Seasonal Total Nitrogen and Phosphorus Variation, Speciation, and Composition in the Maowei Sea Affected by Riverine Flux Input, South China Sea

Human activities have altered global nutrient cycling and have significantly changed marine systems. This is evidenced by the significant changes in nitrogen and phosphorus availability. The Maowei Sea (MWS) is the largest oyster culture bay in southwest China. From August 2018 to May 2019, the spatial and temporal nutrient concentrations and fluxes in MWS using system-wide scale seasonal data were assessed from river estuaries and adjacent coastal waters. The annual average concentrations of total nitrogen (TN) and total phosphorus (TP) in the three estuaries of Maolingjiang River (MLJR), Dalanjiang River (DLJR) and Qinjiang River (QJR) were 3.00 mg/L and 0.183 mg/L, respectively. Therein, the highest TN and TP concentrations were in DLJR, the lowest TN concentration was in MLJR, and the lowest TP concentration was in QJR. DIN and DIP were the main forms of TN and TP, accounting for 80.9% and 59.4%, respectively. The main form of DIN in MLJR and QJR was NO3−, accounting for 86.8% and 84.4%, respectively, while the main form of DIN in DLJR was NH4+, accounting for 55.9%. The annual flux of pollutants discharged into MWS from the three estuaries is 10,409.52 t for TN and 556.21 t for TP. The month with the largest contribution to the annual load was July, accounting for 29.2% and 24.2% of TN and TP, respectively, and the fluxes of TN and TP were significantly different among the three seasons (p p < 0.05) in the concentrations of TN and TP among the three seasons. The annual average N/P ratios of the river water and seawater were 43 and 18, respectively, which were higher than the Redfield ratio (N/P = 16), indicating that the growth of phytoplankton in MWS may be limited by phosphorus. Eutrophication owing to nutrient pollution in the three estuaries may be persistent in adjacent coastal waters, and land–ocean integrated mitigation measures should be taken to effectively improve the water quality in the river estuary and coastal water