Impacts of Spatial and Temporal Variation on the Benthic Nitrogen Cycle in the York River Estuary

Changing the nitrogen and organic matter (OM) load of a marine ecosystem can dramatically alter its function. Tight benthic-pelagic coupling is a key characteristic of shallow microtidal systems like the York River Estuary (YRE), VA, where ammonification due to remineralization of OM or ammonium uptake during photosynthesis by microphytobenthos (MPB) on the benthic surface can be significant sources, or sinks, for inorganic and organic nitrogen. To examine how benthic nitrogen fluxes vary spatially and temporally in the YRE, as well as to determine the relative importance of the various nitrogen transformations taking place in the estuary, a series of field samplings and laboratory incubations were performed. Remineralization was found to increase with sediment organic content and warmer temperatures, suggesting that organic matter entering the YRE from its watershed or produced in the estuary by primary production drives the benthic nitrogen cycle. Benthic uptake was not a significant sink of ammonium, with most of the NH 4+ produced in the benthos fluxing into the water column. Nevertheless, benthic uptake displayed variation between stations. Finally, nitrification was a more important sink of ammonium than uptake by the MPB