A Practitioner's Guide to Freshwater Biodiversity Conservation

By Nicole Silk

A Practitioner's Guide to Freshwater Biodiversity Conservation brings together knowledge and experience from conservation practitioners and experts around the world to help readers understand the global challenge of conserving biodiversity in freshwater ecosystems. More importantly, it offers specific strategies and suggestions for managers to use in establishing new conservation initiatives or improving the effectiveness of existing initiatives.

The book: offers an understanding of fundamental issues by explaining how ecosystems are structured and how they support biodiversity; provides specific information and approaches for identifying areas most in need of protection; examines promising strategies that can help reduce biodiversity loss; and describes design considerations and methods for measuring success within an adaptive management framework.

The book draws on experience and knowledge gained during a five-year project of The Nature Conservancy known as the Freshwater Initiative, which brought together a range of practitioners to create a learning laboratory for testing ideas, approaches, tools, strategies, and methods.

For professionals involved with land or water management-including state and federal agency staff, scientists and researchers working with conservation organizations, students and faculty involved with freshwater issues or biodiversity conservation, and policymakers concerned with environmental issues-the book represents an important new source of information, ideas, and approaches.

• Language: English
• Publisher: Island Press
• Release date: Apr 10, 2013
• ISBN: 9781597266192

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Directors

Preface and Acknowledgments:

The Development of A Practitioner’s Guide to Freshwater Biodiversity Conservation

A Practitioner’s Guide to Freshwater Biodiversity Conservation was developed to help conservation practitioners become better able to meet the challenges of freshwater biodiversity conservation. It should also help practitioners identify conservation actions that will make the greatest contribution towards stemming freshwater biodiversity decline and establish a process for refining and improving upon these actions over time. Conservation practitioners are defined here as anyone pursuing conservation goals or objectives at a particular place—professional staff of conservation organizations and government agencies, water resource managers, policy level decision-makers, etc.

This guide presents information about the global challenge of freshwater biodiversity conservation, explores how ecosystems are structured and function to support this biodiversity, and explains approaches for identifying the most important biodiversity to protect across large geographic areas as well as at specific locations. These preliminary chapters provide important contextual information followed by a detailed review of the four primary causes of freshwater biodiversity decline (water use and management, invasive alien species, land use and management, and overharvesting and fisheries management) and a wide range of promising strategies at various institutional and geographic scales for abating these threats and conserving freshwater biodiversity. The final chapter of this guide describes design considerations and methods for measuring freshwater conservation success within an adaptive management framework.

The collective wisdom gained through The Nature Conservancy’s Freshwater Initiative contributed a great deal to the structure and content of this guide. The Freshwater Initiative was launched by The Nature Conservancy in 1998 to build organizational capacity in freshwater biodiversity conservation. Originally designed as a five year program with a lofty goal of raising $10 million, the Initiative included three components: identifying areas of freshwater biodiversity importance for future conservation action and developing tools and methods to help others identify freshwater priorities; developing breakthrough strategies to common causes of freshwater biodiversity decline as well as methods for more effective applied freshwater biodiversity conservation; and creating a Freshwater Learning Center to build a community of freshwater conservation practitioners, offer opportunities for collaboration and skill-building, and develop products (articles, guidance documents, videos, web sites, etc.) for sharing knowledge and lessons learned across a variety of audiences. Although only 15 people, as a result of the focused creativity harnessed through this team, between 1998 and 2003 the Initiative had access not only to available literature and experts, but also to conservation planners and practitioners from around the globe. Working with these conservation planners and practitioners, and through a series of direct and virtual interactions, the Initiative effectively became a learning laboratory to test ideas, approaches, tools, strategies, and methods. Towards the end of the Freshwater Initiative, staff also developed a course on freshwater biodiversity conservation and disseminated regionally customized versions of this course to hundreds of practitioners within and outside of the United States. This guide is, in essence, the companion text for that course, complete with some of the best information from around the world. It delivers what Freshwater Initiative staff believed to be of most use to others engaged in freshwater biodiversity conservation.

Although the Freshwater Initiative officially disbanded in 2003, work on this guide continued through The Nature Conservancy’s Sustainable Waters Program. Contributors to this guide include Nature Conservancy staff, staff from Nature Conservancy of Canada, staff and graduate students affiliated with the University of Georgia’s Institute of Ecology and the University of Michigan’s School of Natural Resources. These contributors represent a wide range of disciplines (ecology, hydrology, economics, law, etc.) and professional backgrounds (field-based conservation practitioners, university professors, professional experts, etc.), which mirror the complexity and multi-disciplinary nature of the challenge of freshwater biodiversity conservation. Contributors central to the content of individual chapters and subchapters include: Chapter 1: Nicole Silk; Chapter 2: Kristine Ciruna and David Braun; Chapter 3: Mark Bryer, David Braun, Mary Khoury, and Jonathan Higgins; Chapter 4: Introduction: Nicole Silk, Kristine Ciruna and David Braun; Chapter 4: subchapter on Water Use and Management: Ronald Bjorkland and Catherine Pringle; Chapter 4: subchapter on Land Use and Management: Rebecca Esselman and David Allan; Chapter 4: subchapter on Invasive Alien Species: Kristine Ciruna; Chapter 4: subchapter on Overharvesting and Fisheries Management: Ronald Bjorkland and Catherine Pringle; Chapter 5: David Braun; Appendix A: Kristine Ciruna and Allison Aldous; and Appendix B: David Braun. In addition to the contributors listed above, this guide was also peer reviewed at various stages by over 40 practitioners working on freshwater conservation challenges at individual projects, building capacity to engage in this type of work, or contributing to the efforts of others. Special thanks are also extended to Nicole Rousmaniere for the graphic design of, assembly of images included within, and friendliness of the final lengthy product, and to Kristine Ciruna for editing contributions and project management assistance. Development of this guide was partially supported by the U.S. Environmental Protection Agency, Office of Water, through Grant X–82773901 and its ammendments.

