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