High Incidence Hypoxia Increases Benthic Nitrogen Retention but Decreases Nitrogen Removal in Seasonally Hypoxic Areas off the Changjiang Estuary

Abstract

AbstractHow the partitioning between dissimilatory nitrate reduction to ammonium (DNRA) and N2 production responds to hypoxia in seasonally hypoxic estuaries has not been well understood. In this study, 15N tracer incubations were used to investigate benthic denitrification, anammox and DNRA rates for three years (2016–2018) across seasonally hypoxic areas off the Changjiang Estuary. Dissolved oxygen contents in bottom water were significantly lower in July than in February/March. Concentrations of NH4+ and NOx– in bottom water throughout the sampling sites did not vary between seasons. Rates of benthic denitrification (0.45–6.55 nmol N g−1 h−1), anammox (0.02–0.30 nmol N g−1 h−1) and DNRA (0.06–0.59 nmol N g−1 h−1) varied largely along the estuary. Denitrification, anammox and DNRA rates did not vary significantly between inshore and offshore sites. However, denitrification rates were significantly lower and DNRA rates were significantly higher in consistently than transiently hypoxic sites. Anammox rates were slightly lower in consistently than transiently hypoxic sites. Ratios of DNRA to N2 production were in a range of 0.015–0.53, and were significantly higher in consistently than transiently hypoxic sites, but did not vary significantly between inshore and offshore sites. The lower temperature sensitivity of denitrification and anammox compared to their Q10 values further indicated that hypoxia may inhibit N2 production. These results suggest that high incidence hypoxia can decrease denitrification and anammox rates but increase DNRA rates with important implications on N retention versus removal, thereby exacerbating eutrophication and hypoxia in estuarine and coastal environments.