Sustaining Healthy Freshwater Ecosystems
Sustaining Healthy Freshwater Ecosystems Issues in Ecology Published by the Ecological Society of America Number 10, Winter 2003
Issues in Ecology Number 10 Winter 2003 Sustaining Healthy Freshwater Ecosystems SUMMARY Fresh water is vital to human life and economic well-being,and societies extract vast quantities of water from rivers,lakes,wetlands,and underground aquifers to supply the requirements of cities,farms,and industries.Our need for fresh water has long caused us to overlook equally vital benefits of water that remains in stream to sustain healthy aquatic ecosyste There isgrowingre gnition that functionally intact and biologicallyc mplexfres provide many econor mically servicestos es include contro transportation,recreation,purification of human and industrial wastes,habitat for plants and animals,and production of fish and other foods and marketable goods.Over the long term,intact ecosystems are more likely to retain the adaptive capacity to sustain production of these goods and services in the face of future environmental disruptions such as climate change.These ecosystem benefits are costly and often impossible to replace when aquatic systems are degraded.For this reason,deliberations about water allocation should always include provisions for maintaining the integrity of freshwater iificeidee indicats thatqi cystem can be protected or restored byrthe oll Rivers,lakes,wetlands,and their connecting ground waters are literally the "sinks"into which landscapes drain.Far from being isolated bodies or conduits,freshwater ecosystems are tightly linked to the watersheds or catchments of which each is a part,and they are greatly influenced by human uses or modifications of land as well as water.the stream network itself is important to the continuum of river processes Dynamic patterns of flow that are maintained within the natural range of variation will promote the integrity nd sust nab y of fr aquatic system Aquatic ecosystem sadditionally require that sedim nts and orelines,heat and light properties,chemical an nutrient inputs,and plant and animal populations fluctuate within natural ranges,neither experiencing excessive swings beyond their natural ranges nor being held at constant levels. Failure to provide for these natural reauirements results in loss of species and ecosystem services in wetlands rivers.and lakes.Scientifically definina requirements for protectina or restoring aquatic ecosystems.however.is only a first step new policy and man proaches will equired.Current piecemeal and consumption-oriented o water policy ca olve th e problems r inc easingly degraded freshwat ecosystems begin to ress how wat is viewed and anage in the United States 1e0 1)Framing national,regional,and local water management policies to explicitly incorporate freshwater ecosystem 2)Defining water resources to include watersheds.so that fresh waters are viewed within a landscape or ecosystem context instead of by political iurisdiction or in geographic isolation. 3)Increasing communication and education across disciplines,especially among engineers,hydrologists. economists,and ecologists,to facilitate an int grated view of freshwater orts using well-grounded ecolo 月gn Maintaining and protecting remaining freshwater ecosystems that have high integrity. Cover-(1)Rio Grande at Bandelier National Monument,New Mexico.Photo courtesy Jim Thibault,University of New Mexico Biology Department:(2)Rio Grande near Bernalillo,New Mexico.Photo courtesy Anders Molles,son of Manuel C.Molles,Jr. University of New Mexico Biology Department:(3)Dry Rio Grande at Bosque del Apache National Wildlife Refuge,July 17. 2002.Photo courtesy Jennifer Schuetz,University of New Mexico Biology Department
1 Issues in Ecology Number 10 Winter 2003 Sustaining Healthy Freshwater Ecosystems SUMMARY Fresh water is vital to human life and economic well-being, and societies extract vast quantities of water from rivers, lakes, wetlands, and underground aquifers to supply the requirements of cities, farms, and industries. Our need for fresh water has long caused us to overlook equally vital benefits of water that remains in stream to sustain healthy aquatic ecosystems. There is growing recognition, however, that functionally intact and biologically complex freshwater ecosystems provide many economically valuable commodities and services to society. These services include flood control, transportation, recreation, purification of human and industrial wastes, habitat for plants and animals, and production of fish and other foods and marketable goods. Over the long term, intact ecosystems are more likely to retain the adaptive capacity to sustain production of these goods and services in the face of future environmental disruptions such as climate change. These ecosystem benefits are costly and often impossible to replace when aquatic systems are