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

Abstract

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 μM 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 N2 fluxes, determined from N2: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 N2 loss from sediments in response to deployment of oysters is not guaranteed and may depend on site-specific biogeochemistry.