Spatiotemporal variation, composition of DIN and its contribution to eutrophication in coastal waters adjacent to Hainan Island, China

The past few decades have seen a massive increase in global coastal eutrophication, for which nitrogen enrichment is usually the primary causative agent. Based on field monitoring data from coastal waters during dry, wet, and normal flow seasons in 2016, the distribution characteristics of dissolved inorganic nitrogen (DIN) and its influencing factors were studied. The level of DIN pollution was evaluated, and its contribution to the eutrophication in the coastal waters was analyzed. Furthermore, the main source and relationships of DIN were also discussed. The results showed that the concentration of DIN ranged from 0.008 mg/L to 0.384 mg/L, with an average of 0.114 ± 0.087 mg/L. Areas of high DIN concentration (>0.2mg/L) were located in coastal waters adjacent to Haikou City and Qiongzhou Strait in all seasons. The species and composition of DIN in the coastal waters showed seasonal variation. N-NO3− and N-NH4+ were the main species of DIN. The contribution of DIN to eutrophication ranged from 21.2% to 47.8%, with a mean value of 34.6% ± 4.2%. Under poor nutrient condition (EI