Freshwater Ecology: Concepts and Environmental Applications of Limnology

By Walter K. Dodds and Matt R. Whiles

Freshwater Ecology, Second Edition, is a broad, up-to-date treatment of everything from the basic chemical and physical properties of water to advanced unifying concepts of the community ecology and ecosystem relationships as found in continental waters.With 40% new and expanded coverage, this text covers applied and basic aspects of limnology, now with more emphasis on wetlands and reservoirs than in the previous edition. It features 80 new and updated figures, including a section of color plates, and 500 new and updated references. The authors take a synthetic approach to ecological problems, teaching students how to handle the challenges faced by contemporary aquatic scientists.This text is designed for undergraduate students taking courses in Freshwater Ecology and Limnology; and introductory graduate students taking courses in Freshwater Ecology and Limnology.

• Expanded revision of Dodds' successful text.
• New boxed sections provide more advanced material within the introductory, modular format of the first edition.
• Basic scientific concepts and environmental applications featured throughout.
• Added coverage of climate change, ecosystem function, hypertrophic habitats and secondary production.
• Expanded coverage of physical limnology, groundwater and wetland habitats.
• Expanded coverage of the toxic effects of pharmaceuticals and endocrine disrupters as freshwater pollutants
• More on aquatic invertebrates, with more images and pictures of a broader range of organisms
• Expanded coverage of the functional roles of filterer feeding, scraping, and shredding organisms, and a new section on omnivores.
• Expanded appendix on standard statistical techniques.
• Supporting website with figures and tables - http://www.elsevierdirect.com/companion.jsp?ISBN=9780123747242

• Language: English
• Publisher: Academic Press
• Release date: Nov 3, 2010
• ISBN: 9780080884776

Walter K. Dodds

Walter. K. Dodds received his Ph.D. in Biology in 1986 from the University of Oregon. From 1987 to 1990 he was a post doctoral fellow in the Department of Biology at Montana State University. In 1990 he accepted an Assistant Professor position in the Division of Biology at Kansas State University, in 1995 he was promoted to Associate Professor and in 2002 to full Professor. Over the years, Dodds has taught Limnology, Advanced Aquatic Ecology, Microbial Ecology, Principles of Biology, Conservation Biology, Environmental Problems, Origins of Life, Herbivory, Presentations in Ecology, Aquatic Ecology, Stream Ecology, Algal Identification, Algal Ecology, Bacteriology and Freshwater Biology. He has professional memberships in the American Association for the Advancement of Science, the American Society of Limnology and Oceanography, the American Society of Microbiology, the North American Benthological Society, the Phycological Society of America and Sigma Xi. Dodds has grants from agencies including the National Science Foundation, the United States Environmental Protection Agency, the United States Geological Survey, the Kansas Department of Wildlife and Parks and the Kansas Department of Health and Environment. He has been involved in the Konza Prairie Long-Term Ecological Research (LTER) program and provides leadership for the Konza LTER Aquatic and Hydrological Group and the Konza LTER Research Experience for Undergraduates program. Dodds’ recent research has focused on Aquatic Ecology on Konza Prairie, Nitrogen Uptake Retention and Cycling in Stream Ecosystems, Quality and Quantity of Suspended Solids in Kansas Rivers, and Nutrients and Algae in Streams. Dodds has been invited to present seminars at over 20 US agencies and universities, as well as agencies and universities in Australia, New Zealand and Canada. He has presented at numerous national and international scientific conferences and has produced over 80 peer reviewed publications.

Book preview

Front-matter

Freshwater Ecology

AQUATIC ECOLOGY Series

Series Editor

Prof. James H. Thorp

Kansas Biological Survey, and Department of Ecology and Evolutionary Biology University of Kansas Lawrence, Kansas, USA

Groundwater Ecology

Janine Gilbert, Dan L. Danielopol, Jack A. Stanford

Algal Ecology

R. Jan Stevenson, Max L. Bothwell, Rex L. Lowe

Streams and Ground Waters

Jeremy B. Jones, Patrick J. Mulholland

Ecology and Classification of North American Freshwater Invertebrates, Third Edition

James H. Thorp, Alan P. Covich

Freshwater Ecology

Walter K. Dodds, Matt R. Whiles

Aquatic Ecosystems

Stuart E. G. Findlay, Robert L. Sinsabaugh

Tropical Stream Ecology

David Dudgeon

The Riverine Ecosystem Synthesis

James H. Thorp, Martin C. Thoms, Michael D. Delong

Freshwater Ecology

Concepts and Environmental Applications of Limnology

Second Edition

Walter K. Dodds and Matt R. Whiles

AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD • PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Academic Press is an imprint of Elsevier

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Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

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Library of Congress Cataloging-in-Publication Data

Dodds, Walter Kennedy, 1958–

Freshwater ecology : concepts and environmental applications of limnology / Walter K. Dodds, Matt R. Whiles. — 2nd ed.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-12-374724-2 (hardcover : alk. paper) 1. Freshwater ecology—Textbooks. 2. Limnology—Textbooks.

I. Whiles, Matt R. II. Title.

QH541.5.F7D63 2002

577.6—dc22

2010009285

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

For information on all Academic Press publications visit our Web site at www.elsevierdirect.com

Printed in the United States of America

10 11 12 13 9 8 7 6 5 4 3 2 1

Table of Contents

Preface

Acknowledgments

Chapter 1: Why Study Continental Aquatic Systems?

Chapter 2: Properties of Water

Chapter 3: Movement of Light, Heat, and Chemicals in Water

Chapter 4: Hydrologic Cycle and Physiography of Groundwater Habitats

Chapter 5: Hydrology and Physiography of Wetland Habitats

Chapter 6: Physiography of Flowing Water

Chapter 7: Lakes and Reservoirs: Physiography

Chapter 8: Types of Aquatic Organisms

Chapter 9: Microbes and Plants

Chapter 10: Multicellular Animals

Chapter 11: Evolution of Organisms and Biodiversity of Freshwaters

Chapter 12: Aquatic Chemistry and Factors Controlling Nutrient Cycling: Redox and O2

Chapter 13: Carbon

Chapter 14: Nitrogen, Sulfur, Phosphorus, and Other Nutrients

Chapter 15: Unusual or Extreme Habitats

Chapter 16: Responses to Stress, Toxic Chemicals, and Other Pollutants in Aquatic Ecosystems

Chapter 17: Nutrient Use and Remineralization

Chapter 18: Trophic State and Eutrophication

Chapter 19: Behavior and Interactions among Microorganisms and Invertebrates

Chapter 20: Predation and Food Webs

Chapter 21: Nonpredatory Interspecific Interactions among Plants and Animals in Freshwater Communities

Chapter 22: Complex Community Interactions

Chapter 23: Fish Ecology and Fisheries

Chapter 24: Freshwater Ecosystems

Chapter 25: Conclusions

Appendix: Experimental Design in Aquatic Ecology

Glossary

References

Taxonomic Index

Subject Index

Colour Plates

Preface

For the Student

This book was written for you. We obtained as much student input as possible by having student reviewers assess the text and the approach used in it. The idea for the text was based on teaching students who were not satisfied with the existing texts. Teaching aquatic ecology and limnology has made it clear to us that most students enter ecological sciences for practical reasons. They often are concerned about conservation of resources from a classical perspective (e.g., fisheries program) or from an environmental issue perspective. Most existing texts limit the applied aspects of aquatic ecology to a section at the end.