degraded. For this reason, deliberations about water allocation should always include provisions for maintaining the integrity of freshwater ecosystems. Scientific evidence indicates that aquatic ecosystems can be protected or restored by recognizing the following: • Rivers, lakes, wetlands, and their connecting ground waters are literally the “sinks” into which landscapes drain. Far from being isolated bodies or conduits, freshwater ecosystems are tightly linked to the watersheds or catchments of which each is a part, and they are greatly influenced by human uses or modifications of land as well as water. The stream network itself is important to the continuum of river processes. • Dynamic patterns of flow that are maintained within the natural range of variation will promote the integrity and sustainability of freshwater aquatic systems. • Aquatic ecosystems additionally require that sediments and shorelines, heat and light properties, chemical and nutrient inputs, and plant and animal populations fluctuate within natural ranges, neither experiencing excessive swings beyond their natural ranges nor being held at constant levels. Failure to provide for these natural requirements results in loss of species and ecosystem services in wetlands, rivers, and lakes. Scientifically defining requirements for protecting or restoring aquatic ecosystems, however, is only a first step. New policy and management approaches will also be required. Current piecemeal and consumption-oriented approaches to water policy cannot solve the problems confronting our increasingly degraded freshwater ecosystems. To begin to redress how water is viewed and managed in the United States, we recommend: 1) Framing national, regional, and local water management policies to explicitly incorporate freshwater ecosystem needs. 2) Defining water resources to include watersheds, so that fresh waters are viewed within a landscape or ecosystem context instead of by political jurisdiction or in geographic isolation. 3) Increasing communication and education across disciplines, especially among engineers, hydrologists, economists, and ecologists, to facilitate an integrated view of freshwater resources. 4) Increasing restoration efforts using well-grounded ecological principles as guidelines. 5) Maintaining and protecting remaining freshwater ecosystems that have high integrity. 6) And recognizing human society’s dependence on naturally functioning ecosystems. Cover—(1) Rio Grande at Bandelier National Monument, New Mexico. Photo courtesy Jim Thibault, University of New Mexico Biology Department; (2) Rio Grande near Bernalillo, New Mexico. Photo courtesy Anders Molles, son of Manuel C. Molles, Jr., University of New Mexico Biology Department; (3) Dry Rio Grande at Bosque del Apache National Wildlife Refuge, July 17, 2002. Photo courtesy Jennifer Schuetz, University of New Mexico Biology Department
Issues in Ecology Number 10 Winter 2003 Sustaining Healthy Freshwater Ecosystems by Jill S.Baron,N.LeRoy Poff,Paul L.Angermeier,Clifford N.Dahm,Peter H.Gleick,Nelson G.Hairston,Jr..Robert B. Jackson,Carol A.Johnston,Brian D.Richter,Alan D.Steinmar INTRODUCTION that focuses primarily on maintaining the lowest acceptable water quality and minimal flows,and protecting single species Fresh water is vital to human life and economic well- rather than aquatic communities.A fundamental change in being,and societies draw heavily on rivers,lakes,wetlands, water management policies is needed,one that embraces a and underground aquifers to supply water for drinking. much broader view of the dynamic nature of freshwater irrigating crops,and running ndustrlal processes.Ihe benenit resources and the short-and long-term benefits they provide of these extractive uses of fresh water have traditionally Our current educational practices are as inadequat f water that remains as management policies to the cha es Hydrologis engineer managers, e who design and ma 96 th eco wa d rarely d00 ut the o taught (Figur seque sider the c tical role of ter ir tra society or to d the human and industrial wastes habitat for institutions that manage wate plants and animals and production of fish omists dev Der and politician and other foods and marketable aoods seldom project far enough into the futur These human benefits are what ecologists Figure 1-Freshwater ecosystems pro to fully account for the potential call ecolonical services defined as "the vide economically valuable commodities ecological costs of short-term plans.Few conditions and processes through which and services to humans (drinking water. Americans are aware of the infrastructure natural ecosystems,and the species that rrigation.transportation.recreation etc.).as well as habitat for plants and that brings them pure tap water or make them up,sustain and fulfill human r wastes away and fewer stil life."Over the lona term.healthy freshwater animals. carries