Pelagic N2 fixation dominated by sediment diazotrophic communities in a shallow temperate estuary

Estuaries receive substantial anthropogenic nitrogen loading and are mainly considered net nitrogen sinks. While several studies have identified diverse diazotrophic communities in estuarine sediments, the role of pelagic diazotrophs in these systems is not well understood. We investigated the links between diazotrophic community composition, nitrogenase (nifH) gene expression, N-2 fixation, and environmental conditions in Narragansett Bay (USA). Pelagic N-2 fixation rates ranged between 0.02 and 9.41 nmol N L-1 d(-1) and correlated significantly with fluctuations in diazotroph community composition. These fluctuations were also correlated with temperature, salinity, and mean sea level. The dominant sequences in our pelagic samples were related to sequences previously detected in the bay's sediments and were dominated by nifH gene Clusters I and III. We interpret this as a coupling between sediment and pelagic diazotroph communities and speculate that resuspension plays an important role for pelagic N-2 fixation in shallow estuarine environments such as Narragansett Bay. For instance, the finding of active sulfate reducers in the oxygenated water illustrates that the sediment-pelagic coupling can impact nutrient cycling in shallow environments. The pelagic N-2 fixation measured during our study period showed only a minor contribution (< 1%) to the total estimated nitrogen load to Narragansett Bay. However, with intensifying nitrogen management in estuaries, the need to constrain the rates of pelagic N-2 fixation in these systems will be essential for estimating nitrogen fluxes within the bay and to the adjacent coastal ecosystems.