Similar Oxygen Sensitivities of Different Steps of Denitrification in Estuarine Waters

Hypoxia is observed and projected to expand in many aquatic environments, largely due to excess anthropogenic nutrient inputs and climate change, thus influencing biogeochemical processes. Denitrification, generally an anaerobic process, removes bioavailable nitrogen and produces nitrous oxide (N₂O). However, limited observations of the effect of oxygen on denitrification restrict our ability to estimate changes in the amount of bioavailable nitrogen and N₂O emissions under anthropogenic perturbations and climate change. Here, we show that all denitrification steps increased, while the N₂O production yield from denitrification decreased with decreasing oxygen in Chesapeake Bay - the largest estuary in the United States. The different steps of denitrification responded similarly to oxygen changes in Chesapeake Bay, unlike open ocean oxygen minimum zones, with implications for the accumulation or depletion of denitrification intermediates such as nitrite and N₂O. Our observations also suggest that current model parametrizations of denitrification in Chesapeake Bay likely overestimate denitrification and nitrogen removal in the presence of oxygen, which would bias the evaluation of nutrient cycling, ecosystem productivity, and the extent of hypoxia. Overall, our newly derived oxygen sensitivities of denitrification could be used to improve model parametrizations of denitrification and constrain the nitrogen budget and N₂O emissions in estuarine and coastal environments experiencing hypoxia.