Issues in Ecology Number 10 Winter 2003 Ontario demonstrate the consequences of excess inputs of the potential to push functionally intact freshwater ecosystems nutrients and toxic contaminants.as well as non-native species beyond the bounds of resilience or sustainability.threatening introductions and over-fishing (BOX 3).Onondaga Lake,New their ability to provide important goods and services on both York,which was polluted with salt brine effluent from a soda short and long time scales.Further,introduction of non-native ash industry,likewise responded with marked changes in the species that can thrive under the existing or altered range of plankton and fish communities,including invasions by non-native environmental variation can contribute to the extinction of native fish species.Among U.S.lakes identified by the EPA as impaired species,severely modify food webs,and alter ecological processe in 1996,excess nutrients contributed to more than half of the such as nutrient cycling.Exotic species are often successful in water quality problems. More than half of agricultural and modified systems,where they can be difficult to eradicate urban streams sampled by the U. Geological Survey were trations that exceed quidelines TOOLS AVAILABLE FOR RESTORATION Plant and Animal Assemblages ecosyst h a more na y of species that lives in any give and susta prevent in The sullablly ofae One tech that riv ecosystem for any particular species is dictated by the environmental conditions management tar ts for streamflow that is water flow sediment variability over time temperature.light,and nutrient patterns sed for several rivers.including th -and the presence of,and interactions among.other species in the system.Thus River in North Carolina,and the vast both the habitat and the biotic Colorado River system in the West Thes community provide controls and variable streamflow techniques seek a feedbacks that maintain a diverse range balance between water delivery needs for of species.The high degree of natural power generation or irrigation.and ir variation in environmental conditions in stream ecoloaical needs for flow variabilit -Freshwater ecosystems fresh waters across the United States Figure 5- that displays a certain timing.frequency provide habitats to plants and ani promotes high biological diversity.In fact. Human activities duration,and rate of change characteristi North American fresh water use plac ater habitats are virtually unrivaled in diversity of fish, many of these reshwater species of the natural system(Figure 6).Restoring crayfish,amphibian,and aquatic xtinction. Photo courte this flow variability helps to reconnec reptile olog Survey. South dynamic riparian and groundwater systems enabling water to mov he Program(NAWQA). more naturally through all dim are processe decomp aegoasgnaa of water Other restor tion,both fron often perfo point sou age pipe off fron with capacity to adap sources is a kind of i Water Act nd Safe Dr tinue durin en ntal stross and toxin to this is connectivity am water bodies.which allows ply the maiority of pollutants to freshwater ecos species to move to more suitable habitat as environmental some situatio ns best manao conditions change Human activities that alter freshwater environmental practices includeer sion contro and moderate application conditions can greatly change both the identity of the species of fertilizers.pesticides and herbicides. Best ma in the community and the functioning of the ecosystem(Figure practices require willing farmers,however.and willingness is 5).Excessive stressor simplification of natural complexity has often a response either to economic incentives or to stringent 7 Issues in Ecology Number 10 Winter 2003 Ontario demonstrate the consequences of excess inputs of nutrients and toxic contaminants, as well as non-native species introductions and over-fishing (BOX 3). Onondaga Lake, New York, which was polluted with salt brine effluent from a soda ash industry, likewise responded with marked changes in the plankton and fish communities, including invasions by non-native fish species. Among U.S. lakes identified by the EPA as impaired in 1996, excess nutrients contributed to more than half of the water quality problems. More than half of agricultural and urban streams sampled by the U. S. Geological Survey were found to have pesticide concentrations that exceed guidelines for the protection of aquatic life. Plant and Animal Assemblages The community of species that lives in any given aquatic ecosystem reflects both the pool of species available in the region and the abilities of individual species to colonize and survive in that water body. The suitability of a freshwater ecosystem for any particular species is dictated by the environmental conditions – that is, water flow, sediment, temperature, light, and nutrient patterns — and the presence of, and interactions among, other species in the system. Thus, both the habitat and the biotic community provide controls and feedbacks that maintain a diverse range of species. The high degree of natural variation in environmental conditions in fresh waters across the United States promotes high biological diversity. In fact, North American freshwater habitats are virtually unrivaled in diversity of fish, mussel, crayfish, amphibian, and aquatic reptile species compared with anywhere else in the world. The biota, in turn, are involved in shaping the critical ecological processes of primary production, decomposition, and nutrient cycling. Within a body of water, species often perform overlapping, apparently redundant roles in these processes, a factor that helps provide local ecosystems with a greater capacity to adapt to future environmental variation. High apparent redundancy (that is, species richness or biodiversity) affords a kind of insurance that ecological functions will continue during environmental stress. Critical to this is connectivity among water bodies, which allows species to move to more suitable habitat as environmental conditions change. Human activities that alter freshwater environmental conditions can greatly change both the identity of the species in the community and the functioning of the ecosystem (Figure 5). Excessive stress or simplification of natural complexity has the potential to push functionally intact freshwater ecosystems beyond the bounds of resilience or sustainability, threatening their ability to provide important goods and services on both short and long time scales. Further, introduction of non-native species that can thrive under the existing or altered range of environmental variation can contribute to the extinction of native species, severely modify food webs, and alter ecological processes such as nutrient cycling. Exotic species are often successful in modified systems, where they can be difficult to eradicate. TOOLS AVAILABLE FOR RESTORATION Despite widespread degradation of freshwater ecosystems, management techniques are available that can restore these systems to a more natural and sustainable state and prevent continued loss of biodiversity, ecosystem functioning, and ecological integrity. One technique, for example, involves restoring some of the natural variations in stream flow, based on the understanding that river systems are naturally dynamic. New statistical approaches to setting management targets for streamflow variability over time have been applied to or proposed for several rivers, including the Flathead River in Montana, the Roanoke River in North Carolina, and the vast Colorado River system in the West. These variable streamflow techniques seek a balance between water delivery needs for power generation or irrigation, and in￾stream ecological needs for flow variability that displays a certain timing, frequency, duration, and rate of change characteristic of the natural system (Figure 6). Restoring this flow variability helps to reconnect dynamic riparian and groundwater systems with surface flows, enabling water to move more naturally through all the spatial dimensions that are essential to fully functional ecosystems. Other restoration efforts target pollution, both from point sources such as effluent from industrial or sewage pipes and nonpoint sources such as fertilizer runoff from urban lawns and rural croplands. Point sources of water pollution are readily identified, and many have been controlled, thanks in large part to the federal Clean Water Act and Safe Drinking Water Act. Nonpoint sources of nutrients and toxins now supply the majority of pollutants to freshwater ecosystems. In some situations, best management practices have succeeded in reducing runoff of agricultural pollutants. These practices include erosion control and moderate applications of fertilizers, pesticides and herbicides. Best management practices require willing farmers, however, and willingness is often a response either to economic incentives or to stringent Figure 5—Freshwater ecosystems provide habitats to plants and animals. Human activities and water use place many of these freshwater species at risk of extinction. Photo courtesy the U.S. Geological Survey, South Platte National Water Quality Assessment Program (NAWQA)