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Summary The acid test of our understanding is not whether we can take ecosystems to bits on pieces of paper, however scientifically, but whether we can put them together in practice and make them work. A. D. Bradshaw, 1983 INTRODUCTION Aquatic ecosystems perform numerous valuable environmental func- tions. They recycle nutrients, purify water, attenuate floods, aug- ment and maintain streamflow, recharge ground water, and provide habitat for wildlife and recreation for people. Rapid population in- creases in many parts of the United States-accompanied by intensi- fied industrial, commercial, and residential development-have led to the pollution of surface waters by fertilizers, insecticides, motor oil, toxic landfill leachates, and feedlot waste. At the same time that water pollution and releases of nutrient-laden municipal sewage ef- fluents have increased, water consumption has also increased, thus reducing the flows available for the dilution of wastes. Increased sediment delivery resulting from urban construction, agriculture, and forestry also has resulted in greater turbidity and sedimentation in downstream channels, lakes, and reservoirs, with attendant losses of water storage and conveyance capacity, recreational and aesthetic values, and quantity and quality of habitat for fish and wildlife. Increased demands for drainage of wetlands have been ac-