Composition and distribution of nutrients and environmental capacity in Dapeng Bay, northern South China Sea

Seawater physicochemical parameters and environmental capacity are important ecological indicators and typical features of the marine environment. It has great significance in the marine material cycle and ecological health. In September 2021 (wet season) and March 2022 (dry season), two voyage investigations were conducted at 12 stations (D1-D12) on Dapeng Bay (DPB), northern South China Sea. The distribution of nutrient, water-quality status, environmental capacity, and impact of ecological environment were discussed. Results showed that NH₄-N was the main form of dissolved inorganic nitrogen (DIN) during the wet season, with concentrations ranging from 0.008 mg/L to 0.109 mg/L, accounting for ~53 % of DIN. Conversely, NO₃-N was the main form of DIN during the dry season, with concentrations ranging from 0.005 mg/L to 0.117 mg/L, accounting for ~50 % of DIN. The DIP concentration ranged from 0.002 mg/L to 0.019 mg/L, accounting for ~51 % and 31 % of the total dissolved phosphorus in the wet and dry seasons, respectively. The distributions of NH₄-N, NO₃-N, NO₂-N, and DIP were relatively similar, decreasing from the inner bay to the outer bay. The eutrophication indices of 12 stations <1, indicating a poor eutrophication state. Single-factor indices including chemical oxygen demand (COD), DIN, and dissolved inorganic phosphorus (DIP) were less than the class I seawater-quality standard. However, except for station D1, the overall water quality was good. Dissolved oxygen with DIP had a significantly negative correlation during the dry season, indicating that DIP was primarily dominated by marine biological activity and organic-matter decomposition. The remaining environmental capacities of COD, DIN, and DIP in DPB were calculated to be 13,742, 1418, and 141 tons, respectively. Based on the functional-zone division of the sea area, the remaining environmental capacities of COD, DIN, and DIP were exceeded 75 % of the total environmental capacity. This study provided a scientific basis for the protection of marine ecological environment and the sustainable development of DPB.