We hope this guide helps you in your efforts.

—NICOLE SILK

Chapter 1

The Global Challenge of Freshwater Biodiversity Conservation

Nicole Silk

OVERVIEW AND LEARNING OBJECTIVES

This chapter briefly examines the need for and importance of freshwater biodiversity conservation. The chapter begins with an overview of the value of global freshwater resource, examines current trends in the decline of freshwater biodiversity, and considers prospects for improvement from a global perspective.

THE FRESHWATER CRISIS: WHY SHOULD WE CARE?

The Global Importance of Freshwater and Its Biodiversity

The most critical component for human survival is access to a sufficient supply of freshwater. However, the supply of freshwater is surprisingly low globally. Approximately 70% of Earth’s surface is covered by water, yet only 2.5% of Earth’s water is freshwater (McAllister et al. 1997). Most of the freshwater is locked in polar ice caps, stored in underground aquifers (many with recharge cycles measured in millennia), or part of soil moisture and permafrost. Only 0.01% of Earth’s water is available as freshwater in rivers and lakes (McAllister et al. 1997). This one-hundredth of a percent of Earth’s water that occupies only 0.8% of the Earth’s surface (McAllister et al. 1997) provides us with a vast array of environmental services:

Waste disposal;

Energy to fuel our electricity needs;

Transportation corridors to carry raw products and finished goods to customers;

Drinking water to quench our thirst;

Irrigation for agriculture and aquaculture;

Water to make manufactured products;

Places to recreate; and

Aesthetic, spiritual, and religious values.

Freshwater ecosystems also support an exceptional concentration of biodiversity. Species richness is greater relative to habitat extent in freshwater ecosystems than in either marine or terrestrial ecosystems. Freshwater ecosystems contain approximately 12% of all species, with almost 25% of all vertebrate species concentrated within these habitats (Stiassny 1996). The richness of freshwater species includes a wide variety of plants, fishes, mussels, crustaceans, snails, reptiles, amphibians, insects, microorganisms, birds, and mammals that live beneath the water or spend much of their time in or on the water. Many of these species depend upon the physical, chemical, and hydrologic processes and biological interactions found within freshwater ecosystems to trigger their various life cycle stages.

As with terrestrial species richness, freshwater species richness increases strongly toward the equator —there are many more freshwater species in the tropics than in temperate regions. Preliminary estimates suggest that the world’s tropics may be home to a disproportionately large share of the world’s freshwater-species richness. Central Africa, Southeast Asia, and northern South America have high numbers of fish species, including many endemic species that occur only in limited areas (Revenga et al. 2000). However, certain freshwater groups, such as freshwater crayfishes, are much less diverse in the tropics than temperate regions. Indeed, some temperate regions are exceptionally high in aquatic biodiversity. The United States, for example, ranks first in the world in the diversity of freshwater mussels, snails, and salamanders as well as three groups of freshwater insects: caddisflies, mayflies, and stone-flies. U.S. waters contain approximately 30% of the world’s freshwater mussel species.

Despite what is known, much of the diversity of life that thrives in the world’s freshwaters remains largely unknown, unstudied, and unquantified due to the challenges of sampling, the focus on objectives other than biodiversity conservation, and limited resources for inventory.

What is clear is that freshwater biodiversity is a story not only of species, but of communities and ecosystems. The history of geologic and climatic change and species evolution in every area of the globe has produced dynamic assemblages of species that interact with each other and with physical environments in unique ways:

Pacific salmon return to the headwaters of the Columbia River Basin to spawn and die, completing their lifecycles. Through this journey they also bring vast quantities of nutrients from the ocean as their spent bodies decay, enriching freshwater and terrestrial life in these systems.

The floodwaters of the Amazon River enable huge numbers of fish access to bottom-lands necessary for feeding and reproduction. These floodwaters also push vast quantities of organic matter from these bottomlands back into the river, where it becomes food for other organisms and those that feed on them.

The native freshwater mussels of North America need species-specific host fish to reproduce. These mussels release thousands of microscopic larvae called glochidia, some of which find their way to the gills of host fish. The unsuspecting host fish then carries these glochidia to new habitats. Eventually, the glochidia drop from the host fish, settle into their new homes, and grow into adult mussels.

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Illustration courtesy of Toronto and Region Conservation Authority.

Such tales of complex interactions emerge wherever freshwater ecosystems are studied.

All terrestrial organisms (plants, animals, etc.) also need freshwater for their survival. River, lake, and wetland ecosystems support almost all terrestrial animal species, since these species depend on freshwater ecosystems for water, food, and various aspects of their life cycles. Finally, all freshwater that runs into our rivers, lakes, and wetlands also eventually ends up in the ocean. There, the endless cycle of life and death in the freshwater world provides a steady stream of incoming food, sediments, and nutrients to nearshore marine ecosystems. The diversity of life in the world’s freshwaters, estuaries, and the sea are thus closely linked as well. Given the range of life dependent upon freshwater ecosystems, it is clear that we can easily lose tremendous biodiversity by the deterioration of just a few freshwater ecosystems.