The aim of this text is to incorporate discussion of the issues as they arise when the basic materials are being covered. This allows you to see the applications of difficult topics immediately and, we hope, provides additional impetus for doing the work required to gain an understanding. We also attempted to use the broadest possible approach to freshwater ecosystems; scale and linkages among systems are important in ecology. Most students in ecological courses had some interest in the natural world as children. They spent time exploring under rocks in streams, fishing, camping, hiking, or swimming, which stimulated a love of nature. This book is an attempt to translate this basic affinity for aquatic ecosystems into an appreciation of the scientific aspects of the same world. It is not always easy to write a text for students. Instructors usually choose a text, giving the students little choice. Thus, some authors write for their colleagues, not for students. We tried to avoid such pressures and attempted to tailor the approach to you. We hope you will learn from the materials presented here and that they will adequately supplement your instructor’s approach. When you find errors, please let us know. This will improve any future editions. Above all, please appreciate the tremendous luxury of being a student and learning. You are truly fortunate to have this opportunity and we are grateful for the time you take with this text.

For the Instructor

We hope this book will make your job a little easier. The chapters are short, mostly self-contained units to allow the text to conform to a wide variety of organizational schemes that may be used to teach about freshwaters. This will also allow you to avoid sections that are outside the scope of the course you are teaching. However, environmental applications are integrated into the text because we do not view the basic science and applications as clearly separate. We attempt to create instructional synergism by combining applied and basic aspects of aquatic ecology. Describing applications tends to stimulate student interest in mastering difficult scientific concepts. A variety of pedagogical approaches are used in an attempt to engage student interest and facilitate learning. These include sidebars, biography boxes, and method boxes. In the second edition we have also added advanced sections for areas where you might want the students to get a little more in depth but in areas that we often skip in our own lectures for a first class in freshwater ecology. We also include an appendix on experimental design in ecological science and a glossary because we have found many advanced undergraduates have little exposure to the practical side of how science is done. It is always difficult to know what to include and where to go into detail. Detailed examples are supplied to enforce general ideas. The choices of examples, perhaps not always the best overall, are the best we could find while preparing the text. Suggestions for improvements in this and any other areas of the text are encouraged and appreciated, and instructors using the first edition did just that. We apologize for any errors.

Why did we write this? In our experience, teaching limnology/freshwater ecology is more work than teaching other courses because of the breadth of subject, differential preparation of students, and associated laboratories but always seems to be the most fun. Of course it is fun; it is the best subject! We hope this book facilitates your efforts to transmit what is so great about the study of freshwater ecology.

Why did We Write a Second Edition?

Asking students to buy a new text rather than used requires a good reason to produce a new edition. In the near decade from the writing of the first edition, many new advances have occurred, and new topics have gained prominence with respect to environmental effects and hot areas of research interest. A second author (Matt R. Whiles) was added to broaden the perspective. In our attempt to cover these new areas we have added about 500 new and updated references, 50 new figures, 30 updated figures, color plates, two new chapters, and expanded the length of the text by roughly 40%. Particularly, we included more emphasis on wetlands and reservoirs than in the previous edition. Numerous errors were found in the first edition by dedicated students in Walter K. Dodds’s classes (with a potential award of test points for each unique error) and instructors across the country who adopted the first edition. We have hopefully corrected these without introducing too many new ones.

Acknowledgments

We thank Dolly Gudder, who was involved in all aspects of the writing and compilation of this book, including proofing the entire text, drafting and correcting all the figures from the first edition, library research, writing the first draft of the index, and obtaining permissions. We are forever in her debt. Alan Covich provided extensive conceptual guidance and proofread the text; his input was essential to producing this work. Eileen Schofield-Barkley provided excellent editorial comments on all chapters. The fall of 1998 Kansas State University limnology class proofed Chapters 1 through 8 and 11 through 18. The Kansas State University limnology classes proofread all chapters (especially Michelle Let) and graciously field tested the text in draft form. The L.A.B. Aquatic Journal Club, Chuck Crumly, Susan Hendricks, Stuart Findlay, Steve Hamilton, Nancy Hinman, Jim Garvey, Chris Guy, and Al Steinman provided suggestions on the first edition. Early helpful reviews on book concepts were provided by James Cotner, David Culver, Jeremey Jones, Peter Morin, Steven Mossberg, Stuart Fisher, Robert Wetzel, and F. M. Williams. The anonymous reviews (obtained by the publisher) are also greatly appreciated, including those who completed a detailed survey on the prospect of a second edition. Many of the good bits and none of the mistakes are attributable to these reviewers. We appreciate the support of the Kansas State University Division of Biology, the Kansas Agricultural Experiment Station, and Southern Illinois University at Carbondale. This is publication 10-207-B from the Kansas Agricultural Experiment Station.

Thanks for corrections from the Freshwater Ecology classes at Kansas State University with particular thanks to Katie Bertrand and Andrea Severson. Thanks to Lydia Zeglin for help on describing molecular methods. Excellent suggestions for corrections of the first edition also came from Robert Humston, David Rogowski, John Havel, Sergi Thomas, Daniel Welsh, Erika Iyengar, Matt McTammany, Jack Webster, and Robert Humston. Kabita Ghimire produced the global distribution maps of freshwater habitats. Keith Gido, James Whitney, and Joe Gerken helped clarify Chapter 23. Thanks to Erika Martin, Joshua Perkin, and Kyle Winders for detailed review of the entire draft manuscript from the second edition. Thanks to Andy Richford who assisted in the development of the ideas for the second edition while he was editor at Elsevier.

