Developing a salinity-based approach for the evaluation of DIN removal rate in estuarine ecosystems

Estuaries play an important role in the removal of overloading nitrogen to relieve the eutrophic pressure of coastal seawater. However, the exact amount of nitrogen removed in estuarine ecosystems is difficult to be estimated because of the complex dynamic mixing process between riverine water and coastal seawater. In this study, a new method was developed to calculate the removal rate of dissolved inorganic nitrogen (DIN) in estuarine waters attributed to the mixing process and was based on the assumption that relative salinity can serve as an indicator of the degree of mixing. This assumption was supported by the experimental results that demonstrated a linear regression relationship between DIN decline and salinity increase Thus, the decreased amount of DIN in mixing waters attributed to the dilution effect could be determined with the salinity as an index. With this model, the DIN removal rate in both Chesapeake Bay and Pearl River Estuary were defined. As predicted, our analysis demonstrated that the DIN removal rate increased gradually from upstream to downstream in both studied estuaries with obvious seasonable variation pattern: high in warm seasons and low in cold seasons. The practical application of this method might be affected by multiple factors, including the geographic landform of estuaries, initial estuaries DIN concentration, the DIN concentration in seawater, DIN importing from tributaries, sewage discharge and hydrodynamic mixing. Therefore, the results supported the hypothesis that estuaries have a strong capability to remove the nitrogen inputted from human activities, especially in warm season and therefore should play an important role in regulating the balance of global nitrogen biogeochemical cycle.