Low denitrification rates and variable benthic nutrient fluxes characterize Long Island Sound sediments

Organic matter (OM) loading drives benthic metabolism and controls nitrogen (N), phosphorus (P), and carbon (C) cycling in coastal sediments. To better understand the influence of OM on benthic metabolism, we conducted sediment core incubations in Long Island Sound, NY (USA), an estuary heavily influenced by anthropogenic nutrient loading. Cores were collected during the summer and winter at five stations along a west to east transect representing a gradient of high to low nutrient inputs and subsequent OM deposition. Here we provide a comprehensive analysis of environmental drivers of benthic fluxes as well as the first directly measured rates of sediment denitrification and N-fixation, thus providing a missing piece of the N budget for Long Island Sound. Our findings show greatest sediment oxygen demand (SOD) in the western basin during the summer. Sediments were a source of dissolved inorganic N and P. Compared to historic SOD and ammonium fluxes, our measurements were about 50% lower, potentially reflecting larger scale perturbations (e.g., warming temperatures, decrease wind speed) or reduced N loading through improved management of wastewater treatment facilities. Sediments were a source of nitrous oxide (mean (± standard error): 8.63 ± 3.18 nmol m −2 h −1 ) and methane (23.80 ± 18.05 nmol m −2 h −1 ). Net sediment denitrification rates were low (10.34 ± 7.61 µmol N 2 –N m −2 h −1 ) and did not vary significantly across stations or seasons. These sediments have a N removal efficiency of ~ 30% with the potential to remove approximately 9% of the total land-based N load entering Long Island Sound.