The Crisis

The decline of freshwater biodiversity has reached alarming rates. The extinction rate of freshwater biodiversity is predicted to be five times faster than all other groups of species (Ricciardi and Rasmussen 1999). Researchers have estimated that during recent decades between 20% and 35% of the world’s freshwater fish species have become endangered, threatened, or extinct (Ricciardi and Rasmussen 1999, Revenga et al. 2000, Gleick et al. 2002). As well, 20% of threatened insects have freshwater larval stages, 57% of freshwater dolphins are vulnerable or endangered and 70% of freshwater otters are vulnerable or endangered (McAllister et al. 1997). Declines in freshwater biodiversity are found in every country, but perhaps are most disturbing in areas with the greatest known current native species richness. In addition to species loss, entire freshwater ecosystems are also in decline. For example, since 1970, the health of the world’s freshwater ecosystems has declined by 50% (Loh 2000). Eighty-five percent of freshwater ecosystems in Latin America and the Caribbean are in critically endangered or vulnerable condition (Olson et al. 1998). Rates of decline in Africa and Asia are similarly precipitous. Actual rates of freshwater biodiversity loss may even be much higher then these estimates since only partial data exists for most species and even less exists for entire freshwater communities and ecosystems. One projection suggests that unless dramatic steps are taken today, 20% of the world’s freshwater fish may become extinct in the next 25–50 years (Moyle and Leidy 1992).

The reasons for this deterioration are easily understood: the world’s growing human population uses freshwater for the vast range of services shown in Table 1-1. In doing so, freshwater ecosystems are disrupted, starved, contaminated, and sometimes completely eliminated. For example, the world’s rivers are now obstructed by more than 45,000 large dams, including 19,000 in China and 5,500 in the U.S. (World Commission on Dams 2000). More than 85% of the world’s large dams have been built during the last 35 years (Postel 1995). Half of the world’s wetlands have been eliminated—to make way for other land uses such as forestry, agriculture, and new homes—or modified to serve water needs by groundwater extraction as well as through the placement of dams and diversions (Revenga et al. 2000).

TABLE 1.1. Freshwater Ecosystem Services

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FIGURE 1-1. Freshwater Fish Species Richness and Endemism

Source: Revenga, C., S. Murray, J. Abramovitz, and A. Hammond. 1998. Watersheds of the World: Ecological Value and Vulnerability. Washington, DC: World Resources Institute. Reproduced with permission of the World Resources Institute.

Collectively, these changes continue to have devastating impacts on native freshwater species that evolved in close interaction with each other and the distinct patterns of chemistry, sediments, hydrology, temperature, and other physical regimes within these freshwater ecosystems. Indeed, as described above, many species require certain combinations of these patterns to trigger critical aspects of their lifecycles. For some species, when these conditions are eliminated, they can no longer reproduce. Further complicating this picture is the loss of native species due to the effects of invasive alien species introductions and poor fisheries management.

The deterioration of freshwater ecosystems results in a loss of freshwater species, communities, and ecosystems. It also results in a loss of all other animals dependent on freshwater, and degrades the ability of these systems to provide the services for humans previously mentioned. Many rivers can no longer provide flood control for downstream communities, since they have been channelized or engineered to stay within their banks and their watersheds have been altered through land clearing, the draining of wetlands, and the expansion of impervious surfaces (e.g., through paving). When the flood comes, it is often larger and more destructive than would have occurred naturally as it moves into the river more quickly, has no place to meander or spread, and moves faster to downstream locations. In many areas of the world, water extracted from rivers, lakes, and groundwater is no longer safe for drinking without additional and often costly treatments. Rates of infection from diseases carried by water are on the rise (Gleick et al. 2002). Many commercial as well as recreational fishing catches in freshwater and marine ecosystems have declined or have been eliminated.

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FIGURE 1-2. Freshwater Ecoregions of North America

Source: Abell, R., D.M. Olson, E. Dinerstein, P. Hurley, J.T. Diggs, W. Eichbaum, S. Walters, W. Wettengel, T. Allnutt, C. Louks, and P. Hedao. 2000. Freshwater Ecoregions of North America: A Conservation Assessment. Island Press, Washington, DC.

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This crisis may become more devastating in the near future given the increasing human demands for water. Annual water consumption has increased worldwide in the last 50 years. In Western Europe between 1950 and 1990, per capita water consumption grew from 100 to 560 cubic kilometers a year. In Asia, consumption has increased from 600 to 5,000 cubic kilometers between 1900 and the mid-1980s. Globally, freshwater withdrawals have almost doubled since 1960 (Loh 2000). Estimates suggest that freshwater use is growing at 2.5 times the rate of human population growth. In fact, some scenarios suggest that water withdrawals will increase 50% in developing countries and 18% in developed countries during the next 25 years, placing even greater pressures on freshwater ecosystems and potentially leading to severe water shortages across two-thirds of the total world human population by the year 2025 (Szollosi-Nagy et al. 1998). And as populations and economies grow, they will place ever increasing demands on freshwater ecosystems for hydropower, transportation, and the disposal of wastes. To all this is added the uncertainty associated with the potential effects of global climate change.

OUR CHALLENGE

Clearly, we must act now if this precious freshwater heritage is to remain. We must alter our practices so that these ecosystems continue to support both freshwater biodiversity and human life. Our survival and obligation of stewardship demands that we help these freshwater ecosystems to remain healthy, properly functioning, and capable of supporting viable populations of native plants and animals. The following chapters of this guide will help conservation practitioners meet this challenge.

