on such uses of wetlands have been discussed by Guntenspergen and Stearns (1981).

Example 4. Analysis of carbon isotopes has shown that particulate organic detritus eroded from tundra peatlands is a significant source of nutrition for food chains leading to fish and ducks in nearby ponds and lakes (Schell, 1983). This sort of information provides a better understanding of food chain dynamics in freshwater habitats, a matter of particular concern for indigenous peoples and sport fishermen.

Example 5. Geochemical study of iron and aluminum in acid Nova Scotian lakes (Urban et al., 1990) has shown that both elements, weathered from upland catchments, are combined with dissolved organic matter (DOM), supplied from nearby peatlands, to form metal humates. Precipitation of such humates from solution may regulate the concentrations of both iron and aluminum, and to a lesser extent DOM, in these acid lakes. DOM concentrations in the lake water are also related to the proportion of the catchment that is covered by peat (Gorham et al., 1986; Kortelainen, 1993a). The importance of such studies is twofold. Dissolved organic matter in these waters consists largely of colored organic (humic) acids that can, like acid rain, acidify fresh waters. Unlike acid rain, however, which releases aluminum from soils and sediments in forms toxic to fish, humic acids form nontoxic complexes with aluminum.

Example 6. Studies of diverse catchments in northwestern Ontario (St. Louis et al., 1994) have revealed that wetlands are important sources of methylmercury to downstream aquatic ecosystems. In its methylated form, mercury can bioaccumulate to toxic levels in aquatic food chains, leading ultimately to humans through fish (Minnesota Fish Consumption Advisory, 1994).

Example 7. Studies of northern pike, an important North American sport fish, show that it depends on wetlands flooded in spring for spawning and nursery areas. This is undoubtedly true for other organisms.

Example 8. The influence of beaver on small streams, ponds, and lakes is profound (Johnston and Naiman, 1990a,b) and can be understood only at the landscape level because these furbearing mammals range over all three types of water body, as well as adjacent uplands. All of these habitats are affected substantially by beaver use, which has an influence that may last as a legacy on the landscape for decades or more after they have left the area.

Example 9. Wetlands can have an appreciable influence on the water budgets of streams and rivers. An extreme example is the White Nile, which flows through a vast wetland, the Sudd. As much as half of the water flowing into the Sudd is lost by evapotranspiration from the wetland (Melack, 1992), which must greatly affect all aspects of the river downstream.