Directly measured net denitrification rates in offshore New England sediments

Continental shelf sediments are important locations for denitrification in the marine environment. Here, we report a robust, directly measured set of sediment net denitrification rates from five sites off the coast of southern Rhode Island (USA). Using the N2/Ar technique, we sampled three of these sites at an almost bimonthly rate from October 2009 to January 2011, and two additional sites during July 2010. Net denitrification ranged from approximately 20 to 75 N2–N μmol m−2 h−1 with a mean of 44±4 N2–N μmol m−2 h−1. These values are similar to other studies that report direct-measurements of sediment net N2 fluxes located along the East coast of the United States. Net sediment denitrification rates were not significantly different between sites, nor did the rates change over the sampled temperature range (5–17 °C). Net denitrification rates were weakly correlated, but exhibited significant relationships with sediment oxygen demand. Positive and negative fluxes of nitrate along the sediment–water interface suggest that both direct and coupled denitrification are occurring. Sediment molar C:N ratios were consistent temporally and spatially, indicating that sediment organic matter quality may be responsible for the similar rates of net denitrification at our sites. If we apply our rates to the 360,000 km2 North East United States shelf area, we calculate that 0.14 Tmol of N per year are removed, accounting for just over 5% of the total N entering the North Atlantic shelf region. Additionally, we predict a historical denitrification rate based on primary production estimates, which indicate that denitrification rates may have been three times higher in the past than our current measurements. Thus, as seen in a nearby coastal system, the nitrogen cycle in offshore New England regions could have changed during the last century.