Evaluating the impact of oyster (Crassostrea virginica) gardening on sediment nitrogen cycling in a subtropical estuary

To quantify the effect of oysters on sediment N cycling, oyster-free cages and cages with adult or juvenile oysters [Crassostrea virginica (Gmelin, 1791)] were suspended above the sediments at two sites in Mobile Bay, Alabama, located in the northern Gulf of Mexico. While hydrogen sulfide (HS-) was below detection limits at Site 2, HS- exceeded 500 mu M prior to the deployment at Site 1 and remained detectable in sediments in the oyster treatments. Oyster mediated N inputs were estimated at 11.4 (SE 0.4) g N m(-2) and 3.2 (SE 0.2) g N m(-2) in the juvenile and adult treatments, respectively. The abundances of genes associated with denitrifiers (nirK), sulfate reducers (dsrB), and bacterial and archaeal nitrifiers (bac_amoA, arc_amoA, respectively) varied over the course of the study, but were not affected by the treatments. Similarly, potential denitrification rates measured during the study were similar in all treatments. Net N-2 fluxes, determined from N-2:Ar ratios using a membrane inlet mass spectrometer were similar among treatments, despite significantly higher sediment chlorophyll-a content in the juvenile treatment. We conclude that the commonly assumed enhanced rates of N-2 loss from sediments in response to deployment of oysters is not guaranteed and may depend on site-specific biogeochemistry.