Observations of Variable Ammonia Oxidation and Nitrous Oxide Flux in a Eutrophic Estuary

Accurate global forecasting of marine nitrous oxide (N 2 O) emissions requires a better understanding of atmospheric N 2 O fluxes from coastal systems, particularly the mechanisms controlling the net balance between N 2 O 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 N 2 O using a combination of gas measurements (N 2 O and N 2 /Ar) and stable isotope tracer incubations. Observed concentrations of N 2 O 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 (NO 2 − ) accumulations of up to 13 μM. During periods of weak vertical mixing, both N 2 O concentrations and ammonia oxidation rates were lower, while lower O 2 concentrations also allowed N 2 O consumption by denitrification. A three-layer box model provided estimates of N 2 O 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 N 2 O consumption below the pycnocline of approximately −3 μmol m −2 day −1 . Our results demonstrate that physical processes affect the net balance between N 2 O production and consumption, making Chesapeake Bay a variable source and sink for N 2 O.