Analysis and Evaluation of Seawater Quality in the Northwest of Hainan

The state of seawater quality has a significant impact on the ecological environment of coastal waters. To assess the current water quality and eutrophication level of the inshore waters in northwestern Hainan, based on the monitoring results of 23 water quality monitoring stations, the single factor standard index method and the water environment comprehensive index method were used to analyze and evaluate the water quality status of this sea area. Additionally, the potential eutrophication of seawater quality was assessed using a classified and graded potential eutrophication evaluation model. The results revealed that the seawater in the northwest of Hainan exhibited low levels of inorganic nitrogen content, heavy metal content, and pH value, with single factor standard index values less than 1, meeting the first class water quality standard. However, there was one station in the western part of the study area where the seawater labile phosphate content exceeded the first class water quality standard. This may be related to local sewage discharge and upwelling in the western Hainan. Furthermore, due to the excessive oily sewage discharge from ships and the convergence of seawater, the oil content at a central station exceeded the first class water quality standard. Moreover, influenced by factors such as seawater temperature, biomass, and the expansion of hypoxic zones in the central and western parts of the Beibu Gulf, the dissolved oxygen content frequently exceeds the standard, leading to water pollution characterized by low dissolved oxygen levels. The further evaluation of potential eutrophication showed that the overall water quality of this sea area was at a poor trophic level, with a relatively low degree of eutrophication. Overall, the comprehensive water quality evaluation index of this sea area was relatively low, indicating that the water quality was excellent and basically meets the requirements of marine functional zones.