their understand the ecological tradeoffs tha ecosvstems are likely to retain the adaptive are made to allow these conveniences capacity to sustain production of these ecological services in the Although the requirements of healthy freshwate face of future environmental disruptions such as climate change ecosystems are often at odds with human activity.this conflic Ecological services are costly and often impossible need not be inevitable The challenge is to determine how to replace when aquatic ecosystems are degraded Yet today. society can extract the water resources it needs whil aquatic ecosystems are being severely altered or destroyed protec ting the important natural complexity and adaptiv at a greaterrate than at any other time in h nan history capacity or tre hwater ecosystems en scientifi and far faster han they are b ing restored.Debates involving nding ma possible to outline here sustainab wate resour e qua y. esnwat osysten timing the ru ate hat ust b onside mong the c tin ecosyste will b erent polic ty. Ame natura pu e informatio ab anageme society d States are able tom po appro thi sed the overall of different rnment entities have y in what wa condition nized that can be discharged into water or how water is used a water movement thro uthe biosphere is highly redistributed.and the goals of one are often at cros altered by human activities purposes with those of others.U.S.laws and water is intensively used by humans concerning water are implem ented in a manaoe ement context poor water quality is pervasive
2 Issues in Ecology Number 10 Winter 2003 Sustaining Healthy Freshwater Ecosystems by Jill S. Baron, N. LeRoy Poff, Paul L. Angermeier, Clifford N. Dahm, Peter H. Gleick, Nelson G. Hairston, Jr., Robert B. Jackson, Carol A. Johnston, Brian D. Richter, Alan D. Steinman INTRODUCTION Fresh water is vital to human life and economic wellbeing, and societies draw heavily on rivers, lakes, wetlands, and underground aquifers to supply water for drinking, irrigating crops, and running industrial processes. The benefits of these extractive uses of fresh water have traditionally overshadowed the equally vital benefits of water that remains in stream to sustain healthy aquatic ecosystems. There is growing recognition that functionally intact and biologically complex freshwater ecosystems provide many economically valuable commodities and services to society (Figure 1). The services supplied by freshwater ecosystems include flood control, transportation, recreation, purification of human and industrial wastes, habitat for plants and animals, and production of fish and other foods and marketable goods. These human benefits are what ecologists call ecological services, defined as “the conditions and processes through which natural ecosystems, and the species that make them up, sustain and fulfill human life.” Over the long term, healthy freshwater ecosystems are likely to retain the adaptive capacity to sustain production of these ecological services in the face of future environmental disruptions such as climate change. Ecological services are costly and often impossible to replace when aquatic ecosystems are degraded. Yet today, aquatic ecosystems are being severely altered or destroyed at a greater rate than at any other time in human history, and far faster than they are being restored. Debates involving sustainable allocation of water resources should recognize that maintenance of freshwater ecosystem integrity is a legitimate goal that must be considered among the competing demands for fresh water. Coherent policies are required that more equitably allocate water resources between natural ecosystem functioning and society’s extractive needs. Current water management policies in the United States are clearly unable to meet this goal. Literally dozens of different government entities have a say in what wastes can be discharged into water or how water is used and redistributed, and the goals of one agency are often at crosspurposes with those of others. U. S. laws and regulations concerning water are implemented in a management context that focuses primarily on maintaining the lowest acceptable water quality and minimal flows, and protecting single species rather than aquatic communities. A fundamental change in water management policies is needed, one that embraces a much broader view of the dynamic nature of freshwater resources and the short- and long-term benefits they provide. Our current educational practices are as inadequate as management policies to the challenge of sustainable water resource management. Hydrologists, engineers, and water managers, the people who design and manage the nation’s water resource systems, are rarely taught about the ecological consequences of management policies. Likewise, ecologists are rarely trained to consider the critical role of water in human society or to understand the