BIOGEOCHEMISTRY OF STORMWATER PONDS: IMPACTS ON NUTRIENT AND ORGANIC MATTER TRANSFORMATIONS AND TRANSPORT IN COASTAL SOUTH CAROLINA

Along the rapidly urbanizing coast of South Carolina, detention ponds are the most common means of controlling stormwater runoff from developed landscapes. These stormwater ponds have thus become a relatively recent, but increasingly significant, component of the terrestrial-aquatic interface in this region. This presentation will highlight results from a number of studies with the goal of assessing: 1) how variable biogeochemical conditions within ponds are across South Carolina’s coastal landscape; 2) how pond biogeochemistry responds to nutrient enrichment associated with development; 3) how effective these ponds are at sequestering these nutrient inputs; 4) and finally how the internal dynamics of ponds ultimately affects their collective impact on coastal receiving waters. In a broad survey of residential stormwater ponds, variability in nitrogen (N) was much less than of that of phosphorus (P), which varied over two orders of magnitude and increased with increasing development density. High nutrient concentrations promoted high phytoplankton production within ponds, with consequent high dissolved organic matter export from ponds. Despite high N:P ratios across the majority of ponds, nutrient amendment bioassays indicated phytoplankton production was most often limited by N availability, with the relative magnitude of the production response predicted by ambient N:P ratios. A study of pond nutrient and organic matter removal efficiency clearly demonstrated that ponds were not especially effective at retaining N inputs from developed landscapes, as compared to their retention of P inputs. Internal pond dynamics also had significant consequences for the magnitude and form of pond nutrient exports, with high internal phytoplankton production effectively transforming inorganic N inputs into dissolved organic N prior to export. These organic N exports were relatively ineffective at stimulating phytoplankton production in marine receiving waters, but they did significantly stimulate heterotrophic bacterial production. Stormwater ponds thus significantly reduce the magnitude of nutrient export to coastal waters, but also greatly alter the form and stoichiometry of nutrient export, with important consequences for the biogeochemistry of coastal receiving waters.