Spatiotemporal variations and impact factors of nutrients in the Sanya Bay, northern South China Sea.

Nutrients play a fundamental role in maintaining coastal ecosystem stability. Based on two cruise observations in the winter of 2020 and the summer of 2021, the spatiotemporal variations of dissolved inorganic nitrogen (DIN) and orthophosphate (PO) and their influencing factors in Sanya Bay were analyzed. Results show that the mean DIN concentrations in the bay are 2.36 μmol/L in winter and 1.73 μmol/L in summer, and the mean PO concentrations are 0.08 μmol/L in winter and 0.04 μmol/L in summer. The nutrient concentrations and composition are significantly affected by the Sanya River. The surface DIN concentrations at the Sanya River estuary are 15.80 and 5.25 times than those inside the bay in winter and summer, respectively. Meanwhile, a high proportion of NO (74%) and a low proportion of NH (20%) in DIN appear near the river estuary, while a low proportion of NO (37%) and a high proportion of NH (53%) occur away from the estuary. In addition, the thermocline promotes the accumulation of NH at the bottom layer in summer. The high proportion of NO is likely not favorable for the survival of coral reefs in eastern bay. Compared with the previous nutrient concentrations, the DIN concentration in the bay has a decrease after 2014, which might benefit from the implementation of the environmental protection policy by government.