| Abiotic and biotic processes within the sediment chemical environment of seven wetlands were examined to understand inter-site differences in phosphorus biogeochemistry and to evaluate the susceptibility of natural processes to changes in surface water quality. The trend of increasing concentration of the pore water Soluble Reactive Phosphorus (SRP) seaward on the urbanized Cooper River Estuary, is partly due to decline in the phosphorus sorption capacity of the respective marsh sediments. The phosphorus binding capacity is sufficient to explain phosphorus limitation in freshwater wetlands and SRP surplus in salt marshes. Phosphatase activity and biotic production estimates from a pristine oligotrophic estuary reveal that, in such ecosystems, there is a differential nutrient limitation of important trophic compartments. Results show that though the plant productivity in such marshes is limited by availability of nitrogen, the microbial compartment is limited by the availability of phosphorus. Further, fertilizing this marsh with phosphorus has altered the rates of nitrogen fixation and potential denitrification suggesting a link between nitrogen and phosphorus biogeochemical cycles. In addition to these natural processes, phosphorus dynamics of an ecosystem is also influenced by human impacts. For instance, 31P NMR analysis of marsh sediments reveals heretofore unreported occurrence of pyrophosphate (PPi) in estuaries. PPi has wide industrial applications and correspondingly, its occurrence is related to the degree of anthropogenic impact around a site. For example, the highest concentrations of PPi in sediment, pore water and surface water was detected from a salt marsh that is heavily impacted by the city of Charleston. In contrast, no PPi was detected from pristine estuaries. The utilization of PPi, the smallest chemical form of polyphosphate, was sensitive to the carbon and nitrogen status of the sediment microbes. Together, these data suggest that intrinsic sediment characteristics and biotic activity are important determinants of phosphorus availability in a wetland. Results further suggest that the full extent of bioavailable phosphorus loading to aquatic ecosystems is unknown. Given that important trophic compartments in coastal ecosystems exhibit phosphorus limitation it will be necessary to examine other 'invisible' forms of bioavailable phosphorus to fully understand the impacts of nutrient loadings in estuaries. |
