chemical data, analyzed in dated peat cores allow reconstruction of historical records of the successional development of the deposit, as well as of past levels of acidity and water table depth, that can serve as baselines against which to assess recent and current impacts of human activity (Gorham and Janssens, 1992b). Pathways of development are driven both by external environmental factors and by the internal dynamics of the system itself.
Wetland productivity ranges from a few hundred grams of dry matter per square meter per year in many peatlands of the northern boreal zone to thousands of grams in coastal salt marshes and inland freshwater marshes (Mitsch and Gosselink, 1993). In northern peatlands, about 8 percent of the 296 grams of carbon fixed annually from the atmosphere is preserved as peat, which forms a carbon pool of 412 × 1015 g (Woodwell et al., 1995). The average annual accumulation rate over the postglacial period is currently thought to be 96 × 1012 g per year, or 230 kg per hectare per year (Gorham, 1995). About 7 percent of carbon in northern peatlands is exported in streams as dissolved organic carbon, with the potential to acidify downstream waters (Gorham et al., 1986; Kerekes et al., 1986), and about 1.4 percent is emitted to the atmosphere as methane (Gorham, 1995). The rest, 84 percent, returns to the atmosphere as carbon dioxide.
Wetlands are important in the cycling of nitrogen and sulfur to the atmosphere because their anoxic soils and peats are habitats well suited to microbes capable of reducing nitrate and sulfate. Denitrification occurs readily in circumneutral waterlogged soils (Kaplan et al., 1979) but less so in acid oligotrophic peatlands (Gorham, 1995). Nitrogen fixation takes place in a variety of ways, more particularly in circumneutral wetlands. Shrubs such as Alnus and Myrica have actinomycete nodules on their roots, and cyanobacteria (blue-green algae) are often important in marshes with standing water (Dickinson, 1984; Mitsch and Gosselink, 1993). Sulfate reduction in waterlogged soils results in the emission of volatile sulfur gases, but relatively few measurements have been made (Castro and Dierberg, 1987; Faulkner and Richardson, 1989). Northern peatlands can be important sinks for nitrogen and sulfur, with a nitrogen stock of about 16 × 1015 g and a sulfur stock of about 1.3 × 1015 g (Gorham, 1991). Their average accumulation rates during the postglacial period are estimated at 10 and 0.83 kg per hectare per year, respectively.
Wetlands are extremely efficient sinks for a diverse array of chlorinated hydrocarbons, including DDT, toxaphene, and PCBs (polychlorinated biphenyls; Rapaport and Eisenreich, 1988). Mercury, on the other hand,