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Threats to freshwater biodiversity include invasive alien species such as zebra mussels, attached to a native pink heelsplitter mussel, (above, photograph by K.S. Cummings, Illinois Natural History Survey), hydrologic alteration by dams and other diversions (upper right, photograph courtesy U.S. Army Corps of Engineers), and overexploitation of water resources for agriculture and other activities (lower right, photograph by Jeff Vanuga/USDA NRCS).

REFERENCES

Gleick, P.H., M. Cohen (contributor) and A.S. Mann (contributor). 2002. The World’s Water 2002–2003. The Biennial Report on Freshwater Resources. Island Press, Washington, DC.

Loh, J. (ed). 2000. Living Planet Report 2000. UNEP-WCMC, WWF-World Wide Fund for Nature, Gland, Switzerland.

McAllister, D.E., A.L. Hamilton and B. Harvey. 1997. Global freshwater biodiversity: Striving for the integrity of freshwater ecosystems. Sea Wind 11 (3): 1–140.

Moyle, P.E. and R.A. Leidy. 1992. Loss of biodiversity in aquatic ecosystems: Evidence from fish faunas. In: P.L. Fiedler and S.K. Jain (Eds.). Conservation Biology: The theory and practice of nature conservation, preservation and management. Chapman and Hall, New York.

Olson, D., E. Dinerstein, P. Canevari, I. Davidson, G. Castro, V. Morisset, R. Abell, and E. Toledo (eds.). 1998. Fresh-water biodiversity of Latin America and the Caribbean: A conservation assessment. Biodiversity Support Program. Washington, DC.

Postel, S.L. 1995. Where have all the rivers gone? World Watch 8: 9–19.

Revenga, C., J. Brunner, N. Henninger, K. Kassem, and R. Payne. 2000. Pilot Analysis of Global Ecosystems: Freshwater Systems. World Resources Institute. Washington, DC. 65 p.

Ricciardi, A., and J.B. Rasumussen. 1999. Extinction rates of North American freshwater fauna. Conservation Biology 13(5): 1220–1222.

Stiassny, M.L. 1996. An overview of freshwater biodiversity: With some lessons learned from African fishes. Fisheries 21(9): 7–13.

Szollosi-Nagy, A., P. Najlis, and G. Bjorklund. 1998. Assessing the world’s freshwater resources. UNESCO. Nature & Resources 34(1).

World Commission on Dams. 2000. Dams and Development: A new framework for decision-making. The Report of the World Commission on Dams.

Chapter 2

Freshwater Fundamentals: Watersheds, Freshwater Ecosystems and Freshwater Biodiversity

Kristine Ciruna and David Braun

OVERVIEW AND LEARNING OBJECTIVES

This chapter explains the form and function of freshwater ecosystems and their associated watersheds. This chapter explores climate and the associated hydrologic cycle, geology, and watershed vegetation, all essential ingredients in shaping the location and characteristics of freshwater ecosystems across the landscape. The chapter also describes the major types of discussion of the adaptation of species to freshwater ecosystems and the abundance and distribution of freshwater biodiversity globally. Figure 2-9 at the end of this chapter explains the context of this information in relation to the entire guide.

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FIGURE 2-1. Large River Watershed

Source: Adapted from USEPA, Office of Water. 1997. Volunteer Stream Monitoring: A Methods Manual.

WATERSHED FORM AND FUNCTION

What is a Watershed?

A watershed is an area of land that collects water arriving as rain and snow and then drains that water to a common outlet at some point along a body of freshwater (Dunne and Leopold 1978). All freshwater ecosystems—lakes, rivers, wetlands—have an associated watershed. Watersheds vary tremendously in size. The watershed at the headwaters of a river may be just one or two hectares (2–5 acres), whereas the watershed of the river that this small stream flows into may contain thousands of hectares of land.

Figure 2-1 represents a watershed for a large river system. The watershed boundary or divide consists of a line of highest land elevation extending around the entire watershed area. Water falling on one side of this divide will eventually flow out of the watershed outlet while water falling on the other side of the divide will drain into another watershed. The diagram in Figure 2-1 also shows that this large river system includes smaller streams or tributaries, headwaters, and wetlands. Each of these parts of the larger system has its own watershed that is a sub-watershed of the larger system. Larger watersheds are commonly made up of smaller sub-watersheds as illustrated in Figure 2-2. The tree-like branching of streams in a watershed is the drainage network of the watershed.

The variables that influence the size of a watershed include geology, climate, and land cover. These variables also determine the speed and direction of water flow within a watershed and the wash of sediments, nutrients, dissolved minerals, and a wide range of plant matter with this water and influencing the water chemistry of freshwater ecosystems within watersheds in the absence of human intervention and alteration. Understanding global processes underlying these variables is essential to understanding watershed form and function.

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FIGURE 2-2. Sub-watersheds Within a Larger Watershed

Global Processes that Shape Watershed Form and Function

Geology and climate are large-scale processes that shape the Earth’s physical features. Finer scale environmental processes and factors shape biological organization across the landscape, from the distribution of genes to ecosystems. These finer scale environmental processes and factors—such as vegetative cover, a river’s flow regime, or a wetland’s nitrogen cycle—also depend on geology and climate within a localized setting.

CLIMATE

Climate refers to an aggregate of both average and extreme conditions of solar radiation, temperature, humidity, precipitation, winds, and cloud cover measured over an extended period of time. The climate of a watershed heavily influences its vegetation communities, streamflow magnitude and timing, water temperature, and many other characteristics of key ecological factors for freshwater ecosystems.

