Effects of marine produced organic matter on the potential estuarine capacity of NOx− removal

Abstract

Dissolved inorganic nitrogen (DIN) is very high in the Pearl River Estuary (PRE) and nitrate (NOx−) removal processes such as denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) are important for determining export of DIN to coastal waters. However, fluxes of NOx− removal and influencing factors in the PRE are still unclear. We conducted 4 cruises at 11 sites in the PRE to investigate potential NOx− removal rates, their contributions, and corresponding gene abundances, and controlling factors in surface sediments (0–5 cm). The results showed that the potential rates of denitrification, anammox, and DNRA as well as their contributions varied spatially and seasonally. Denitrification (1.98 ± 1.7 μg N g−1 d−1) was the major NOx− removal processes (68.43 ± 14.61%) while DNRA (0.45 ± 0.28 μg N g−1 d−1) contributed 22.61 ± 14.89% in NOx− removal. The NOx− removal processes and corresponding gene abundances were correlated with the chlorophyll concentrations in both overlying water and sediment, indicating that marine-produced organic matter was the major driver for benthic NOx− removal processes. In addition, water column turbidity had important effects on primary production, which affects benthic N processes. Our study provides evidences for that the turbidity-regulated primary production in overlying water is the primary driver for benthic NOx− removal processes. The contribution of sediment NOx− removal fluxes to water column NOx− concentration was low in the upper estuary and increased in the lower estuary where marine produced chlorophyll a was higher. However, daily fluxes of NOx− removal were estimated to account for only 0.18–7.22% (mean 1.85 ± 1.62%) of NOx− in the whole overlying water column. This suggests that most riverine NOx− was exported out into the adjacent coastal waters.