, combined with still-high inflation, could see the Federal Reserve delaying a much-anticipated interest rate cut this year.

Federal Reserve chair Jerome Powell on Wednesday made comments described by some economists as “dovish” – in line with cutting interest rates to support the economy. Powell said that recent data on job gains and inflation have come in stronger than expected, but this data “does not materially change the overall picture, which continues to be one of solid growth, a strong but rebalancing labour market and inflation moving down toward 2% on a sometime bumpy path”.

Lee Hardman , a senior currency analyst at MUFG , a Japanese bank, said:

The comments provide further reassurance to market participants that the Fed remains on course to deliver three rate cuts this year with the first cut most likely to be delivered in June. Ahead of the latest nonfarm payrolls report released on Friday, chair Powell emphasised that that the Fed is not concerned by strong growth and job gains.

The US trade deficit in February was slightly bigger than expected, at $68.9bn – $1.6bn more than economists had forecast.

International Paper says it will cut costs but keep DS Smith UK office in bid battle

The DS Smith cardboard box manufacturing plant in Lebanon, Indiana seen in a January 2020 handout picture.

American packaging company International Paper has said it would cut £93m in annual costs in a proposed takeover of British rival DS Smith – which suggests that job cuts may be considered.

International Paper’s £5.7bn bid is trying to gatecrash an agreed £5.1bn deal between DS Smith and British rival Mondi. It said it had made “significant progress […] in reciprocal due diligence”.

The company said that it would keep a “European headquarters” at DS Smith’s current offices in Paddington, London, but that the group would be headquartered in Memphis, Tennessee.

It would also pursue a secondary listing of International Paper shares in London, in a move to try to prevent objections from British investors.

It said those cuts would make “overhead synergies by reducing duplicative corporate and business overhead expenses”. However, it said that DS Smith’s North American manufacturing locations and International Paper’s European manufacturing locations “would continue their respective operations”. It did not mention any factory closures.

Businesses in mergers often cut shared services such as corporate functions like payroll or office supplies, although International Paper did not detail what would be cut. It said the rest of the £376m in savings would come from “operational efficiencies” as well as using its newfound size to get better deals from suppliers.

Mondi shares had fallen 2.8% in morning trading , but they have recovered since International Paper’s statement to top the FTSE 100 with a 4% gain. ( Perhaps Mondi investors think they would be better off not getting into a bidding war with International Paper? )

DS Smith shares also rose by 2.4% as investors hope for a juicy premium.

Bet365, the betting exchange run by Britain’s best-paid woman, Denise Coates, will pay more than £580,000 for failures in anti-money laundering and social responsibility.

The UK Gambling Commission said that Bet365 had checks that were “ineffective at managing money laundering risk”, that it had failed to check new customers were not covered by financial sanctions, and failed to check identity documents.

The company also failed to check properly if early intervention were required to prevent customers losing more money than they could afford.

The regulator said that Hillside (UK Gaming) ENC, which holds a licence for Bet365’s bingo and casino products, will pay £343,035 and Hillside (UK Sports) ENC, which holds a licence to offer betting, will pay £239,085.

However, those amounts will be a drop in the ocean for the hugely profitable company, which is based in Stoke.

Coates’s company paid her a salary of £220m last year, plus dividends of at least £50m .

Kay Roberts , the commission’s executive director of operations, said:

The policy and procedural failings may not have been as severe as those at other gambling businesses in recent years but they were failings nonetheless. We expect high standards from operators in terms of keeping gambling safe, fair and crime-free, and will always take action to correct any failings. This operator is very aware that a repeat of these failings will result is escalating regulatory action.

Chinese companies were already playing an important part in the Thames Water fiasco: China Investment Corporation owns 9% of the shares.

However, the equity owners are likely to take a back seat to the debt holders if the parent company defaults on its debt repayments.

A default – rated as likely by Fitch – would mean that the Chinese banks who are reportedly debt holders could lose money, but it could also give them ownership over Thames Water assets (if the terms of its debts are found to be enforceable).

That could add another layer of political controversy on top of a situation that already features rivers of excrement and cross-party disdain for a key utility. The Daily Telegraph reports that Tim Loughton , a Tory MP who is one of five politicians sanctioned by China in 2021, said:

This is another worrying aspect of the global reach of China’s financial institutions. It’s very unhealthy that a major British utility company could be beholden to a power like China, where we know that they pose a very serious threat to our security. There are plenty of examples overseas where they have lent to infrastructure projects up to the eyeballs only for them to go bust.

