Temporal and Vertical Oxygen Gradients Modulate Nitrous Oxide Production in a Seasonally Anoxic Fjord: Saanich Inlet, British Columbia
Date
2020
Authors
Ji, Qixing
Jameson, Brett D.
Juniper, S. Kim
Grundle, Damian S.
Journal Title
Journal ISSN
Volume Title
Publisher
Journal of Geophysical Research: Biogeosciences
Abstract
Nitrous oxide (N2O) is a strong greenhouse gas and an ozone depleting agent. In marine
environments, N2O 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 15N tracer experiments of N2O production in parallel with measurements of N2O concentration
and natural abundance isotopes/isotopomers in Saanich Inlet, a seasonally anoxic fjord, to investigate how
temporal and vertical oxygen gradients regulate N2O production pathways and rates. In April, June, and
August 2018, the depth of the oxic‐anoxic interface (dissolved oxygen ¼ 2.5 μmol L−1 isoline) progressively
deepened from 110 to 160 m. Within the oxygenated and suboxic water column, N2O supersaturation
coincided with peak ammonium oxidation activity. Conditions in the anoxic deep water were potentially
favorable to N2O production from nitrate and nitrite reduction, but N2O undersaturation was observed
indicating that N2O 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 N2O production from ammonium oxidation and increased water column N2O
supersaturation while inhibiting nitrate and nitrite reduction to N2O. Results from 15N tracer incubation
experiments and natural abundance isotopomer measurements both implicated ammonium oxidation
as the dominant N2O production pathway in Saanich Inlet, fueled by high ammonium fluxes
(0.6–3.5 nmolm−2 s−1) from the anoxic depths. Partial denitrification contributed little to water column N2O
production because of low availability of nitrate and nitrite.
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Citation
Ji, Q., Jameson, B.D., Juniper, S.K., & Grundle, D.S. (2020). Temporal and Vertical Oxygen Gradients Modulate Nitrous Oxide Production in a Seasonally Anoxic Fjord: Saanich Inlet, British Columbia, Journal of Geophysical Research: Biogeosciences, 125. http://doi.org/10.1029/2020JG005631