Oyster reefs have declined globally. Interest in their restoration has motivated research into oyster-mediated ecosystem services including effects on biodiversity, filtration, and nitrogen (N) cycling. Recent evidence suggests oysters may promote denitrification, or anaerobic respiration of nitrate (NO3-) into di-nitrogen gas, via benthic deposition of carbon (C) and N-rich biodeposits. However, the mechanisms whereby biodeposits promote N transformations prerequisite to denitrification (e.g., mineralization and nitrification) are unclear. Previous research has also not measured oysters' influence on N cycling in urbanized areas. In May 2010 we deployed eastern oysters (Crassostrea virginica) in mesh cages above sand-filled boxes at four sites across a nutrient gradient in Jamaica Bay, New York City (New York, USA). Oysters were arranged at four densities: 0, 40, 85, and 150 oysters/m2. For 17 months we measured water-column nutrients and chlorophyll a, every two weeks to monthly. Every two months we measured sediment ash-free dry mass (AFDM), exchangeable ammonium (NH4+), ammonification, nitrification, denitrification potential (DNP), and NO3- and C limitation of DNP. Oysters increased sediment AFDM at three of four sites, with the greatest increase at high density. Oysters did not affect any N pools or transformations. However, variation among sites and dates illustrated environmental drivers of C and N biogeochemistry in this urban estuary. Overall, nitrification was positively related to net ammonification, water column NH4+, and sediment NH4+, but was not correlated with DNP. Denitrification was consistently and strongly NO3- limited, while C was not limiting or secondarily limiting. Therefore, the oyster-mediated increase in AFDM did not affect DNP because C was not its primary driver. Also, because DNP was unrelated to nitrification, it is unlikely that biodeposit N was converted to NO3- for use as a denitrification substrate. Predicting times or sites where denitrification is driven by the C and N species originating from oyster biodeposits remains a challenge under eutrophic conditions. Towards this goal, we synthesized our conclusions with literature predictions in a conceptual model for pathways whereby oysters might influence C and N dynamics differently in oligotrophic relative to eutrophic ecosystems.