Thames Water reportedly owes Chinese banks as Fitch downgrades rating

Another update on Thames Water’s lenders: two Chinese state-owned banks are among the companies that are owed £190m by the UK’s biggest water company, the Financial Times has reported.

The FT reported that Chinese state-owned Bank of China, and Industrial and Commercial Bank of China (ICBC) were lenders, as well as Allied Irish Banks and Dutch lender ING, citing people familiar with the matter.

The debts mean that the two Chinese state-owned companies could end up as shareholders of Thames Water if its parent company is unable to repay the loan.

Fitch Ratings, an influential debt rating agency, on Thursday said it had downgraded the debt of Thames Water’s parent company.

Fitch wrote that even if the lenders do agree to extend Kemble Water Finance Limited’s debts, it would probably still constitute a default. What happens after that would depend on the terms of the debt – and what security the lenders demanded in exchange for the money.

Fitch said:

With Kemble’s shareholders not injecting the £500m of equity into Thames Water Utilities Limited expected for end-March 2024, and Kemble considering it not possible currently to fulfil upcoming interest payments, we believe that a downgrade to [restricted default] has become highly likely. Even assuming that lenders will agree to amend and extend the £190m loan due on 30 April 2024, this agreement would probably constitute a distressed debt exchange under our criteria.

Google considering charging for AI-powered search - FT

Gemini AI is seen on a phone in New York City.

An interesting story on the future of web search: Google is considering charging users for its AI search abilities, according to the Financial Times.

We may have all got used to having all of the world’s information available basically for free, but AI may change that calculation.

For one thing, AI answers are significantly more costly to run on servers than standard calculations. That means more energy: a financial and environmental cost .

But they could also be very attractive for certain “power users” – people with specific needs who might be willing to pay a premium to get enhanced abilities. The FT reported:

Google is looking at options including adding certain AI-powered search features to its premium subscription services, which already offer access to its new Gemini AI assistant in Gmail and Docs, according to three people with knowledge of its plans.

Of course, AI answers will need to address some of the (quite significant) teething problems such as “hallucinations” – not ideal for finding good information. And the FT report flags that if Google gives answers directly it could undermine click-throughs – the source of its hundreds of billions of dollars of ad revenues.

And companies in the information economy might well put up barriers to Google’s search crawlers if they fear their content is being relied upon without fair remuneration. The New York Times has already taken action against Google’s rival, Microsoft and ChatGPT maker OpenAI , for alleged copyright infringement.

A Google spokesperson told the FT:

For years, we’ve been reinventing search to help people access information in the way that’s most natural to them. With our generative AI experiments in search, we’ve already served billions of queries, and we’re seeing positive search query growth in all of our major markets. We’re continuing to rapidly improve the product to serve new user needs. We don’t have anything to announce right now.

Dutch bank ING is one of the lenders owed £190m by Thames Water’s parent company, Kemble Water Finance, Sky News has reported .

Sky News also said that an unidentified large Chinese bank is also said to have a significant position in the loan facility, citing banking sources.

Much of Thames Water’s capital structure is very opaque, giving millions of households in the south-east of England little clue as to who ultimately is due to receive a financial return if the regulator allows the water company to charge higher bills.

The £190m bond is due to be repaid in the next few weeks . A large chunk of the £500m previously promised by Thames Water’s shareholders was earmarked for that purpose, but the company is now scrambling to find new lenders or equity investors to help it fill the gap and avoid a default that could see it taken into temporary public ownership.

ING was approached for comment.

Alex Lawson

Thames Water chief executive Chris Weston will meet union leaders today amid concerns over the future of Britain’s biggest water company.

Last week, its shareholders refused to stump up £500m which had been expected by the end of March. The company has said the industry regulator, Ofwat, is being too stringent, making the company “uninvestible”.

Thames has said it can draw on funds to last until the middle of next year, but there are concerns it could ultimately fall into a government-handled administration.

Representatives from the GMB, Unison and Unite unions will meet at the company’s headquarters, Clearwater Court in Reading, this afternoon.

Ahead of the meeting, GMB national officer, Gary Carter , said he will “demand there are no cuts to work force numbers – or terms and conditions”. He said:

Any cost cutting measures being considered by Thames will only be a sticking plaster and will not address the root cause of the company’s problems – a lack of investment by shareholders stretching back decades.

UK car sales in strongest March since 2019

A photo of new and secondhand cars for sale on a dealership forecourt last year in Ellesmere Port, England.

