Variability of Nitrous Oxide Fluxes From West Falmouth Harbor, Cape Cod, Massachusetts
Abstract
Eutrophic estuaries are potentially important but under-examined sources of the greenhouse gas, nitrous oxide (N2O), to the atmosphere, yet they are also heterogeneous, with potential for N2O discharge from both surface waters and intertidal sediments. In order to investigate the range and variability of N2O fluxes in a nitrogen-enriched estuary, measurements of N2O fluxes were made in West Falmouth Harbor, Massachusetts, during the summer of 2008. West Falmouth Harbor is impacted by increased nitrogen loading through fresh groundwater discharge from upland residential areas and a wastewater treatment plant located within the watershed. Fluxes from intertidal sediments were examined using flux chambers in 42 different sites. These fluxes were highly variable but reached values as high as 3700 μmol N2O/m2/d (average 75 μmol N2O/m2/d). The highest flux measured in West Falmouth Harbor was approximately three orders of magnitude higher than the average flux recently reported for soils, implying that the processes that drive the high fluxes are worthy of attention for understanding controls on fluxes from coastal regions. Results suggest that enhanced N2O fluxes in West Falmouth Harbor may be influenced by the discharge of nitrogen and nitrous oxide-enriched groundwater. N2O concentrations in groundwater ranged from 5-160 times the saturation of nitrous oxide, levels which could drive a flux through surficial sediments. Furthermore, of the sites where visible groundwater discharge was present and resulted in the formation of a pool of fresh groundwater under the flux chamber, 78% produced a significant N2O flux, while only 38% of sites with no visible groundwater discharge produced a detectable N2O flux. Water column N2O fluxes in this location were ~16 μmol N2O/m2/d, assuming a typical estuary gas transfer velocity of 5 cm/hr. Although the intertidal zone contained "hotspots" with very high fluxes, the total flux from the West Falmouth Harbor water column was greater than that from the intertidal zone due to the greater surface area of the water column. The heterogeneity within the intertidal zone presents a challenge when "scaling up" intertidal flux estimates to meaningful averages, and relating fluxes to the processes that control them.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.A33B0233G
- Keywords:
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- 0322 Constituent sources and sinks;
- 0442 Estuarine and nearshore processes (4235);
- 0469 Nitrogen cycling