Walter K. Dodds thanks his teachers over the years who guided him so well down the academic path: Ms. Waln, Steve Seavey, John Priscu, and especially Dick Castenholz and Eric Wickstrom. His students (Chris, Eric, Ken, Michelle, Mel, Randy, Nicole, Bob, Jon, Kym, Jessica, Justin, Alyssa, Kyle, Alex, and all the others) have kept asking the questions that fuel imagination. Walter’s parents initiated his fascination for nature, and the encouragement of his siblings and in-laws kept him going. Dolly made it all possible.

Matt R. Whiles thanks F. E. Anderson, L. L. Battaglia, J. E. Garvey, J. W. Grubaugh, R. O. Hall, A. D. Huryn, K. R. Lips, R. Lira, and S. D. Peterson for valuable input and advice on this work. His early mentors, M. E. Gurtz, C. M. Tate, G. R. Marzolf, and J. B. Wallace provided guidance at critical points of his life and helped him realize that he could actually make a career out of his interests in ecology. His parents, Jim and Jane, and his sister, Wendy, tolerated and even supported his somewhat odd boyhood interests and laid the foundation for all of this. His wife, S. G. Baer, provided endless support, encouragement, and advice, and his recent and current graduate students (Amanda, Catherine, Checo, Dan, David, Eric, Jodi, Kaleb, Kim, Natalie, Therese) provided inspiration and tolerated him throughout this process.

Our children, Hannah, Joey, Sadie, and Rowland, put it all in perspective; the next generation is the reason this text includes environmental applications. We both deeply appreciate the support and love of our families.

Why Study Continental Aquatic Systems?

Human Use of Water: Pressures on a Key Resource 5

What Is the Value of Water Quality? 8

Advanced: Methods for Assigning Values to Ecosystem Goods and Services 12

Climate Change and Water Resources 14

Politics, Science, and Water 16

Summary 17

Questions for Thought 18

Figure 1.1 Crater Lake, Oregon.

As is true of all organisms, our very existence depends on water; we need an abundance of fresh water to live. Although the majority of our planet is covered by water, only a very small proportion is associated with the continental areas on which humans are primarily confined (Table 1.1). Of the water associated with continents, over 99% is in the form of groundwater or ice and is difficult for humans to use. Human interactions with water most often involve fresh streams, rivers, marshes, lakes, and shallow groundwaters; thus, we rely heavily on a relatively rare commodity.

Table 1.1 Locations, Amounts, and Turnover Times of Water Compartments in the Global Hydrologic Cycle

(Data from Todd, 1970; Wetzel, 2001; Pidwirny, 2006; The Hydrologic Cycle, Fundamentals of Physical Geography, 2nd Ed.)

Why study the ecology of continental waters? To the academic, the answer is easy: because it is fascinating and we enjoy learning for its own sake. Thus, the field of limnology¹ (the scientific study of continental waters) has developed. Limnology has a long history of academic rigor and broad interdisciplinary synthesis (e.g., Hutchinson, 1957, 1967, 1975, 1993; Wetzel, 2001). One of the truly exciting aspects of limnology is the synthetic integration of geological, chemical, physical, and biological interactions that define aquatic systems. No limnologist exemplifies the use of such academic synthesis better than G. E. Hutchinson (Biography 1.1); he did more to define modern limnology than any other individual. Numerous other exciting scientific advances have been made by aquatic ecologists, including refinement of the concept of an ecosystem, ecological methods for approaching control of disease, methods to assess and remediate water pollution, ways to manage fisheries, restoration of freshwater habitats, understanding of the deadly lakes of Africa, and conservation of unique organisms.

Biography 1.1 G. Evelyn Hutchinson

George Evelyn Hutchinson (Fig. 1.2) was one of the top limnologists and ecologists of the 1900s, and likely the most influential of the century. His career spanned an era when ecology moved from a discipline that was mainly the province of natural historians to a modern experimental science. In large part, he and his students were responsible for these developments. Born in 1903 in Cambridge, England, Hutchinson was interested in aquatic entomology as a youth and authored his first publication at age 15. He obtained an MA from Emmanuel College of Cambridge University and worked in Naples, Italy, and South Africa before securing a position at Yale University. He remained at Yale until the end of his career, and died in 1991.

Hutchinson’s range of knowledge was immense, and his broad and innovative view of the world enriched his scientific endeavors. He was well versed in literature, art, and the social sciences, and published works on religious art, psychoanalysis, and history, including some of the most widely read and cited ecological works of the century. His four volumes of the Treatise of Limnology are the most extensive treatment of limnological work ever published. His writings on diversity, complexity, and biogeochemistry inspired numerous investigations. Hutchinson organized a research team to study the Italian Lake Ianula in the 1960s; this multidisciplinary approach has since become a predominant mode of ecological research. It is reported that he was always able to find positive aspects of his students’ ideas and encouraged them to develop creative thoughts into important scientific insights. As a consequence, many of Hutchinson’s students and their students are among the most renowned ecologists today.

Figure 1.2 G. Evelyn Hutchinson.

(Courtesy of the Yale Image Library).

Hutchinson earned many major scientific awards in his career, including the National Medal of Science. He wrote popular scientific articles and books that were widely distributed. He was a staunch defender of intellectual activities and their importance in the modern world. Hutchinson’s mastery of facts, skillful synthesis, knack for asking interesting and important questions, evolutionary viewpoint, and cross-disciplinary approach make him an admirable role model for students of aquatic ecology.

Each of these advances will be covered in this book. We hope to transmit the excitement and appreciation of nature that come from studying aquatic ecology. Further justification for study may be necessary for those who insist on more concrete benefits from an academic discipline or are interested in preserving water quality and aquatic ecosystems in the broader political context. There is a need to place a value on water resources and the ecosystems that maintain their integrity and to understand how the ecology of aquatic ecosystems affects this value. Water is unique and has no substitute. A possible first step toward placing a value on a resource is documenting human dependence on it and how much is available for human use.

Humankind would rapidly use all the water on the continents were it not replenished by atmospheric input of precipitation. Understanding hydrologic fluxes, or movements of water through the global hydrologic cycle, is central to understanding water availability, but much uncertainty surrounds some aspects of these fluxes. Given the difficulty that forecasters have predicting the weather over even a short time period, it is easy to understand why estimates of global change and local and global effects on water budgets are beset with major uncertainties (Mearns et al., 1990; Mulholland and Sale, 1998). Scientists can account moderately well for evaporation of water into the atmosphere, precipitation, and runoff from land to oceans. This accounting is accomplished with networks of precipitation gauges, measurements of river discharge, and sophisticated methods for estimating groundwater flow and recharge.

