The tight coupling between nitrogen and carbon cycling has important implications for global climate. These two cycles are under growing anthropogenic pressures (e.g., warming temperatures, changes in nitrogen loading). In this article, we use Narragansett Bay as a model system to explore how large- and small-scale human forcings alter marine sediment nitrogen cycling. We measured net sediment N2 fluxes at two stations in Narragansett Bay over a nine-year period (2005–2013), resulting in observed mean net denitrification rates of 85 µmol m–2 h–1 and 40 µmol m–2 h–1 at the highly impacted and the less impacted sites, respectively. However, mean net N-fixation was essentially the same at each site (–89 and –88 µmol m–2 h–1). We found significant relationships between mean summer (June, July, August) water column chlorophyll and mean summer sediment N2 fluxes. Using these relationships and the long-term chlorophyll record in Narragansett Bay, we predicted summer net sediment N2 fluxes for the last 40 years. This approach suggests that the coastal ocean nitrogen cycle responds rapidly to changes in organic matter availability. If such trends hold true for the global ocean, the marine nitrogen cycle as a whole may also be undergoing significant changes.

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