A 4 yr, spatially extensive study of the Neuse River and Pamlico Sound estuarine system, North Carolina, was used to evaluate the temporal patterns and bottom-up controls of the resident bacterioplankton community. This meso-to eutrophic estuarine system had high concentrations of dissolved and particulate organic matter and supported an active and abundant phytoplankton community. Temporal analysis of bacterioplankton productivity (BP) revealed a strong seasonal pattern, similar to water temperature. Comparisons of BP and several environmental variables grouped by season and year showed little correspondence between the bacteria and either river discharge or organic matter at either scale. Temperature explained about half of the variation in BP data, but without any obvious threshold in the semi-log temperature-productivity relationship, implying the absence of a shift to substrate limitation at higher temperatures. The temperature effect varied in space and was lower at the freshwater station, suggesting differential substrate availability or the presence of a different resident microbial community. Proxies for bacterial substrates, including dissolved organic carbon, particulate organic nitrogen, and chlorophyll a, improved the predictive power in multiple regression models, but at least one-third of the variation in BP remained unexplained. The regression models differed by station group, further highlighting the possible difference in biota and/or resources between the freshwater and brackish sites.

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