The global water budget is the estimated amount of water movement (fluxes) between compartments (the amount of water that occurs in each area or form) throughout the globe (Fig. 1.3). This hydrologic cycle will be discussed in more detail in Chapter 4 but is presented here briefly to introduce discussion of how much water is available for human use. Total runoff from land to oceans via rivers has been reported as 22,000, 30,000, and 35,000 km³ per year by Leopold (1994), Todd (1970), and Berner and Berner (1987), respectively. These estimates vary because of uncertainty in gauging large rivers in remote regions. So, what are the demands on this potential upper limit of sustainable water supply?

Figure 1.3 Fluxes (movements among different compartments) in the global hydrologic budget (in thousands of km³ per year).

(Data from Berner and Berner, 1987).

Human Use of Water: Pressures on a Key Resource

People in developed countries generally are not aware of the quantity of water that is necessary to sustain their standard of living. In North America particularly, high-quality water often is used for such luxuries as filling swimming pools and watering lawns. Perhaps people notice that their water bills increase in the summer months. Publicized concern over conservation may translate, at best, into people turning off the tap while brushing their teeth or using low-flow showerheads or low-flush toilets. Few people in developed countries understand the massive demands for water from industry, agriculture, and power generation that their lifestyle requires (Fig. 1.4).

Figure 1.4 Estimated uses of water (A) and total population and per capita water use (B) in the United States from 1950 to 2000. Note that industrial and irrigation uses of water are dominant. Offstream withdrawals used in these estimates do not include hydroelectric uses.

(Data courtesy of the United States Geological Survey).

Some of these uses, such as domestic, require high-quality water. Other uses, such as hydroelectric power generation and industrial cooling, can be accomplished with lower quality water. Some uses are consumptive and preclude further use of the water; for instance, a significant portion of water used for agriculture is lost to evaporation. The most extreme example of nonrenewable water resource consumption may be water mined (withdrawal rates in excess of rates of renewal from the surface) from aquifers (large stores of groundwater) that have extremely long regeneration times. Such withdrawal is practiced globally (Postel, 1996) and also accounts for a significant portion of the United States’ water use, particularly for agriculture (Fig. 1.5). In many cases use rates exceed the rate at which the aquifer is replenished, and the groundwater is over exploited. Other uses are less consumptive. For example, hydroelectric power consumes less water (i.e., evaporation from reservoirs increases water loss, but much of the water moves downstream).

Figure 1.5 Amounts of surface and groundwater used in the United States from 1950 to 2000. Estimates include only withdrawals and not hydroelectric uses.

(Data courtesy of the United States Geological Survey).

How much water does humankind need? A wide disparity occurs between per capita water use in developed and less developed arid countries, particularly in semiarid countries in which surface water is scarce (Table 1.2). Israel is one of the most water-efficient developed countries, with per capita water use of 500 m³ per year (Falkenmark, 1992), about four times more efficient than the United States. Increases in standard of living lead to greater water demands (per capita water use) if not accompanied by increases in efficiency of use.

Table 1.2 General Ranges of Water Use with Varied Socioeconomic Conditions on a per-Capita Basis; Rates per Country Are Estimates for the Year 2000

(After la Rivière, 1989; Falkenmark, 1992; Gleick et al., 2002)

The maximum total water available for human use is the amount that falls as precipitation on land each year minus the amount lost to evaporation. As mentioned earlier, the maximum amount of water available in rivers is 22,000 to 35,000 km³ per year. However, much is lost to floods in populated areas or flows occurring in areas far removed from human population centers, leaving approximately 9000 km³ per year for all human uses (la Rivière, 1989). Humans cannot sustain use of water greater than this supply rate without additional supplies withdrawn from groundwater at rates greater than renewal, collected from melting ice caps, transported from remote areas, or reclaimed (desalinized) from oceans. These processes are expensive or impossible to sustain in many continental regions without tremendous energy input, as will be discussed later in this chapter.

Predicting future water use is difficult but instructive for exploring possible future patterns and consequences of this use. Total annual offstream withdrawals (uses that require removal of water from the river or aquifer, not including hydroelectric power generation) in the United States in 1980 were 2,766 m³ per person and have decreased slightly since that time, mostly due to a decrease in total industrial use (Fig. 1.4). If all the people on Earth used water at the rate it is currently used in the United States (i.e., their standard of living and water use efficiency were the same as in the United States), over half of all the water produced by the hydrologic cycle would be used. Globally, humans currently withdraw about 54% of runoff that is geographically and temporally accessible (Postel et al., 1996); if all people in the world used water at the per capita rates used in the United States, all the water available (geographically and temporally accessible) would be used.

On a local scale, water scarcity can be severe. Political instability in Africa is predicted based on local population growth rates and limited water supply (Falkenmark, 1992). Similar instabilities are likely to arise from conflicts over water use in many parts of the world (Postel, 1996). Large urban areas have tremendous water demands, and very strong effects on local and regional hydrology, generally decreasing water availability (Fitzhugh and Richter, 2004). For example, in some large arid cities major river flow is almost completely composed of sewage effluent, and some small streams that were ephemeral have become permanent because of runoff from lawn watering. In the arid southwestern United States, uses can account for more than 40% of the supply (Waggoner and Schefter, 1990). In such cases, degradation of water quality has substantial economic consequences.

The population of the earth is currently over 6.7 billion people and is predicted to double over the next 43 years (Cohen, 1995). The global population will have increased more than 10% between the first and second editions of this book. Given the increase in human population and resource use (Brown, 1995), demand for water will only intensify (Postel, 1996). As the total population on Earth expands, the value of clean water will increase as demands escalate for a finite resource (Dodds, 2008). Population growth is likely to increase demand for water supplies, even in the face of uncertainty over climate in the future (Vörösmarty et al., 2000). Increased efficiency has led to decreases in per capita water use in the United States since the early 1980s (Fig. 1.4). Efforts to boost conservation of water will become essential as water becomes more valuable (Brown, 2000).

Despite the existence of technology to make water use more efficient and maintain water quality, the ongoing negative human impact on aquatic environments is widespread. Most uses of water compromise water quality and the integrity of aquatic ecosystems, and future human impact on water quality and biodiversity is inevitable. An understanding of aquatic ecology will assist humankind in making decisions to minimize adverse impacts on our aquatic resources, and will ultimately be required for policies that lead to sustainable water use practices (Gleick, 1998). Accounting for the true value of water will help policy makers decide how important its use is, and students reading this book understand how water is a vital resource to learn about.