institutions that manage water. Economists, developers, and politicians seldom project far enough into the future to fully account for the potential ecological costs of short-term plans. Few Americans are aware of the infrastructure that brings them pure tap water or carries their wastes away, and fewer still understand the ecological tradeoffs that are made to allow these conveniences. Although the requirements of healthy freshwater ecosystems are often at odds with human activity, this conflict need not be inevitable. The challenge is to determine how society can extract the water resources it needs while protecting the important natural complexity and adaptive capacity of freshwater ecosystems. Current scientific understanding makes it possible to outline here in general terms the requirements for adequate quantity, quality, and timing of water flow to sustain the functioning of freshwater ecosystems. A critical next step will be communication of these requirements to a broader community. The American public, when given information about management alternatives, supports ecologically based management approaches, particularly toward fresh water. Several previous studies that have addressed the overall condition of freshwater resources have recognized that • water movement through the biosphere is highly altered by human activities; • water is intensively used by humans; • poor water quality is pervasive; Figure 1—Freshwater ecosystems provide economically valuable commodities and services to humans (drinking water, irrigation, transportation, recreation, etc.), as well as habitat for plants and animals
Issues in Ecology Number 10 Winter 2003 Table 1-Changes in hydrologic flow,water quality,wetland area,and species viability in U.S.rivers,lakes,and wetlands since Euro-American settlement. U.S.Freshwater Resources Pre-settlement Condition Current Conditions Source Undammed rivers (in 48 contiguous states) 5.1 million km 4.7 million km Echeverria et al.1989 Free-flowing rivers that qualify for wild 5.1 million km o 0001 million km USD011982 and scenic status (in 48 contiquous states) Number of dams>2m 0 75.000 CE01995 Volume of water diverted from surface waters 0 10 million mday(1985) Solley et al.1998 Total daily U.S.water use Unknown 1.5 million mday(1995 Solley et al.1998 Sediment inputs to reservoirs not applicable 1.200 million m/year Stallard 1998 River water quality(1.1 million km surveyed) Unimpaired 402.000 km impaired* EPA 1998 Lake water quality*(6.8 million ha surveyed) Unimpaired 2.7million ha impaired" EPA 1998 Wetland acreage(in48 contiguous states) 87 million ha 35 million ha van der Leeden et al.1990 Number of native freshwater fish species 822 species Stein and Flack 1997 Number of native freshwater mussel specie 305 species 157 imperiled or extinct Stein and Flack 1997 Number of native crayfish species 330 species 111 imperiledor extinc Stein and Flack 1997 Number of native amphibian species 242 species 64 imperiled or extinct Stein and Flack 1997 Only 19%(1,16,00 km)of total river km in U.S.were surveyed out of a total of ,792,00 km.Only 0%(6.8 million ha)of tota lake area (16.9 million ha)were surveyed. and freshwater plant and animal species are at maintain natural habitat dynamics that support production greater risk of extinction from human activities and survival of species. Variability in the timing and rate of compared with all other species. water flow strongly influence the sizes of native plant and These and other analyses indicate that freshwater ecosystems animal populations and their age structures,the presence of es,the interact are needed In this paper we describe the requirements for water of ecosystem processes. Periodic and episodic sufficent quaity.amount.timng.and variability patterns alsc quality.physic habita ec al dynamics tha nne in aqu cosy have evol ve ting and ing ter of fre d to the wate REOUIREMENTS FOR FRESHWATER catehmants of which th Water flowing thr FCOSYSTEM INTEGRITY the landscape on its way to the se a moves in three di linking upstream to downstream stream channels to Freshwater ecosystems differ greatly from one floodplains and riparian wetlands and surface waters to another depending on type.location. and around water.materials genera ted across the landscan nevertheless share important features.For one,lake ultimately make their way into rivers lakes and othe wetlands rivers and their connerted around waters share a freshwater ecosystems Thus these systems are areatly common need for water within a certain range of quantity influenced by what happens on the land,including human and quality.In addition,because freshwater ecosystems are activities. dynamic,all require a range of natural variation or We have identified five dynamic environmental factors disturbance to maintain viability or resilience.Water flows that regulate much of the structure and functioning of any that vary both season to season and year to year,for example, aquatic ecosystem,although their relative importance varies are needed to support plant and animal communities and among aquatic ecosystem types(Figure 2).The interaction
