sources such as urban and agricultural runoff from a lake's watershed; and (3) long-range atmospheric transport of contaminants (the most difficult to measure and control). These stresses result in a variety of impacts on lake quality relative to human use and ecological integrity.
The specific impacts of stresses on lake ecosystems depend on the nature of the stress and the characteristics of the lake, but some responses are common to several categories of stress. For example, stress-impacted lakes tend to lose sensitive native species. Their replacement by stress-tolerant native or exotic species often does not fully compensate for the loss and leads to lower biodiversity and simplified food webs. Many types of stress result in loss of habitat; often this is the proximate cause of species losses. Many kinds of stress produce ''nuisance conditions," that is, proliferation of a native or exotic organism or deterioration in a physical-chemical property (such as water clarity) to the extent that beneficial uses of the lake are impaired. Finally, the development of toxic levels of contaminants in biota results not only from direct loading of toxic materials to lakes but also from indirect effects of other stresses (e.g., solubilization of aluminum as pH is decreased by acid deposition).
Of the six categories of stress, problems related to nutrient overenrichment and excessive plant production are probably the most common and have received public and scientific attention for the longest time. Concern about lake eutrophication from municipal wastewater extends back at least to the 1940s and the classic studies of Sawyer (1947) on the relationship between springtime concentrations of inorganic phosphorus and nitrogen and the occurrence of algal blooms in summer. By the 1960s, widespread concern existed about increasing eutrophication of the Great Lakes, and nutrient enrichment problems were recognized in numerous inland lakes. A large-scale research program funded primarily by federal agencies was undertaken on eutrophication in the 1960s and 1970s. This program led to improved understanding of the extent of the problem in U.S. lakes, delineated specific causes of the problem in some lakes, generated quantitative relationships between rates of nutrient loadings (especially of phosphorus) to lakes and water column responses in the lakes, and developed techniques to restore lakes degraded by eutrophication.
Eutrophication results in numerous ecological and water quality changes in lakes. The chain of events leading to use impairment is