What Is the Value of Water Quality?

Accurate accounting for the economic value of water includes determining both the immediate benefit and how obtaining a particular benefit alters future use. Consumption and contamination associated with each type of use dictate what steps will be necessary to maintain aquatic ecosystems and water quality and quantity. Establishing the direct benefits of using water, including patterns and types of uses, is also necessary. Elucidation of benefits will allow determination of economic value of water and how its use should be managed.

Availability of water is clearly essential, but the quality of water also must be considered. Aquatic ecosystems provide us with numerous benefits in addition to direct use. Estimates of the global values of wetlands ($3.2 trillion per year) and rivers and lakes ($1.7 trillion per year) indicate the key importance of freshwaters to humans (Costanza et al., 1997). These early estimates will increase as researchers are able to assign values to more ecosystem goods and services (Dodds et al., 2008). Ecosystem goods and services are those provided by the ecosystem that are of value to humans. Freshwater aquatic ecosystems have the greatest value per unit area of all habitats. Estimates suggest that the greatest values of natural continental aquatic systems are derived from flood control, water supply, and waste treatment. The value per hectare is greater for wetlands, streams, and rivers than for any terrestrial habitats. The next section details how these values are actually assigned.

We explore values of aquatic ecosystems because monetary figures can influence their perceived value to society. Assigning values to ecosystems can provide evidence for people advocating minimization of anthropogenic impacts on the environment. Ignoring ecosystem values can be particularly problematic because perceived short-term gain often outweighs poorly quantified long-term harm when political and bureaucratic decisions are made about resource use. For example, restored wetlands may not be as valuable as conserved wetlands (Dodds et al., 2008), so policy decisions should be made accordingly. The concept of no net loss when a constructed wetland can be substituted for a wetland that is removed for development may not capture all the values a natural wetland provides to society.

Quantification of some values of water is straightforward, including determining the cost of drinking water, the value of irrigated crops, some costs of pollution, and direct values of fisheries. Others may be more difficult to quantify. What is the value of a canoe ride on a clean lake at sunset or of fishing for catfish on a lazy river? What is the worth of the species that inhabit continental waters including nongame species?

What is the actual value of water? The local price of clean water will be higher in regions in which it is scarce. Highly subsidized irrigation water sells for about $0.01 per m³ in Arizona, but clean drinking water costs $0.37 per m³ in the same area (Rogers, 1986). Drinking water costs between $0.08 and $0.16 per m³ in other areas of the United States (Postel, 1996). At the rate of $0.01 per m³, and assuming that people on Earth use 2,664 km³ per year for irrigation (according to the Food and Agriculture Organization for 2001), the global value of water for irrigation can be estimated as $26 billion per year. This is probably an underestimate; in the 1970s in the United States, 28% of the $108 billion agricultural crop was irrigated (Peterson and Keller, 1990). Thus, $30 billion worth of agricultural production in one country alone could be attributed to water suitable for irrigation. Worldwide, 40% of the food comes from irrigated cropland (Postel, 1996). World grain production in 1995 was 1.7 × 10¹² kg (Brown, 1996). In the 1990s, grain had a value of $0.50 per kilogram, and $340 billion per year came from irrigated cropland globally. This value has increased at least 15% by 2008.

Use of freshwater for irrigation does not come without a cost. Agricultural pesticide contamination of groundwater in the United States leads to total estimated costs of $1.8 billion annually for monitoring and cleanup (Pimentel et al., 1992). Erosion related to agriculture causes losses of $5.1 billion per year directly related to water quality impairment in the United States (Pimentel et al., 1995). This estimate includes costs for dredging sediments from navigation channels and recreation impacts, but it excludes biological impacts. These estimates illustrate some of the economic impetus to preserve clean water.

The economic value of freshwater fisheries, including aquaculture, worldwide is over $21 billion per year (Table 1.3). This economic estimate includes only the actual cash or trade value of the fish and crustaceans. In many countries, sport fishing generates considerable economic activity. For example, in the United States, $15.1 billion was spent on goods and services related to freshwater angling in 1991 (U.S. Department of the Interior and Bureau of the Census, 1993). In addition, 63% of nonconsumptive outdoor recreation visits in the United States included lake or streamside destinations, presumably to view wildlife and partake in activities associated with water (U.S. Department of the Interior and Bureau of the Census, 1993). Many of these visits result in economic benefits to the visited areas. Maintaining water quality is vital to healthy fisheries and healthy economies. Pesticide-related fish kills in the United States are estimated to cause $10 to 24 million per year in losses (Pimentel et al., 1992). Finally, maintaining fish production may be essential to ensuring adequate nutrition in developing countries (Kent, 1987). Thus, the value of fisheries exceeds that of the fish. Managing fisheries clearly requires knowledge of aquatic ecology. These fisheries and other water uses face multiple threats from human activities.

Table 1.3 Global Fisheries Production Relying on Freshwater in 2005

(Food and Agriculture Administration, 2007)

Sediment, pesticide, and herbicide residues; fertilizer runoff; other nonpoint runoff; sewage with pathogens and nutrients; chemical spills; garbage dumping; thermal pollution; acid precipitation; mine drainage; urbanization; and habitat destruction are some of the threats to our water resources. Understanding the implications of each of these threats requires detailed understanding of the ecology of aquatic ecosystems. The effects of such human activities on ecosystems are linked across landscapes and encompass wetlands, streams, groundwater, and lakes (Covich, 1993). Management and policy decisions can be ineffective if the linkages between systems and across spatial and temporal scales are not considered (Sidebar 1.1). Effective action at the international, federal, state, and local governmental levels, as well as in the private sector, is necessary to protect water and the organisms in it. Success generally requires a whole-system approach grounded with sound scientific information (Vogt et al., 1997). Productive application of science requires explicit recognition of the role of temporal and spatial scale in the problems being considered and the role of the human observer (Allen and Hoekstra, 1992). Thus, we attempt to consider scale throughout the book. As discussed later, understanding of the mechanisms of problems such as nutrient pollution, flow alteration in rivers, sewage disposal, and trophic interactions has led to successful mitigation strategies. Many of our rivers are cleaner than they were several decades ago. Future efforts at protection are more likely to be successful if guided by informed aquatic ecologists interested in safeguarding our water resources.