3 Issues in Ecology Number 10 Winter 2003 • and freshwater plant and animal species are at greater risk of extinction from human activities compared with all other species. These and other analyses indicate that freshwater ecosystems are under stress and at risk (Table 1). Clearly, new management approaches are needed. In this paper we describe the requirements for water of sufficient quality, amount, timing, and flow variability in freshwater ecosystems to maintain the natural dynamics that produce ecosystem goods and services. We suggest steps to be taken toward restoration and conclude with recommendations for protecting and maintaining freshwater ecosystems. REQUIREMENTS FOR FRESHWATER ECOSYSTEM INTEGRITY Freshwater ecosystems differ greatly from one another depending on type, location, and climate, but they nevertheless share important features. For one, lakes, wetlands, rivers, and their connected ground waters share a common need for water within a certain range of quantity and quality. In addition, because freshwater ecosystems are dynamic, all require a range of natural variation or disturbance to maintain viability or resilience. Water flows that vary both season to season and year to year, for example, are needed to support plant and animal communities and maintain natural habitat dynamics that support production and survival of species. Variability in the timing and rate of water flow strongly influence the sizes of native plant and animal populations and their age structures, the presence of rare or highly specialized species, the interactions of species with each other and with their environments, and many ecosystem processes. Periodic and episodic water flow patterns also influence water quality, physical habitat conditions and connections, and energy sources in aquatic ecosystems. Freshwater ecosystems, therefore, have evolved to the rhythms of natural hydrologic variability. The structure and functioning of freshwater ecosystems are also tightly linked to the watersheds, or catchments, of which they are a part. Water flowing through the landscape on its way to the sea moves in three dimensions, linking upstream to downstream, stream channels to floodplains and riparian wetlands, and surface waters to ground water. Materials generated across the landscape ultimately make their way into rivers, lakes, and other freshwater ecosystems. Thus these systems are greatly influenced by what happens on the land, including human activities. We have identified five dynamic environmental factors that regulate much of the structure and functioning of any aquatic ecosystem, although their relative importance varies among aquatic ecosystem types (Figure 2). The interaction Table 1— Changes in hydrologic flow, water quality, wetland area, and species viability in U.S. rivers, lakes, and wetlands since Euro-American settlement. *Only 19% (1,116,500 km) of total river km in U. S. were surveyed out of a total of 5,792,400 km. Only 40% (6.8 million ha) of total lake area (16.9 million ha) were surveyed. U. S. Freshwater Resources Pre-settlement Condition Current Conditions Source Undammed rivers (in 48 contiguous states) 5.1 million km 4.7 million km Echeverria et al. 1989 Free-flowing rivers that qualify for wild 5.1 million km 0.0001 million km US DOI 1982 and scenic status (in 48 contiguous states) Number of dams >2m 0 75,000 CEQ 1995 Volume of water diverted from surface waters 0 10 million m3 day-1(1985) Solley et al. 1998 Total daily U. S. water use Unknown 1.5 million m3 day-1(1995) Solley et al. 1998 Sediment inputs to reservoirs not applicable 1,200 million m3 /year Stallard 1998 River water quality*(1.1 million km surveyed) Unimpaired 402,000 km impaired* EPA 1998 Lake water quality*(6.8 million ha surveyed) Unimpaired 2.7 million ha impaired* EPA 1998 Wetland acreage (in 48 contiguous states) 87 million ha 35 million ha van der Leeden et al. 1990 Number of native freshwater fish species 822 species 202 imperiled or extinct Stein and Flack 1997 Number of native freshwater mussel species 305 species 157 imperiled or extinct Stein and Flack 1997 Number of native crayfish species 330 species 111 imperiled or extinct Stein and Flack 1997 Number of native amphibian species 242 species 64 imperiled or extinct Stein and Flack 1997
