Metabolic Responses of Estuarine Microbial Communities to Inputs of Surface Runoff and Groundwater from Contrasting Landscapes

Growing interest in groundwater discharge has led to evidence of significant nutrient inputs to coastal settings from groundwater relative to surface water pathways, but with limited understanding of the extent to which nutrients from each flowpath influence downstream functional responses of microbial communities. Dissolved organic carbon (DOC) and nutrient concentrations of surface water run-off and groundwater in an anthropogenically-developed and a relatively pristine tidal creek basin near Myrtle Beach, South Carolina were determined and changes in productivity and biomass of phytoplankton and bacterioplankton in response to these inputs were observed during two microcosm incubation experiments (August and October, 2011). RMANOVA, Cluster analyses, and correlations were used to relate microbial responses to the initial treatment conditions. Rainwater in the developed basin that would otherwise enter the groundwater is largely rerouted into the surface flowpath by stormwater infrastructure, bypassing ecosystem buffers and filtration mechanisms. Subsequently, surface runoff from the developed basin was enriched in nutrients and DOC and yielded the highest production rates compared to developed groundwater, pristine surface runoff and groundwater, and a deionized water control. Primary and bacterial productivities were correlated in August and correlated best with inorganic nitrogen concentrations, indicating that nitrogen was likely the primary limiting nutrient. Bacterial productivities were more significantly related to initial treatment conditions than phytoplankton. In both experiments, treatments with highest DOC yielded the most bacterial productivity relative to primary productivity. High DOC concentrations may simultaneously absorb light, hindering phytoplankton, and initially stimulate bacteria, before phytoplankton have produced a large pool of labile photosynthetic leachate