Sidebar 1.1 Valuation of Ecosystem Services: Contrasts of Two Desired Outcomes

Ecosystem services refer to the properties of ecosystems that confer benefit to humans. Here, we contrast two types of watershed management and some economic considerations of each. The first case involves the effects of logging on water quality and salmon survival on the northern portion of the Pacific coast of North America, and the second involves water supply in some South African watersheds. The preferred management strategies are different, but both rely on understanding ecosystem processes related to vegetation and hydrologic properties of watersheds. When watersheds have more vegetation, particularly closer to streams, they have lower amounts of runoff and less sediment in the runoff. Removal of streamside vegetation is a major concern for those trying to conserve salmon.

Several species of salmon are considered endangered and the fish have direct effects on the biology of the streams in which they spawn (Willson et al., 1998). Sport and commercial fisheries have considerable value on the northwest coast of North America. Dams that prevent the passage of adult fish and habitat degradation of streams are the two main threats to salmon survival in the Pacific coastal areas. Logging (Fig. 1.6), agriculture, and urbanization lead to degradation of spawning habitat. The main effects of logging include increased sedimentation and removal of habitat structure (logs in the streams). These factors both decrease survival of eggs and fry. Even moderate decreases in survival of young can have large impacts on potential salmon extinction (Kareiva et al., 2000).

Figure 1.6 A clear cut across a small stream.

(Courtesy of the United States Forest Service).

Economic analysis of efforts to preserve salmon populations includes calculation of the costs of modifying logging, agriculture, and dam construction and operation as opposed to the benefits of maintaining salmon runs. The economic benefits of salmon fisheries are estimated at $1 billion per year (Gillis, 1995). Costs of modifying logging, agriculture, and dam construction and operation probably exceed the direct economic value of the fishery.

The second case involves shrubland watersheds (fynbos) in South Africa that provide water to large agricultural areas downstream and considerable populations of people in urban centers and around their periphery (van Wilgen et al., 1996). Introduced weed species have invaded many of these shrubland drainage basins (watersheds or catchments). The weeds grow more densely than the native vegetation and reduce runoff to streams. Also, about 20% of the native plants in the region are endemic and thus endangered by the weedy invaders.

Costs of weed management are balanced against benefits from increased water runoff. Costs associated with weed removal are offset by a 29% increase in water yield from the managed watersheds. Given that the costs of operating a water supply system in the watershed do not vary significantly with the amount of water yield, the projected costs of water are $0.12 per m³ with weed management and $0.14 per m³ without it. Other sources of water (recycled sewage and desalinated water) are between 1.8 and 6.7 times more expensive to use. An added benefit to watershed weed control is protection of native plant species. Thus, weed removal is economically viable.

The two cases illustrate how ecosystem management requires understanding of hydrology and biology. In the case of the salmon, vegetation removal (logging) is undesirable because it lowers water quality and reduces reproductive success. In the South African shrublands, removal of introduced weeds is desirable because it increases water yield. Knowledge of the ecology of the systems is essential to making good decisions.

Advanced: Methods for Assigning Values to Ecosystem Goods and Services

An entire field of defining ecosystem goods and services has arisen as a branch of ecological economics. In this section we discuss how methods are actually assigned, and how this relates to freshwaters. A simple calculation can illustrate how valuable water is. About 9,000 km³ of water are available each year to humans through the global hydrologic cycle when and where we need it. It takes 2,443 J of energy to evaporate each gram of water. This means that 2.19 × 10²² J of energy are required each year to move water from the ocean into the atmosphere. Humans currently use about 5 × 10²⁰ J of energy each year, mostly from fossil fuels. Thus it would take about 40 times our global energy use to evaporate all the water available, or 16 times to evaporate all the water we actually use. The average elevation on Earth is roughly 800 m, and lifting the 9,000 km³ of water to that elevation would require about 10% of human energy use (this does not account for frictional loss in pipes or inefficiencies of pumps) that would be required if humans needed to pump the water from sea level to the areas they inhabit. Classical economics generally assumes that the water is simply available and assigns no value to it, even though to replicate the ecosystem service of providing fresh water from the ocean would require a substantial amount of the world’s economy to provide if the global hydrologic cycle did not.

Several schemes have been constructed to value ecosystem goods and services. Probably the most widely agreed upon scheme was developed in conjunction with the Millennium Ecosystem Assessment (MEA, 2003). This scheme recognizes utilitarian (provisioning, regulating, and supporting aspects) and nonutilitarian values (ethical, religious, cultural, and philosophical aspects). Utilitarian values can be broken down into direct use values, indirect use values, and option values. Direct use values include consumptive (harvesting fish or aquatic plants) and nonconsumptive (water sports, recreation) use values. Indirect use values include water purification, water supply, and other ecosystem processes that benefit humanity. Option values are values that are not used currently, but may be used in the future (e.g., maintaining a fishery so your offspring can use it). Different values are determined with appropriate methodology. With respect to direct uses, consumptive values can be directly estimated by the market value of the item of interest.

Nonconsumptive values can be determined based on the amount of money people spend to partake in particular activities and surveys to gauge how much they would be willing to pay to maintain that activity (Wilson and Carpenter, 1999). For example, one can calculate how much property value would increase if a lake was cleaner, or how many more recreation days would be spent on a clean lake compared to a polluted lake coupled with the average amount of money spent per recreational visit. In an example of determining a relationship between an economic benefit and an ecosystem attribute, Michael et al. (1996) demonstrated that a 1-m increase in lake clarity translated into increased property values of $32 to $656 per meter of frontage. Thus, it can be established how much people are willing to pay for some aesthetic values.

Indirect uses can also be quantified. For example, if water pollution is of concern, the amount of money required to treat the water to make it usable would be quantified. In another example, flood regulation is an important property of some wetlands because they allow the water to spread out and cause less damage downstream. The cost of protecting downstream areas from floods if the wetlands were removed can be used to assign values for wetland flood protection.

The idea of quantifying ecosystem goods and services is controversial in the field of economics (e.g., Bockstael et al., 2000). Critics argue that discounting (the lost cost of investment opportunity) is ignored. The argument is fundamentally philosophical, with proponents for conserving economic values of ecosystem goods and services arguing that benefits are accrued to society rather than individuals, so benefits are worth as much if they are used immediately or withheld as option values.

The second critique of valuation of ecosystem goods and services is that they do not have exact value (e.g., using willingness to pay survey methods). Economists who argue this are more comfortable assuming that values of ecosystem goods and services are externalities, or economic values that are not readily assigned a value. This critique is based on the argument that a poorly constrained estimate of value is worse than no estimate at all.

