Substantial nitrous oxide emissions from intertidal sediments and groundwater in anthropogenically-impacted West Falmouth Harbor, Massachusetts

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Large N2O emissions were observed from intertidal sediments in a coastal estuary, West Falmouth Harbor, MA, USA. Average N2O emission rates from 41 chambers during summer 2008 were 10.7mol N2Om-2h-1±4.43μmol N2Om-2h-1 (standard error). Emissions were highest from sediments within a known wastewater plume, where a maximum N2O emission rate was 155μmol N2Om-2h-1. Intertidal N2O fluxes were positively related to porewater ammonium concentrations at 10 and 25cm depths. In groundwater from 7 shoreline wells, dissolved N2O ranged from 488% of saturation (56nM N2O) to more than 13000% of saturation (1529nM N2O) and was positively related to nitrate concentrations. Fresh and brackish porewater underlying 14 chambers was also supersaturated in N2O, ranging from 2980% to 13175% of saturation. These observations support a relationship between anthropogenic nutrient loading and N2O emissions in West Falmouth Harbor, with both groundwater sources and also local N2O production within nutrient-rich, intertidal sediments in the groundwater seepage face. N2O emissions from intertidal "hotspot" in this harbor, together with estimated surface water emissions, constituted 2.4% of the average overall rate of nitrogen export from the watershed to the estuary. This suggests that N2O emissions factors from coastal ecosystems may be underestimated. Since anthropogenic nutrient loading affects estuaries worldwide, quantification of N2O dynamics is warranted in other anthropogenically-impacted coastal ecosystems.

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