Effects of River Input on the Inorganic Nitrogen Components in Estuary–Bay Waters: Zhanjiang Bay, China

Rivers discharge a substantial amount of nutrients into the bay, which constitutes a key area of research in water pollution and environmental management. However, research on the influence of river input on various nitrogen components in semi-enclosed bays like the Zhanjiang Bay remains limited. This study conducted on-site monitoring and cultivation experiments in Zhanjiang Bay. Consequently, a systematic investigation was conducted on the influence of river inputs on the migration and change in different nitrogen components, primarily dissolved inorganic nitrogen (DIN). The results showed that the DIN flux during the rainy season was the greatest input flux, amounting to 9724 t, which accounted for 45.6% of the annual DIN flux. The fluxes of NO2-N and NO3-N were the highest during the rainy season, accounting for 49% and 52% of the total annual flux, respectively. Based on on-site cultivation under different salinity conditions (5, 15, and 25), the DIN content decreased much faster in lower-salinity waters, with the highest average daily change rate of −16.3 μmol/dm3 in a salinity of 5. This indicated that hydrodynamic mixing and exchange processes have a significant impact on the variations in inorganic nitrogen components. Therefore, the results of this study provide valuable insights into the migration and change in inorganic nitrogen components in estuarine water bodies. It also holds significance for guiding the treatment of environmental pollution in estuaries, such as addressing the sewage discharge from the steel industry.