Issues in Ecology Number 10 Winter 2003 of these drivers in space and time defines the dynamic nature Focusina on one factor at a time will not vield a of freshwater ecosy stems true picture of ecosystem functioning.Evaluating freshwater 1.The flow pattern defines the rates and pathways ecosystem integrity requires that all five of these dynamic by which rainfall and snowmelt enter and circulate within environmental factors be integrated and considered jointly. river channels,lakes,wetlands,and connecting ground waters,and also determines how long water is stored in Flow Patterns these ecosystems. 2 Sediment and organic matter inouts provide raw An evaluation of the characteristics required for healthy materials that create physical habitat structure,refugia, functionina can beain with a description of the natural or substrates,and spawning grounds and supply and store historical flow pattermns for streams,rivers,wetlands and lakes nutrients that sustain aquatic plants and animals. Certain aspects of these patterns are critical for regulating Temperature and light characteristics requlate biological productivity (that is,the growth of algae or the metabolic processes.activity levels,and productivity of phytoplankton that form the base of aquatic food webs)and aquatic organisms. biological diversity.particularly for rivers.These aspects include Chemical and nutrient conditions regulate pH. base flow,annual or frequent floods,rare and extreme flood plant and animal productivity,and water quality. events. seasonality of flows,and annual variability (BOX 1) The plant and anima assemblage influences Sucn ractors are also relevant for evaluating the integrity o ecosystem proces community structure and because flow patterns and hydrope (tha ations in water le these factors vary within deine the yea ates,as well as types a aqu such as nd and 2in atter or ences uctivity and is ann ogen highsr lows in flows.te erature.and other surroundin LOW REGIME WATER QUALITY Sediment Biotic The rmal/Light Assemblage Chemical/Nutrier Inputs Flux Functional Aquatic Ecosystems Short-term ng-term Sustainability Goods and Services And Adaptive Capacity Figure 2-Conceptual model of major forces that influence freshwater ecosystems
4 Issues in Ecology Number 10 Winter 2003 of these drivers in space and time defines the dynamic nature of freshwater ecosystems: 1. The flow pattern defines the rates and pathways by which rainfall and snowmelt enter and circulate within river channels, lakes, wetlands, and connecting ground waters, and also determines how long water is stored in these ecosystems. 2. Sediment and organic matter inputs provide raw materials that create physical habitat structure, refugia, substrates, and spawning grounds and supply and store nutrients that sustain aquatic plants and animals. 3. Temperature and light characteristics regulate the metabolic processes, activity levels, and productivity of aquatic organisms. 4. Chemical and nutrient conditions regulate pH, plant and animal productivity, and water quality. 5. The plant and animal assemblage influences ecosystem process rates and community structure. In naturally functioning freshwater ecosystems, all five of these factors vary within defined ranges throughout the year, tracking seasonal changes in climate and day length. Species have evolved and ecosystems have adjusted to accommodate these annual cycles. They have also developed strategies for surviving – and often requiring — periodic hydrologic extremes caused by floods and droughts that exceed the normal annual highs or lows in flows, temperature, and other factors. Focusing on one factor at a time will not yield a true picture of ecosystem functioning. Evaluating freshwater ecosystem integrity requires that all five of these dynamic environmental factors be integrated and considered jointly. Flow Patterns An evaluation of the characteristics required for healthy functioning can begin with a description of the natural or historical flow patterns for streams, rivers, wetlands and lakes. Certain aspects of these patterns are critical for regulating biological productivity (that is, the growth of algae or phytoplankton that form the base of aquatic food webs) and biological diversity, particularly for rivers. These aspects include base flow, annual or frequent floods, rare and extreme flood events, seasonality of flows, and annual variability (BOX 1). Such factors are also relevant for evaluating the integrity of lakes and wetlands because flow patterns and hydroperiod (that is, seasonal fluctuations in water levels) influence water circulation patterns and renewal rates, as well as types and abundances of aquatic vegetation such as reeds, grasses, and flowering plants. Furthermore, the characteristic flow pattern of a lake, wetland, or stream critically influences algal productivity and is an important factor to be considered when determining acceptable levels of nutrient (nitrogen and phosphorus) runoff from the surrounding landscape. Figure 2— Conceptual model of major forces that influence freshwater ecosystems. FLOW REGIME WATER QUALITY Sediment Flux Chemical/Nutrient Flux Thermal/Light Inputs Biotic Assemblage Functional Aquatic Ecosystems Short-term Goods and Services Long-term Sustainability And Adaptive Capacity