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

Abstract

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.

from Energy Ecology Environment Ambio via Terpsi Hori on Inoreader http://bit.ly/2F0SKQB