AME 76:1-14 (2015)  -  DOI: https://doi.org/10.3354/ame01764

ABSTRACT: In blackwater systems of the southeastern US, dissolved organic carbon (DOC) contributes a major portion to the total dissolved organic matter pool. The primary DOC source is terrestrial vegetation, with phytoplankton contributing less. Thus, upland development may reduce terrestrial DOC inputs, thereby affecting bacterial abundances. Conversely, development and runoff may increase nitrogen (N) and phosphorus (P) inputs, fueling phytoplankton growth and algal-derived DOC. Yet, the variability of DOC, bacteria, and phytoplankton has not been fully assessed across diverse land uses. We investigated seasonal (July 2012 to May 2013) levels of DOC, bacteria, and phytoplankton biomass (chl a) in response to N and P additions at 4 coastal South Carolina sites: a forested/agricultural creek, an urbanized creek, a forested creek, and a detention pond. DOC concentrations were highest at the least developed site (forested creek), suggesting the influence of surrounding land. DOC was significantly and positively correlated with precipitation but negatively correlated with salinity, suggesting that rainfall affected DOC mobilization. Chl a was highest during summer and positively correlated with temperature, whereas bacterial abundances were generally negatively correlated with salinity. During experiments, chl a was often greater in addition treatments than controls, especially at the urbanized creek and detention pond. In certain N-amended treatments, particularly those containing urea, both DOC and chl a became elevated following incubation. These results indicate that urea stimulated phytoplankton biomass and possibly a greater contribution of phytoplankton-derived DOC to the total DOC pool. Our findings suggest that biogeochemical cycling of DOC may become altered in developing coastal regions.

KEY WORDS: Dissolved organic carbon ^. Bacteria ^. Phytoplankton ^. Estuaries ^. South Carolina