Temporal and Vertical Oxygen Gradients Modulate Nitrous Oxide Production in a Seasonally Anoxic Fjord: Saanich Inlet, British Columbia
Qixing Ji, Brett D. Jameson, S. Kim Juniper, Damian S. Grundle
Abstract
Abstract Nitrous oxide (N 2 O) is a strong greenhouse gas and an ozone depleting agent. In marine environments, N 2 O is produced biologically via ammonium oxidation, nitrite, and nitrate reduction. The relative importance of these principle production pathways is strongly influenced by oxygen availability. We conducted 15 N tracer experiments of N 2 O production in parallel with measurements of N 2 O concentration and natural abundance isotopes/isotopomers in Saanich Inlet, a seasonally anoxic fjord, to investigate how temporal and vertical oxygen gradients regulate N 2 O production pathways and rates. In April, June, and August 2018, the depth of the oxic‐anoxic interface (dissolved oxygen = 2.5 μmol L −1 isoline) progressively deepened from 110 to 160 m. Within the oxygenated and suboxic water column, N 2 O supersaturation coincided with peak ammonium oxidation activity. Conditions in the anoxic deep water were potentially favorable to N 2 O production from nitrate and nitrite reduction, but N 2 O undersaturation was observed indicating that N 2 O consumption exceeded rates of production. In October, tidal mixing introduced oxygenated water from outside the inlet, displacing the suboxic and anoxic deep water. This oxygenation event stimulated N 2 O production from ammonium oxidation and increased water column N 2 O supersaturation while inhibiting nitrate and nitrite reduction to N 2 O. Results from 15 N tracer incubation experiments and natural abundance isotopomer measurements both implicated ammonium oxidation as the dominant N 2 O production pathway in Saanich Inlet, fueled by high ammonium fluxes (0.6–3.5 nmol m −2 s −1 ) from the anoxic depths. Partial denitrification contributed little to water column N 2 O production because of low availability of nitrate and nitrite.