The UK car industry recorded its strongest March sales since 2019, before the coronavirus pandemic, as companies kept replacing their fleets.

Sales of battery electric vehicle (BEV) registration volumes were at their highest ever recorded levels, but market share fell by one percentage point from the same month last year, down to 15.2%. Battery cars remain more expensive than petrol and diesel equivalents, although a wave of cheaper Chinese cars is expected to change that in the coming months.

March sales are particularly important because of the issuance of new number plates, which means that some buyers hold off for vehicles that will retain their residual prices slightly longer.

New car sales rose 10.4% in March compared with the same month last year, according to the Society of Motor Manufacturers and Traders, a lobby group. 317,786 new cars were sold with a ‘24 plate.

However, that was still 30.6% below pre-pandemic levels.

The SMMT said that fleet buyers were making up for lost time during the years of disrupted supply during the pandemic, but added that registrations by private buyers fell by 7.7%. The lobby group cited a “challenging economic backdrop of low growth, weak consumer confidence and high interest rates” for the decline.

Mike Hawes , the SMMT ’s chief executive, said:

Market growth continues, fuelled by fleets investing after two tough years of constrained supply. A sluggish private market and shrinking EV market share, however, show the challenge ahead. Manufacturers are providing compelling offers, but they can’t single-handedly fund the transition indefinitely. Government support for private consumers – not just business and fleets – would send a positive message and deliver a faster, fairer transition on time and on target.

Amazon cuts hundreds of jobs at cloud unit after ditching 'just walk out' stores

A photo of the UK's first branch of Amazon Fresh, which opened in Ealing area of London in 2021.

Amazon has announced it is cutting hundreds of jobs in its cloud unit, Amazon Web Services (AWS), some of them affected by a decision to ditch its “just walk out” shops.

The stores gave customers the uncanny experience of walking out of shops with their items without scanning them or approaching a cashier (whether human or computer). However, while it seemed to be completely automated, it actually relied on human reviewers in India watching video and manually tagging items.

Industry publication Geekwire reported that AWS’s cuts will include several hundred jobs in its sales, marketing, and global services organisation, and a few hundred jobs on its physical stores technology team, citing executives’ emails to staff. The technology had been offered to retailer clients, but no businesses outside Amazon had agreed to take the technology on.

An AWS spokesperson said, in a statement to news organisations:

These decisions are difficult but necessary as we continue to invest, hire, and optimize resources to deliver innovation for our customers.

The firm also said “it will continue to hire and grow, especially in core areas of our business”, adding that there are thousands of jobs available and it is working to find internal opportunities for employees whose roles are affected.

Kalyeena Makortoff

NatWest’s incoming chair Rick Haythornthwaite is stepping down from his chairmanship of Ocado next year, dodging potential criticism of over-boarding he prepares to lead one of the UK’s largest banks.

It was the last major commitment hanging over the former Centrica boss, given that he’d announced back in September that he was prepared to ditch all of his other positions to focus on his role at Natwest.

However, he’ll stay on as Ocado’s chair for another year, saying he will not seek re-election at the food retailer’s AGM in 2025 – which could leave the chairman relatively stretched as he tries to fill the shoes of longstanding NatWest chair Howard Davies.

Haythornthwaite explained his decision in a statement on Thursday:

Since the announcement of my appointment as Group Chair of NatWest I have given extensive thought to my workload, listened to all parties and reflected on how I ensure that I deliver effectively on all fronts. With the benefit of time and greater visibility of the expected growth in requirements of the publicly listed portfolio, it has become evident that pressure on my time is likely to increase over the medium term. Given that Ocado has a strong and stable board, a high-quality management team as well as good momentum in business performance, I have made public my intention to step down a year from now to ensure that the company has sufficient time for a measured chair succession.

Co-op grocery profits up despite sale of petrol chain

Sarah Butler

The Co-op’s grocery chain increased profits as it signed up 1 million new members last year after investing more than £70m in cutting prices last year including introducing special discounts for members.

Profits rose 11% year-on-year to £154m despite a 6.4% fall in sales to £7.3bn driven by the sale of the Co-op’s petrol forecourt chain to Asda. Underlying sales rose 4.3%, excluding the impact of that deal, although that was still well behind the pace of grocery inflation.

Shirine Khoury-Haq, the chief executive of the Co-operative Group, said the Co-op had faced “challenging trading conditions, volatile markets and ongoing financial headwinds” in the past year “but would now “continue to move our Co-op into a position of growth, with our member-owners firmly at the heart of all we do.”

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