Understanding how estuarine hydrology controls ammonium and other inorganic nitrogen concentrations and fluxes through the subtropical Jiulong River Estuary, SE China under baseflow and flood-affected conditions

Higher nitrogen fluxes through estuaries increase the risk of harmful algal blooms, may expand eutrophication and can cause hypoxia within estuaries and the adjacent coastal areas. However, the key factors controlling dissolved inorganic nitrogen (DIN) concentrations and export from hydrologically dynamic and turbid estuarine systems are still poorly understood. A series of cruises with high spatial resolution under different hydrological conditions were conducted in 2015-2016 across the Jiulong River Estuary (JRE) continuum, including the estuarine turbidity maximum (ETM). During baseflow, ETMs were more intense during spring tides than neap tides due to stronger net sediment resuspension. The turbidity maxima were stronger and generally further downstream under flood-affected conditions. Based on the distribution of ammonium on the salinity gradient in the low salinity region of the estuary (<2 PSU), we grouped all the cruises into NH4 Addition Pattern (AP) and NH4 Removal Pattern (RP). During baseflow, AP occurred during neap tides and RP during spring tides. An important source of ammonium to the water column was from resuspended sediments and their pore waters. Based on property-salinity plots, nitrification was likely one of the most important transformation processes in the turbid water column of the JRE, resulting in the net removal of ammonium and the net addition of nitrite. It was more intense during spring tides because there were more suspended particles carrying nitrifying bacteria. There was a major addition of DIN from estuarine processes in addition to the extra nitrogen flushed from the catchment during flood-affected flow, in particular during the first flood of the year, compared with a comparatively minor addition during baseflow. This additional DIN was likely from the breakdown products of particulate organic nitrogen accumulated in sediments which were then resuspended under flood-affected conditions.