problems—including sterility, birth defects, unsuccessful hatching, and deformities in offspring—in fish-eating birds (Gilbertson et al., 1991). In northern Canada, indigenous peoples who eat large quantities of aquatic organisms often have unacceptably high concentrations of mercury in their blood and hair (Lockhart, 1995).

The likelihood of synergistic interactions among diverse pollutants from atmospheric sources needs far more investigation. For example, a number of organochlorine compounds are now known to have similar effects on enzyme and hormone systems, indicating that they may have additive or synergistic effects (Colborn and Clement, 1992). Another example of such synergism is mercury and acid rain. Bioaccumulation of mercury along food chains depends in the first instance on its transformation by microbes from the inorganic form to methylmercury, which has greatly increased mobility and biological uptake. Several studies have shown that the rate of methylation increases upon lake acidification (Xun et al., 1987; Ramlal et al., 1993); this may provide an explanation for the typically higher concentrations of mercury observed in fish from lakes with low pH (Wiener et al., 1990).

Limnologists, in association with a variety of other scientists (fisheries biologists, toxicologists, aquatic chemists, hydrologists, atmospheric scientists), have been centrally involved in the study and solution of problems caused by acid rain and airborne toxic compounds. Table 3-1 chronicles some of the key scientific discoveries concerning the effects of acid rain and toxic air pollutants on aquatic ecosystems.

BIOLOGICAL CHANGES IN AQUATIC ECOSYSTEMS

All of the physical and chemical changes described above can markedly affect the populations that inhabit inland waters. Physical changes (such as dam construction and global warming) and chemical inputs (from acid rain, runoff, direct waste discharges, airborne toxic compounds, etc.) can change the structure of the aquatic food web and create conditions in which native species cannot survive. In addition to causing biological changes via physical and chemical changes, humans have affected aquatic biota in more direct ways: by introducing exotic species, altering aquatic communities to support game fish, and causing the extinction of native species.

Exotic Species Introduction

Sometimes intentionally and sometimes inadvertently, humans have substantially expanded the geographic ranges of many species. For example, about 20 percent of the 5,523 vascular plant species of eastern North America have been introduced (Fernald, 1950). In many cases, species



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