Effects of Inter-tributary Dissolved Organic Carbon Variability on Heterotrophic Microbial Communities in Upper Winyah Bay, SC

Winyah Bay, SC, is a riverine-dominated estuary whose four main blackwater tributaries enter the estuary within eight kilometers of one another. These rivers\u27 differing watershed land use patterns allow for comparisons of bacterial activity and carbon properties under different allochthonous loading regimes. This study examined dissolved organic carbon (DOC), characterized by concentration, lability as measured by 21-day consumption bioassays, and UV-Visible absorption spectra; bacterial community composition based on terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rDNA; community metabolic diversity as measured by Biolog GN2 plates; and associated nutrient and in situ data. Strong seasonal shifts were observed in DOC concentration, measures of DOC quality, and corollary factors, while DOC lability appeared to be impacted by discrete events. In situ DOC concentrations were measured between 475 and 2000 µmol/1 in the rivers and 550 to 1100 µmol/1 in the Bay. Between 1.9 and 11.9% of total DOC was consumed in the bioassays, and consumption was independent of in situ DOC concentrations. While lability differed significantly by date, it was not significant between sites. Lability in the system as a whole was best explained by in situ pH, chlorophyll a, and carbon-specific a355, while explanatory factors of lability differed across individual sites. Biolog plates indicated significant differences in community metabolic activity between sites and dates, although these were to strongly correlated with environmental or carbon variables. Gini coefficients, a measure of the metabolic diversity of a community, showed that sites with greater carbon lability also had more metabolically diverse communities. T-RFLP analysis showed significant differences in phylogenetic community structure between sites and dates, with several carbon and environmental factors relating to these differences. T-RFLP communities were positively correlated with environmental variables excluding carbon but not including carbon, possibly due to a reduced data set. Metabolic community structure as measured by Biolog plates, however, was never significantly correlated with environmental variables. T-RFLP and Biolog measures of community structure were positively correlated both on their own and after correcting for environmental variables. This study did not support the overall hypothesis that land use on the watershed scale would drive carbon lability. However, it suggests that carbon lability may be achieve relatively equivalent values in neighboring systems, despite the probability that lability in each of those systems is being driven by a different suite of mechanisms. Additionally, both phylogenetic structure and community metabolic diversity were significantly related to carbon lability. Finally, the significant correlation between T-RFLP and Biolog measures of community structure further supports the paradigm that phylogeny and catabolic genes are, at the least, loosely related