is more mobile. St. Louis et al. (1994) report that despite relatively low total yields of mercury in waters flowing from catchments dominated by wetlands, yields of methylmercury were relatively high compared to yields from upland catchments. A purely upland catchment appeared to retain or demethylate methylmercury, whereas catchments containing wetlands were net producers of methylmercury. Rudd (1995) has reviewed the sources of methylmercury to freshwater ecosystems.
In recent times wetlands—whether pristine or not—have been seen to have a broad array of values to human society (reviewed by Greeson et al., 1979, and Sharitz and Gibbons, 1989; see also Richardson, 1994). Among the physical values are such properties as shoreline stabilization, flood-peak reduction, and ground water recharge. On the other hand, peatlands can also serve as physical barriers to human movement, as they do in northern Canada and Russia and as they did for Napoleon in his Russian campaign of 1812 (Chandler, 1966). Environmental problems of transport, pipeline building, and waste disposal have been described by Radforth and Brawner (1977).
Wetlands function chemically to improve water quality as filters, transformers, and sinks for materials delivered to them by human activities. For instance, they can filter 60-90 percent of suspended solids from added wastewater and as much as 80 percent of sediment in runoff from agricultural fields (Richardson, 1989). Wetlands to which nitrogen fertilizer is added can transform it by microbial denitrification to gaseous molecular nitrogen and nitrous oxide at rates substantially higher than those in similar unfertilized wetlands (Martikainen et al., 1992). As mentioned earlier, wetlands are sinks for carbon, sulfur, and nitrogen, and they retain in their peat deposits or volatilize a good deal of these materials when added from human by-products such as acid deposition (Bayley et al., 1987), agricultural runoff, and sewage wastewater (Richardson, 1989).
Wetlands also provide a variety of biological benefits to human society. In the natural state, these include substantial forest resources and minor agricultural resources such as marsh hay, wild rice, and cranberries. After drainage, both forestry and agriculture are possible on a broader scale. Waterfowl and furbearers such as muskrat, beaver, and mink inhabit many natural wetlands, and many riparian wetlands serve as protective nurseries and food sources for young fish prior to moving out into open water.
Aesthetic values of wetlands include unique plants, animals, and scenery. The patterned nature of many large northern peatlands as seen from the air also provides an aesthetically as well as scientifically valuable experience (see, for example, the illustrations in Wright et al., 1992). In