Nonutilitarian values are more difficult to quantify. Still, people are willing to pay to protect aspects of the environment that are important to them for ethical, religious, cultural, or philosophical reasons. For example, many people are willing to allot tax dollars to help protect parks and natural areas even if they are unlikely ever to visit those areas.

The fact remains that many aspects of freshwater environments are highly valued by humanity. Rivers and lakes are some of the most desirable places to live and recreate, and we have multiple dependencies on them as a source of water. As ecological economics matures as a field, more concrete values will continue to be assigned to freshwater ecosystem goods and services.

Climate Change and Water Resources

There is now consensus among scientists that human-induced climate change is warming the planet (Intergovernmental Panel on Climate Change (IPCC), 2007), although some of the details remain uncertain. Knowledge of the basics of freshwater ecology will be essential if future scientists, such as the students reading this book, hope to be able to deal with the consequences of global climate change. The earth’s warming climate is expected to negatively affect the quality and quantity of freshwater resources. Along with direct effects of warming temperatures, climate models forecast changes in regional precipitation patterns and overall higher variability in precipitation that will lead to increased frequency, magnitude, and unpredictability of flooding and droughts in many regions. Reductions in snow pack in mountain regions, which are already occurring at an alarming rate, will cause reduced stream flows. Warming will also result in earlier melting of snow pack, causing changes in the seasonal hydrology of receiving streams (Barnett et al., 2008). Climate change will decrease freshwater availability in many areas because of increased losses to evaporation and human use, and warming water temperatures are expected to have both additive and synergistic effects with other stressors such as nutrient pollution and the spread of exotic species. Smaller bodies of water will likely be affected first by climate change because they have less thermal and hydrologic buffering capacity and are more highly influenced by local precipitation patterns (Heino et al., 2009).

Warming temperatures will exacerbate current water pollution problems because increased water loss will reduce the volumes of water in lakes, streams, and wetlands, effectively concentrating pollutants and reducing the flushing of these materials (Whitehead et al., 2009). Along with the obvious ecological consequences, water treatment costs will increase accordingly with increasing levels of pollutants. Freshwater habitats in many regions are already stressed from increasing nutrient inputs from human activities; this coupled with reduced water volumes and warming water temperatures will fuel growth of undesirable algae (Wrona et al., 2006). Algal growths will lead to changes in community structure and reduced oxygen availability.

Predicting exact responses of different freshwater habitats to the combined effects of warming and other human impacts is difficult. For example, increased nutrient levels and water temperatures might initially increase biodiversity in a cold, oligotrophic lake, whereas similar changes to a eutrophic lake would likely result in a decrease in biological diversity because of reduced oxygen storage capacity (Heino et al., 2009). Warming may also directly influence the availability of nutrients and other materials. For example, in arctic regions, predicted melting of upper layers of permafrost will liberate phosphorus, which can lead to cascading effects on the productivity of regional streams and lakes (Hobbie et al., 1999).

Climate change will alter seasonal patterns in lakes and extend the ice-free season and period of summer stratification of lakes in higher latitudes. As temperatures warm, lake mixing patterns will change, with extended periods of summer stratification. Extended stratification will increase the probability of anoxia in cooler, deep water habitats that cold-water species need to survive.

Losses in biodiversity, and thus ecosystem function and stability, as a result of climate change and other human impacts are expected to be among the greatest in freshwater habitats (Xenopoulos et al., 2005). Growth rate, adult size, and ultimately fecundity of most aquatic organisms are all influenced by temperature. Even subtle changes in average or maximum water temperatures can have measurable effects on the biota; in some cases these effects are sublethal, but can still be serious. For aquatic insects such as many mayflies, warming water temperatures just 2 to 3°C above optimal can greatly reduce the number of eggs produced by females (Vannote and Sweeney, 1980; Firth and Fisher, 1992), which has important implications for future mayfly populations, predatory fish production, and overall ecosystem health. Warming can also desynchronize life cycles and seasonal phenologies of consumers and resources, a pattern that has already been documented with some flowers and pollinators (Memmott et al., 2007).

Global climate change has led us into a no-analog world, where predicting hydrology based on past patterns is difficult or impossible (Milley et al., 2008). Thus, a mechanistic understanding of hydrologic process is necessary to predict the future availability of water in various regions as well as patterns of drought and flooding that will influence human society.

Politics, Science, and Water

Given the undeniable importance of water to human affairs, the fact that water issues are political is scarcely surprising. Globally, water could well become the first limiting resource for humanity (Dodds, 2008). More than 2 billion people on Earth live in areas where water is scarce (Oki and Kanae, 2006) and more than a billion lack adequate safe drinking water (Pimentel et al., 2004). Locally, many communities are limited by water. Most major cities on earth are located near water because people need an adequate supply, and rivers and lakes served as primary transportation avenues before road, rail, and air travel were available. Wetlands also have historic importance; the ancient Chinese built one of the world’s great civilizations fueled for centuries with rice harvested from wetlands.

Now, particularly in arid regions, politics and water are deeply intertwined. In the arid western United States, water law is convoluted and water rights transcend most other property rights. Communities appropriate water from far away, and subsidize its use to encourage development. However, water is not just limiting in the United States; conflicts over water have occurred throughout recorded history (Gleick, 2008). Cut-off of water was used as a military tool as early as 2,500 bc, and has been used repeatedly to this day. Violent clashes have occurred over water repeatedly, including between Ethiopia and Kenya in 2006, between villagers in Kashmir India in 2002, between Chinese farmers in 1999 (with riot damages reaching $1 million), and in 1993 the Iraqi government started draining the Mesopotamian marshes to punish people living in the area. Political conflict over water across borders occurs in many places including India–Pakistan, Israel–Jordan, and Mexico–United States. The degree of conflict will only increase with more people vying for a finite amount of water and water pollution and exploitation decreasing the availability of clean water.

As we harm the quality of water, even less becomes available for use and it also becomes less suitable to support native biota. When water is a more limited commodity its market value swells, even more so as the economic values associated with ecosystem goods and services provided by water are increasingly recognized. The United States’ Clean Water Act, the European Union’s Water Framework Directive, India’s system of Water Quality Standards, and the Law of the People’s Republic of China on Prevention and Control of Water Pollution are just a few examples of how governments are attempting to protect water quality. Such protection requires sound scientific understanding of how aquatic ecosystems function and humans influence that function.

