Long-term variation of dissolved organic carbon and its influencing factors in Sanya Bay*

To explore the spatiotemporal changes, trends, and environmental impact factors of dissolved organic carbon (DOC) in Sanya Bay, water physicochemical parameters collected from 2008 to 2023 in Sanya Bay were used as the input data for analysis based on stepwise regression analysis (SR) and random forest algorithm (RF). The results showed that the DOC concentration increased significantly from 2008 to 2018 due to coastal development in Sanya. A subsequent decline in DOC concentration might have resulted from the combined effects of reduced land-based inputs during the COVID-19 pandemic and the government’s efforts to strengthen the management of the marine environment. High-value zones of DOC were observed on the coastal areas of Sanya Bay. The mean value of DOC concentration was (2.00 ± 1.34) mg·L-1, which maintained at a lower level compared to other semi-enclosed bays. Both SR and RF results indicated that DOC concentrations generally exhibited an initial increase followed by a decrease with rising temperature, total alkalinity, and silicate concentration, and showed significant negative correlations with total phosphorus, dissolved oxygen, and phosphate. These findings demonstrate that temperature, total alkalinity, total phosphorus, silicate, dissolved oxygen, and phosphate are the main factors affecting DOC in Sanya Bay.