Eastern oyster (Crassostrea virginica) filtration, biodeposition, and sediment nitrogen cycling at two oyster reefs with contrasting water quality in Great Bay Estuary (New Hampshire, USA)

Abstract

Benthic deposition of carbon (C) and nitrogen (N)-rich oyster biodeposits may increase denitrification, or anaerobic respiration of nitrate (NO3 −) to di-nitrogen gas (N2). However, environmental drivers of C and N dynamics in oyster biodeposits and reef-adjacent sediments require clarification. In July 2012, we collected intact sediment cores adjacent to and 15–20 m away from two oyster reefs (Crassostrea virginica) in Great Bay, New Hampshire, USA: one reference site and one site with cultural eutrophication. We also measured seston, chlorophyll a, and in situ oyster feeding and biodeposition. Cores were incubated in continuous-flow chambers where inflow water received 15N-ammonium (NH4 +), 15NO3 −, or no isotopes (control). We quantified fluxes of dissolved nutrients and gasses (oxygen, 28N2, 29N2, 30N2, and argon) after 24 h. Finally, we measured size-fractionated sediment organic matter. At the eutrophic site, abundant phytoplankton in the 5–28 µm size range was correlated with enhanced oyster feeding rates and biodeposit quality (lower C:N). This site had greater denitrification rates in reef-adjacent cores relative to distal cores. Low production of 29,30N2 in 15NH4 + amended cores suggested water column or biodeposit NH4 + were unlikely to be converted to N2. At both sites, reef-adjacent cores had more shell and higher 29,30N2 production with 15NO3 − addition relative to distal cores, suggesting direct denitrification enhancement near reefs. Oysters likely increased sediment N2 production via high quality biodeposits (eutrophic site), and NO3 − diffusion via structural complexity of reef-adjacent sediment (both sites). Overall, results suggest oyster-mediated ecosystems services may be expected to vary with environmental conditions.