Managing aquatic resources successfully requires scientific understanding of freshwaters. Through scientific study of freshwaters, students may come to understand the beauty and complexity of a crucial part of the natural world. Part of studying freshwaters is learning the intrinsic value, in addition to the utilitarian value, of water. We hope that this text will serve as a resource for future stewards of our freshwaters.

Summary

1. Clean water is essential to human survival, and we rely most heavily on continental water, including streams, lakes, wetlands, and groundwater.

2. The global renewable supply of water is about 39,000 km³ per year, and humans use about 54% of the runoff that is reasonably accessible. Thus, clean water is one resource that will be limited severely with future growth of the human population and increases in the standard of living. Local problems with water quality and supply may lead to political instability.

3. Economic analysis of the value of clean water is difficult, but factors to consider include the value of clean water for human use, the value of fisheries, and recreational use of aquatic habitats. The global benefits of these uses translate into hundreds of billions of dollars each year. Intangible benefits include preservation of nongame species and native ecosystems.

4. The study of the ecology of inland waters will lead to more sound decisions regarding aquatic habitats as well as provide a solid basis for future research.

Questions for Thought

1. Why are you interested in studying aquatic ecology, and is such study important?

2. What is the difference between fluxes and compartments in water cycles, and what types of units are typically used to describe them?

3. What are some potential economic benefits to maintaining water quality?

4. What are the potential dangers of approaching conservation of aquatic resources from a purely economic viewpoint?

5. To whom does fresh water really belong?

6. List three trade-offs that are potentially involved in protecting native species in regulated rivers by attempting to mimic natural water discharge patterns.

¹ The term limnology includes saline waters (Wetzel, 2001) and all other continental waters, but limnology courses traditionally do not cover wetlands, groundwater, and even streams. Thus, this book is titled Freshwater Ecology.

Properties of Water

Chemical and Physical Properties 20

Advanced: The Nature of Water 26

Relationships among Water Viscosity, Inertia, and Physical Parameters 27

Movement of Water 31

Advanced: Equations Describing Properties of Moving Water 37

Forces That Move Water 41

Summary 43

Questions for Thought 44

Figure 2.1 Bubbles formed in ice. This image reflects several properties of water. Surface tension forces the bubbles to be spherical, and gas that can dissolve in liquid water cannot do so in solid water.

(Courtesy of Steven Lundberg).

Unique physical properties dictate how water acts as a solvent and how its density responds to temperature. These physical properties have strong biological implications, and knowledge of water’s characteristics forms the foundation for aquatic science. Physical properties of water are so central to science that they are used to define several units of measurement, including mass, heat, viscosity, temperature, and conductivity. Properties of water influence how it controls geomorphology, conveys human waste, links terrestrial and aquatic habitats, and influences evolution of organisms. In this chapter we explore how viscosity and inertia of water vary with scale, temperature, and relative velocity related to aquatic ecology. Movement of water is discussed in the final section, including how flowing water interacts with solid surfaces.

Chemical and Physical Properties

One of the many unusual properties of water is that it exists in liquid form at the normal atmospheric temperatures and pressures encountered on the surface of Earth (Table 2.1). Most common compounds or elements take the form of gas or solid in our biosphere (exceptions include mercury and organic compounds). Polarity of the water molecule and hydrogen bonding dictates the range of temperatures and pressures at which water occurs in a liquid state as well as additional vital distinguishing characteristics. Because oxygen atoms attract electrons, the probability is greater that electrons will be nearer to the oxygen than the hydrogen atoms. The angle of attachment (104.5°) between the two covalent bonds, one for each of the hydrogen atoms attached to the oxygen, means a slight positive charge near the hydrogen atoms, to one side of the molecule. This unequal distribution of charge leads to each water molecule exhibiting polarity. The negative region near the oxygen attracts positive regions near the hydrogen atoms of nearby water molecules resulting in hydrogen bonding (Fig. 2.2).

Table 2.1 Some Properties of Water

(Adapted from Berner and Berner, 1987)

Figure 2.2 Schematic of hydrogen bonding among water molecules. The black lines represent covalent bonds; the dashed lines represent hydrogen bonds. This is an approximate two-dimensional representation. In water, three-dimensional cage-like structures are formed. In liquid water, these structures form and break up very rapidly.

Hydrogen bonding becomes more prevalent as water freezes but also occurs in the liquid phase (Luzar and Chandler, 1996; Liu et al., 1996); without hydrogen bonding, water would be a gas at room temperature. When water freezes, the molecules form tetrahedral aggregates that lead to decreased density, mass per unit volume. Thus, pure ice has a density of 0.917 gcm−3 at 0°C, which is significantly less dense than liquid water at any temperature (Fig. 2.3A). Lower density as a solid than as a liquid is another unusual aspect of water; most compounds are denser in solid than in liquid phase. There are several forms of crystal ice with varied density, but the ecological relevance of these different forms is minor.

Figure 2.3 The density of water as a function of temperature from freezing to 80°C (A), from 0 to 40°C (B), and from 0 to 10°C to focus on the region of maximum density (C). At 0°C, ice forms with a density of 0.917 g per milliliter.

(Data from Cole, 1994).

The density of liquid water, which is influenced by temperature and dissolved ions, can control the physical behavior of water in wetlands, groundwater, lakes, reservoirs, rivers, and oceans. Density also influences water flow and viscosity. Differences in density are important because less dense water floats on top of water with greater density. Such density differences can maintain stable layers. Formation of distinct stable layers is called stratification. Stratification is discussed in detail in Chapter 7 because it can control water movement and distribution of chemicals and organisms in lakes.

Maximum density of pure water occurs at 3.98°C (Fig. 2.3B, C). As water cools below 3.98°C, hydrogen bonding begins to arrange the water molecules into the crystal structure of ice, leading to more space between the water molecules and a less dense fluid. As water is heated above 3.98°C, it has a continuously greater decrease in density per degree temperature increase above 3.98°C (Fig. 2.3). Dissolved ions also increase water density. This density increase that occurs at ionic concentrations found in some natural saline lakes can easily overcome or enhance temperature effects on stratification (Fig. 2.4).

Figure 2.4 Comparison of density change caused by temperature to that changed by salinity. The range of variation in density with temperature is represented by the gray box. Seawater has an approximate salinity of 3.5% (indicated by the point bounded by error bars); saline lakes can exceed this value by many times. (Data from Dean, 1985).

Water is also one of the best solvents known and can dissolve both gases and ions. The solvent properties of water have greatly influenced geologic weathering of the earth’s surface by dissolving ions from rocks. Weathering is the natural source to the biosphere of