Observations of Variable Ammonia Oxidation and Nitrous Oxide Flux in a Eutrophic Estuary
Accurate global forecasting of marine nitrous oxide (N2O) emissions requires a better understanding of atmospheric N2O fluxes from coastal systems, particularly the mechanisms controlling the net balance between N2O production and consumption. The objective of this study was to examine how physical and biological processes in the eutrophic Chesapeake Bay estuary influence the temporal and spatial variability of N2O using a combination of gas measurements (N2O and N2/Ar) and stable isotope tracer incubations. Observed concentrations of N2O varied considerably in both space and time with the highest concentrations (up to 20.9 nM) across the pycnocline. Ammonia oxidation rates ranged from 14.3 to 108.9 nM h−1 and were highest following wind events that mixed oxygenated surface water below the pycnocline into ammonium-rich bottom waters, resulting in nitrite (NO2−) accumulations of up to 13 μM. During periods of weak vertical mixing, both N2O concentrations and ammonia oxidation rates were lower, while lower O2 concentrations also allowed N2O consumption by denitrification. A three-layer box model provided estimates of N2O production at the surface and across the pycnocline of 4 μmol m−2 day−1 and 21 μmol m−2 day−1, respectively, and an estimate of N2O consumption below the pycnocline of approximately −3 μmol m−2 day−1. Our results demonstrate that physical processes affect the net balance between N2O production and consumption, making Chesapeake Bay a variable source and sink for N2O.
KeywordsChesapeake Bay Nitrification Denitrification Greenhouse gases
We thank the captain and crew of the R/V Hugh R. Sharp and Chief Scientist Michael Roman for allowing our participation on the cruises, which were funded by United States National Science Foundation (NSF) award OCE-1259691. We thank W. Boicourt and M. Scully for providing current velocity and wind data for the sampling area. We are grateful to James Pierson, Wen-Cheng Liu, Catherine Fitzgerald, and Hannah McFadden for sampling assistance at sea. We thank Todd Kana for valuable advice on the MIMS data and Jeff Cornwell, Larry Sanford, and the members of SCOR working group #143 for discussions. This work was supported by funds from the NSF to A.E.S. (OCE-1260006 and OCE-1437310) and startup funds from the University of Maryland Center for Environmental Science. S.M.L. was supported by a Horn Point Laboratory graduate student fellowship, a Maryland SeaGrant graduate fellowship, and the Interdepartmental Graduate Program in Marine Science at the University of California, Santa Barbara. N.J.N. was supported by NSF (OCE-1259691 and OCE-1334398). A.W.F. was supported by NSF OCE-1061609 awarded to W. Boicourt, L. Sanford, and Ming Li and a Horn Point Laboratory graduate student fellowship. R.J.F. was supported by NSF grants OCE-1260006, DEB-091918, and DEB-1252923.
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Conflict of Interest
The authors declare no conflict of interest.
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