Vertical dissolved inorganic nitrogen fluxes in Marsh and Mudflat areas of the Yangtze Estuary

Nitrogen is a dominant macronutrient in many river-dominated coastal systems, and excess concentrations can drive eutrophication, the effects of which can include hypoxia and algal blooms. The Yangtze River in China transports a large amount of dissolved inorganic N. Therefore, it is important to understand the role of the marsh and mudflat areas within the estuary on processing this exogenous N load. In situ dissolved inorganic nitrogen (DIN) fluxes across the sediment-water interface were determined monthly at Chongming Island at two sites (a vegetated marsh and an unvegetated mudflat) and were compared with rates from a previously published laboratory incubation study by our research group. Results from the in situ study showed that NO₃^- flux rates comprised the major component of total DIN flux, ranging from 55 to 97%. No significant difference was observed in the N flux rates between the marsh and mudflat sites. Overall, sediment at both sites served as a sink of DIN from surface water with mean flux rates of -178 μmol m^-2 h^-1 and -165 μmol m-2 h^-1 for the marsh and mudflat, respectively. In general, DIN flux rates were not significantly correlated with DIN concentrations and other measured parameters (temperature, dissolved oxygen, salinity, and pH) of surface water. The in situ measured fluxes of NO₃^- and NO₂^- in this study were not significantly different from those of our previous laboratory incubation (p > 0.05), whereas NH₄⁺ fluxes in situ were significantly lower than those from the laboratory core incubations (p < 0.05). This result suggests that caution should be used when extrapolating rates from laboratory incubation methods to the field because the rates might not be equivalent.