| Trends in tidal marsh metabolism, measured as net carbon assimilation and sediment heterotrophy, in South Carolina estuaries were documented. Observations made in marshes of varying salinity and across estuaries that contrasted in disturbance history and river influence suggested that the concentration of chemical constituents in marsh sediments resulted from (1) an interaction between local biogeochemical processes, with nutrient concentrations increasing and N:P ratios decreasing with increasing salinity, and (2) the nature of allocthonous loading, which varied naturally among estuaries as a result of watershed differences. Marsh nutrient availability was significantly higher in a pristine, unobstructed, black-water river estuary compared to a dammed, urbanized estuary, which, in turn, was higher than in an estuary that received little freshwater input.; These contrasts among marshes were coupled with controlled experiments to study the interaction between nutrient availability and salinity on the marsh carbon cycle. The combined effects of biology, nutrient availability, and salinity dictated the relative heterotrophy of sediments. After considering the spatial-dependent effects of these factors on heterotrophy, it was concluded that nutrient loading and rising sea level, could shift the carbon balance of marsh sediments toward greater turnover, thereby decreasing carbon and nutrient burial and/or the exchange rate of organic material between the marsh and the water column. Likewise, natural differences along the salinity gradient resulted in carbon assimilation differences among marshes with the greatest assimilation occurring in brackish marshes, which translated into these marshes accounting for 57 to 71% of total marsh plant production in two estuaries. Plant production, however, was unaffected by the differences in nutrient availability among marshes of similar salinity, suggesting that other factors were more limiting. It was demonstrated that even if nutrients were limiting, a tripling of inorganic nutrient loading would be necessary to elicit a response by the marsh plants.; The balance between assimilatory and degradative metabolic processes determines the role of the marsh in estuaries. A marsh mesocosm study highlighted the importance of a spatial separation in carbon storage, which resulted from differences in decomposition between species, and had important implications for the quantity and quality of carbon and nutrients available for export to the adjacent estuarine water column in natural marshes. |