Climate not only shapes the average weather for a watershed, but also the extremes of weather that can occur from one year to the next, or from one decade or even century to the next. Extreme events, such as floods, droughts, destructive downpours, and ice build-up, may not happen every year or decade, but they do occur. Such disturbances are natural phenomena which influence a watershed. The species present in any freshwater ecosystem will have evolved ways to cope with (or even take advantage of) these kinds of disturbances or else they would not exist in the watershed climate-driven disturbances act like giant biological filters, temporarily or even permanently driving out species that cannot tolerate or protect themselves from extreme conditions, clearing out established vegetation and animal colonies, reshaping the physical habitat, and opening up new opportunities for colonization. These dynamics are natural features of freshwater ecosystems.

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FIGURE 2-3. Summary of the Hydrologic Cycle

Source: The Federal Interagency Stream Restoration Working Group. 1998. Stream Corridor Restoration: Principles, Processes, and Practices

Hydrologic Cycle

The hydrologic cycle—the continuous cycling of freshwater from the atmosphere to the earth to the oceans and back again—is intimately tied to climate and forms the backbone of all freshwater ecosystems. The details of the hydrologic cycle—the pattern of precipitation, evaporation, runoff, groundwater recharge and return flow, and concentration of the water in lakes, rivers, and wetlands—will differ from one watershed to another because of differences in watershed climate, geology, and vegetation.

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The flows of the San Pedro River in Arizona are dependent upon groundwater. Photo by Harold E. Malde

As depicted by the hydrologic cycle (see Figure 2-3), when water falls to Earth’s surface as precipitation, it can do one of many things. The water may simply evaporate again. It may fall directly onto a body of freshwater and from there either evaporate or flow downward through a river system back to the ocean. It may fall as snow or freeze upon hitting the ground, and remain in this condition until weather conditions allow it to melt. After hitting the ground, or after melting, it may flow over the land surface as runoff, eventually becoming part of a lake, river, or wetland. Lakes, rivers, and wetlands are actually ecological terms for the different forms of surface water within a watershed. Precipitation may also soak into the ground (either immediately or after melting), where it adds to soil moisture. Once it soaks into the ground or flows into a water body, the water may also be extracted and used by plants before returning to the atmosphere as water vapor through plant transpiration.

Precipitation Patterns

Precipitation in a watershed directly influences the patterns of variation in streamflow and the water levels in lakes and wetlands. Precipitation affects the topography of a watershed by continually eroding and depositing materials as it moves across the land surface. Precipitation also strongly influences the chemistry of the water flowing through a watershed. While in the atmosphere, water droplets absorb gases, including airborne pollutants, and dust particles. Droplets swept up off the ocean surface by the wind carry dissolved ocean salts with them. As a result, the water that returns to Earth as precipitation contains not just water, but a dilution of acids, gases, and salts. Once on the ground additional minerals, gases, and organic matter from soils and underlying geologic materials in the watershed combine with these other substances to supply essential chemicals (e.g., carbon, silicon, nitrogen, and phosphorous) to freshwater ecosystems. These chemicals dissolved in the water link the hydrologic cycle to other nutrient cycles and contribute to the distribution and abundance of all living things.

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FIGURE 2-4. Confined and Unconfined Aquifers

Source: T.C. Winter, J.W. Harvey, O.L. Franke, W.M. Alley. 1998. Ground Water and Surface Water: A Single Resource. USGS Circular 1139. Denver: USGS.

Groundwater

Water that soaks into the ground usually continues to percolate further downward, adding to or recharging the water stored in the geologic materials of the watershed as groundwater. Groundwater is water that fills the spaces between soil particles and the fractures in rock formations below the ground. Approximately 22% of all freshwater on the planet occurs as groundwater compared to 0.01% in lakes and rivers (McAllister et al. 1997).

Many types of soils and rock formations hold groundwater and therefore are effective aquifers. An aquifer is a soil or rock formation that both holds a substantial amount of groundwater and is permeable enough for water to be extracted. For example, basalt is a dominant type of surficial aquifer in the northwestern U.S. and many other parts of the world. It is composed of many tiny, interconnected fractures that can hold large volumes of water. Unconsolidated materials such as sand and gravel, semi-consolidated sand, sandstone, and carbonate rock can also be effective aquifers. Other rocks or soil formations that are extremely dense, contain few fractures, or have extremely small pore spaces, such as granite and clay, are generally not very permeable and therefore do not make effective aquifers. These latter types of materials can act as barriers to groundwater movement, preventing surface water from percolating far into the ground or preventing groundwater from seeping back to the ground surface. An impermeable geologic layer that prevents deep groundwater from mixing with groundwater in some higher layer or from seeping back to the ground surface is called a confining layer. The groundwater confined in this manner is generally under pressure.

Groundwater flows from points of recharge (water infiltrating the ground) to points of discharge to surface water ecosystems and generally flows downward with gravity. However, groundwater trapped beneath a confining layer can develop a great deal of pressure. When this pressurized water meets a fracture in the confining layer, it will force its way upward, sometimes emerging at the ground surface as a spring or geyser. Groundwater also flows more quickly through some geologic formations than through others, depending on the permeability of the formations. As a result, the route that groundwater travels underground—called its flow path—is often complex. Groundwater flow paths differ greatly in length, depth, and travel time, from one aquifer and watershed to another. In fact, groundwater systems are structurally complex. Aquifers exist at many different levels below Earth’s surface, depending on the different layers of geologic materials. Many aquifers and several confining layers are often stacked up layer upon layer, each with its own geological characteristics (Figure 2-4). Similarly, an unconfined aquifer is one that is not capped by a confining layer. Generally, the aquifer nearest the ground surface, also referred to as a surficial aquifer, is an unconfined aquifer; the top of groundwater in such an unconfined aquifer is called the water table. Surficial aquifers which interact directly with river, lakes, and wetlands can have a huge influence on these freshwater ecosystems through their influence on patterns of water flow, temperature, and chemistry within freshwater ecosystems.

