It is critical to realize, however, that the physics, chemistry, and biology of a water body are interrelated. Changes in the physical landscape surrounding water bodies can affect the chemical inputs to them, which in turn can affect aquatic biota. Similarly, changes in the chemical composition and biota of an aquatic ecosystem can affect the physical landscape.

Much of the research described in this chapter has been conducted in response to problems caused by human activities. In a general sense, this research could be considered goal oriented (or directed research) rather than basic research conducted for the pursuit of knowledge itself. Nonetheless, much of this work has contributed to the understanding and solution of aquatic ecosystem problems because it advanced understanding of the fundamental behavior of these ecosystems. Similarly, many of the research needs identified in this chapter address basic limnological questions even though the results could be applied toward the solution of practical problems. (For more detailed information about research needs in limnology, see Naiman et al., 1995, and background papers at the end of this report.)

PHYSICAL CHANGES IN WATERSHEDS AND WATER BODIES

In many locations, the most serious causes of water quality decline are not direct inputs of pollutants but indirect effects resulting from changes in the landscape and atmosphere surrounding the water body and alteration of the water's natural flow path. Countless freshwater systems also have been affected by direct physical alterations to the shoreline or shape of the water body. For example, vegetation along lake and stream banks often is cleared to allow recreational or commercial access. Outlets to lakes often are dammed to provide downstream flow controls and allow water-level regulation in the lake. Channels are constructed between lakes and rivers, and littoral areas of lakes are dredged to allow ship and boat traffic. In addition, wetlands often are drained for agriculture and forestry. These physical changes can have subtle or dramatic impacts on the structure and functions of aquatic ecosystems, depending on the severity of the change. In many cases, the impacts are caused by excessive diversion of water from a stream for crop irrigation or other water supply purposes to the extent that so-called in-stream uses of the water (for example, maintenance of fish populations) may be impaired. Limnologists have made and continue to make critical contributions toward understanding how water bodies are disrupted by physical changes to the water bodies themselves or to their watersheds.

Dam and Impoundment Building

More than 80,000 dams exist in the United States (Frederick, 1991), creating impoundments that range in size from small millponds to large



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