Microbial productivity in the Neuse River and Pamlico Sound estuarine system: Patterns and perturbations

The spatiotemporal patterns of estuarine microbial communities under a variety of conditions is essential to a better understanding of overall ecosystem function and its influence on adjacent coastal areas. In 1999, three sequential hurricanes impacted the Neuse River and Pamlico Sound system and the effects on water quality and the phytoplankton community in the sound were followed for over two years. Pre-storm conditions returned after a month for nutrients and after from 6--8 months for salinity and phytoplankton biomass. Phytoplankton community structure appeared to be still changing at the end of the study. The storm floods generated 2--3 times the annual nitrogen loading to the sound, bypassing the sub-estuary filtration. The patterns and controls of bacterioplankton were examined during 2002--2005 along the salinity gradient in the same system. Bacterioplankton productivity (BP) was similar to measurements from other temperate estuaries and had about 50% of its variation explained by temperature. Dissolved and particulate organic matter showed a small interactive effect with temperature, but much of the remaining variation was left unexplained. Overall, there was a mid-estuarine peak in BP that corresponded to peak phytoplankton productivity and biomass, and the location of these peaks related to annual discharge. This pattern disappeared at the scale of individual research trips and when the system was impacted by another major hurricane. Variation with depth was large and BP was often higher in bottom or pycnocline waters, correlating with stratification intensity and particulate carbon concentrations. The effect of temperature varied by location, with the upstream, freshwater station having a lower effect than the rest, possibly due to substrate limitation. Data from this study fit the phytoplankton--bacterioplankton relationship seen in cross-system analyses, although the freshwater site again appeared independent of the other sites. Water column respiration was found to be similar to benthic respiration rates and was used to calculate bacterial growth efficiency and carbon demand (BCD). At all the downstream marine stations, BCD was approximately equal to phytoplankton production, whereas it was several times that at the freshwater site indicating support of bacteria by allochthonous organic matter...