Confined aquifers that occur only deep in the ground are regional aquifers. Water movement into these deep aquifers and then back to the surface can take thousands or even millions of years. Intermediate level aquifers are generally closer to the surface than regional aquifers and generally have a water residence time of hundreds of years. Water in surficial aquifers may have a residence time of days to years. All aquifers interact not only with surface water but with each other; except where confining layers prevent it, aquifers constantly exchange their water supply with each other. Patterns within both deeper and more surficial aquifers are often unrelated to watershed boundaries. Eventually (sometimes in a few days, sometimes only over millions of years), all this groundwater will seep back out to the Earth’s surface and combine with other waters. Some seeps out beneath the ocean along the coast and the rest discharges into rivers, lakes, and wetlands contributing to their water levels and flow patterns.

GEOLOGY

The geology of a landscape influences its freshwater characteristics in seven crucial ways. First, geologic history determines the location of the landscape’s watersheds and the freshwater ecosystems within them. Specifically, uplifting, faulting, warping, and erosion of the Earth’s crust determines watershed boundaries and the avenues down which a river will run; depressions on the surface of Earth’s crust provide the location for the formation of a lake and so forth.

Second, geology determines the detailed spatial patterns of flow of water across and below the land’s surface within each watershed (Figure 2-5). The permeability of soil and underlying rock formations dictates whether water hitting the land as rain or snow will soak into the ground to become groundwater or remain on the surface as runoff. As noted earlier, groundwater typically does not remain locked underground, but eventually seeps back out to the land surface along geologic fractures or at low points across a watershed, where it helps form and sustain rivers, lakes, and wetlands. Rivers and streams in watersheds composed of mainly permeable soil and rock formations tend to have fairly constant minimum amount of flow throughout the year driven by groundwater contributions—called baseflow. These rivers rarely ever run dry because a significant fraction of the precipitation within such watersheds infiltrates the ground and sustains the groundwater system. The resulting slow seepage of groundwater back to the land surface provides a stable source of flow year-round. However, even streams with strong base flows have water levels that fluctuate, due to the effects of snow melt and rainfall. At the opposite extreme, rivers that have watersheds composed of impervious soils and underlying rock formations (or parking lots!) tend to be very flashy or prone to fast runoff and may become dry during prolonged periods of little or no precipitation.

Third, the geology of a landscape determines not only whether water infiltrates to become groundwater, but where that recharge can take place, how the water moves beneath the ground, and where the groundwater can return to the land surface. As noted above, different soils and rock formations at and beneath the land surface differ in their permeability and hence in their ability to recharge, store, and transmit groundwater. The distribution of geologic formations also determines where and how easily groundwater may re-emerge at the land surface as seeps or springs. As a result, neither the recharge nor the reemergence (discharge) of groundwater takes place evenly across any watershed.

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FIGURE 2-5. Influence of Basin Surficial Geology on Water Flow

Fourth, geology also strongly influences the water chemistry of freshwater ecosystems. As water moves through a watershed, both across the land surface and through the groundwater system, it dissolves some of the minerals and organic matter in its path, as noted above. These dissolved materials react with each other, with plant and animal life in the water, with, gases dissolved from the air, and with other ingredients brought along from the atmosphere to create a unique pattern of water chemistry in every watershed. Water that enters a surface water body from the groundwater system typically contains higher concentrations of dissolved minerals than does runoff water because of the longer time it spends in contact with the watershed’s geologic materials. Water temperature, one aspect of water chemistry, is also affected by geology. Groundwater generally is cooler than surface water in a watershed during warm seasons and may in fact be warmer than surface water fed streams during cold seasons as it stays nearly the same temperature year-round. In volcanic regions groundwater may become quite hot. By determining how and where groundwater discharges into surface waters, the geology of the watershed influences where and to what extent these releases may modify surface water temperature.

Fifth, the geology of a watershed determines the types, sizes, and shapes of the mineral materials that create the physical habitat substrate of every freshwater body—the shores and bottoms of lakes, the banks and beds of streams and rivers, and the sub-soils of wetlands. Most important among these materials are the sediments that get washed into rivers and lakes and carried along by currents in these waters. Water can exert a lot of force on these materials, loosening and carrying them off when flowing fast, for example, during a severe storm, and dropping them again when the flow abates. This almost constant reworking of the landscape and the substrate of the water bodies is a distinctive characteristic of all freshwater ecosystems, and the way these processes shape the physical habitat of these ecosystems depends strongly on the geologic materials available.

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Topographic features of a large river watershed include headwaters, streams of varying sizes and gradients, as well as lakes and wetlands. Photographs by National Park Service (top left), Harold E. Malde (top right, middle left, bottom left, bottom right), Brian Richter/TNC (middle right).

Sixth, the geology of a watershed determines where natural breaks may occur within the drainage system, creating natural barriers to the movement of freshwater species. Such breaks may consist of waterfalls or severe rapids or may consist of zones of porous geologic materials that allow the water to sink in rapidly to recharge the groundwater system and potentially cause the stream to sink out of sight altogether.

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Beaver dam. Photograph by Charlie Ott.

