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

TitleObservations of Variable Ammonia Oxidation and Nitrous Oxide Flux in a Eutrophic Estuary
Publication TypeJournal Article
Year of Publication2019
AuthorsLaperriere SM, Nidzieko NJ, Fox RJ, Fisher AW, Santoro AE
JournalEstuaries and Coasts
Date PublishedJan

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 $μ$M. 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 $μ$mol m−2 day−1 and 21 $μ$mol m−2 day−1, respectively, and an estimate of N2O consumption below the pycnocline of approximately −3 $μ$mol 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.