Finally, geology determines the amount and distribution of relief, or variation in elevation, present across a watershed. Relief shapes several features of freshwater ecosystems. For example, the overall slope of the watershed determines how rapidly water runs off into its water bodies, and the gradient of a river determines how fast the river will flow and the size of a river’s bottom substrate material. The shape and gradient of a river valley also determine how readily the river may flood the land when the water runs high. Lakes and most wetlands will form in areas of flat topography where water can pool. Changes in topography, such as a break in slope along a hillside, may determine where a wetland will form. Lastly, topography is critically important to the connectivity of freshwater features. Changes in topography, such as the creation of levees along a river’s course, will dramatically alter the river’s connection with its floodplain, specifically the transfer of water, nutrients, sediments, and species between the river and its floodplain.

WATERSHED VEGETATION

Except in the most harsh mountain and desert climates, watersheds are covered with vegetation that evolved over time through a unique combination of geologic, and climatic factors. The vegetation across a watershed shapes freshwater ecosystems in four ways as described below and illustrated in part by Figure 2-6.

First, the vegetation of a watershed exerts a strong influence on water chemistry. Plant matter becomes incorporated into the soil as it decays. As it decays, it releases a wide range of organic compounds that percolate further into the soil and are also washed out of the soil by precipitation and flooding. These dissolved organic materials vary in their chemistry, depending on the kinds of plants and soils present across the landscape; all are rich in carbon, but some may be rich in nitrogen, some highly acidic, and some even toxic to aquatic life. Once they wash into a freshwater ecosystem, they help shape the chemistry of the water and also provide an important source of nutrients for microorganisms, helping drive the food chain of the ecosystem. Vegetation may also provide shade, modifying water temperature within an ecosystem.

Second, the vegetation of a watershed, particularly the vegetation living near freshwater bodies, provides a steady source of plant litter—leaves, twigs, fruits, and even whole bushes and trees—that may fall or be washed into these freshwater bodies. This detritus plays two crucial roles in freshwater ecosystems: as food for freshwater life and as solid material that adds complexity to the physical habitat. Seeds and fruits that fall into the water rapidly become food for fishes and waterfowl; and any plant material that falls into the water also rapidly becomes food for microbes, insect larvae, snails, and other aquatic invertebrates. In some freshwater ecosystems, these inputs of organic matter may be the most important source of carbon in the entire ecosystem, and carbon is a crucial building block for the entire food chain. As part of the physical habitat, plant detritus can serve as raw material for nests and the protective casings of some insect larvae. Larger material may become wedged along the shoreline, bank, or bed of a freshwater body, creating shade and shelter. At the largest scale, the trees and tree limbs or large woody debris that falls into a stream can create large eddies and pools, shelter and basking surfaces for many fish and reptiles, perching surfaces for waterfowl, and even temporary dams. Beavers in the Northern Hemisphere deliberately carry woody debris into streams in order to build their dams and lodges, exerting a strong influence on the shape of the freshwater ecosystem. The removal of beavers from much of their range by trapping has resulted in dramatic changes in watershed dynamics.

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FIGURE 2-6. Diagram Showing How Watershed Vegetation Shapes Freshwater Ecosystems

Source: Peter B. Bayley. 1995. Understanding Large River-Floodplain Ecosystems. BioScience, vol. 45, March 1995, p. 154. Copyright, American Institute of Biological Sciences.

Third, the vegetation of a watershed strongly influences the way that water behaves on the land surface after it falls as precipitation or is released by the melting of snow or ice. The more dense the vegetation and plant litter covering a watershed, the more it slows down and holds back runoff and delays the time it takes for a drop of water to wash into a freshwater body after hitting the ground. This effect of vegetation on storm runoff detention plays a crucial role in shaping the hydrology and chemistry of freshwater ecosystems. By slowing runoff, it spreads out the time it takes for the precipitation from a storm event to reach its destination in a stream, river, lake, or wetland. This moderates the potential for flooding and also gives the water more time to soak into the soil itself, as well as to react chemically with the soil. The vegetation of a watershed also plays an important role in shaping soil permeability. Soils with lots of root activity, and soils that attract burrowing animals because of their covering vegetation, are naturally more permeable than other soils.

Finally, vegetation plays a key role in each watershed’s micro-climate. Plants pull water from the soil through their roots and transpire it from their leaves. Plants in fog-prone regions have evolved ways to collect the cloud moisture directly onto their leaves and needles, directing the moisture to then fall to the ground to help moisten the soil and be available to plant roots. The more heavily vegetated a watershed, therefore, the more water gets taken out of the soil, and sometimes out of the air itself, and released back to the atmosphere without ever reaching a freshwater body. The amount of and type of vegetation in a watershed, consequently, has a large effect on the humidity of the air over a watershed, which itself exerts a strong influence on the kinds of plant and animal life that can live in that environment. In extreme cases, such as in the cloud forests of Central and South America and in the forests of giant redwoods of the North American Pacific Coast, watershed vegetation creates the very micro-climate on which the forest ecosystem depends, which in turn shapes the hydrology of the watersheds’ rivers, lakes, and wetlands as well.

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Cloud forest, Tariquia Flora and Fauna Reserve, Bolivia. Photograph by Ivan Arnold.

The preceding text of this chapter discussed the formation and function of watersheds and the importance of climate and the associated hydrologic cycle, geology, and watershed vegetation in shaping the loca tion and characteristics of freshwater ecosystems on the landscape. The following section provides an overview of the major types of freshwater ecosystems and the key ecological factors that determine their ability to remain healthy and able to support resident biodiversity.

A freshwater ecosystem consists of a group of strongly interacting freshwater and riparian/near-shore species and communities linked by shared physical habitat, environmental regimes, energy exchanges, and nutrient dynamics. Freshwater ecosystems vary in their spatial extent, can have indistinct boundaries, and can be hierarchically nested within one another depending on spatial scale. Conserving freshwater biodiversity is best accomplished not by conserving habitat for a few species and communities alone, but by conserving entire freshwater ecosystems.

FRESHWATER ECOSYSTEMS AND THEIR VARIABILITY

Types of Freshwater Ecosystems

The features that perhaps most distinguish freshwater ecosystems from terrestrial ecosystems are their variability in form and their dynamic nature. Freshwater ecosystems are extremely dynamic in that they often change where they exist (e.g., a migrating river channel) and when they exist (e.g., seasonal ponds) in a time frame that we can experience. Freshwater ecosystems are nearly always found connected to and dependant upon one another, and as such they form drainage networks that constitute even larger ecological systems. Freshwater ecosystems exist in many different forms, depending upon their underlying climate, geology, vegetation, and other features of the watersheds in which they occur. Many classifications exist to describe in great detail freshwater and freshwater-related ecosystems (i.e., Cowardin et al. 1979, Maxwell et al. 1995, and Higgins et al. 2003). In very general terms, however, freshwater ecosystems fall into three major groups:

Standing-water ecosystems (e.g., lakes and ponds);

Flowing-water ecosystems (e.g., rivers and streams); and

Freshwater-dependent ecosystems that interface with terrestrial ecosystems (e.g., wetlands and riparian areas).

Cave systems often have both standing and flowing water and unique aquatic biota that have evolved in them. The resulting underground freshwater ecosystems present a unique set of challenges to conservation, which this guide cannot address due to its highly specialized demands. Estuaries, formed where rivers and streams discharge to the sea, constitute another type of freshwater-influenced ecosystem with complex conservation challenges beyond the scope of this guide.

LAKES AND PONDS

Lakes and ponds are inland depressions containing standing water derived from glaciers, river drainage, surface water runoff or groundwater seepage. They vary in size from less than a hectare to large bodies of open water covering thousands of square kilometers such as the Great Lakes bordering the U.S. and Canada. Lakes in temperate climates display seasonal characteristics. The difference between a lake and a pond is in the depth and not the size. In a pond, sunlight reaches all the way to the bottom, whereas in a lake the light does not reach the bottom. It is possible, therefore, for a pond to be larger in surface area than a lake. The lack of light at the bottom of lakes means that plants do not grow there. This in turn affects the distribution of plant-eating organisms and also carnivores. Given the greater complexity of lakes, the remainder of this section focuses on lakes.

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Link Lake, Deschutes National Forest, Oregon, Photograph by Charlie Ott.

Three distinct layers of water temperature develop in lakes during summer months. The top layer (the epilimnion) is the warmest, followed by the middle layer (the metalimnion), with the bottom layer (hypolimnion) being the coldest. Since sunlight is the strongest in the top layer, the majority of a lake’s biomass (phytoplankton) occurs within the top layer of the lake during the summer. During spring and fall, cooler air temperatures cause the top layer of water to cool, resulting in greater mixing of the lake waters. As a result, the water temperature is more uniform throughout lakes due to increased water flow (mixing), allowing for fish and other wildlife to occur in all areas. In areas with cold winters, lakes can form distinct temperature layers, providing protection for organisms needing to stay within warmer waters: water that cools to 4°C or below is lighter than warmer water and therefore floats to the surface carrying ice or forms a cold top layer and organisms can stay well below this surface water where the environment is warmer. In tropical climates, of course, the temperature patterns in lakes tend to behave as if it were summer year-round.

Although called standing water bodies, the water in lakes and ponds does not stand still. The water arrives as precipitation, direct runoff, and inflow from streams and areas of groundwater discharge and leaves as evaporation, outflow to streams, and groundwater recharge. The flushing rate of water through a lake is greatest in lakes connected to river ecosystems where river inflows and outflows create a constant flow pattern. Wind and changes in water temperatures cause further mixing. In larger lakes, all of these same factors apply, but temperature differences with depth can also create distinct temperature zones and deeply circulating currents, prevailing winds can create steady currents, particularly along the shoreline, and the pull of the moon’s gravity can actually create modest lake tides. The patterns of water flow in ponds and lakes play a crucial role in shaping the availability and spatial distribution of habitat over time.

RIVERS AND STREAMS

True flowing-water ecosystems—rivers and streams— are distinguished by water moving along distinct channels within a watershed, creating rich but often challenging habitat for a unique spectrum of organisms. Factors such as the amount and depth of water flowing, the source of the water, how the flow naturally changes over time, and the location of the ecosystem within a watershed all combine to further determine the type of the river ecosystem and the organisms living in it. Nearly all stream and river ecosystems are directly connected to other streams and rivers (and ponds, lakes, and wetlands as well), and eventually to estuarine and marine ecosystems. This connectivity serves a critical role in shaping the movement of energy, material, and biota throughout the entire system. Riparian ecosystems—the ecosystems of streamside lands, especially those subject to flooding—serve as transition zones between streams, rivers and the terrestrial world, providing key habitat (e.g., floodplains as fish nurseries), material inputs

(e.g., woody debris for structure and nutrients), and other ecological services (e.g., temperature control from shading) to these ecosystems. Rivers and streams are so closely linked to their adjacent floodplain riparian corridors, it often makes sense to treat the river and floodplain as a single ecosystem. This is particularly true in large river valleys where seasonal flooding covers vast areas of the adjacent lowlands for months at a time, such as along portions of the Amazon River and Rio