Biogeochemical processes and eutrophication status of nutrients in the northern Beibu Gulf, South China

Land-source nutrient inputs into coastal areas have increased remarkably in recent decades. To investigate whether the rapid development of the coastal areas in Beibu Gulf, a newly developing industry and port in South China, has accelerated the eutrophication of seawater in the coastal gulf, the nutrients and other physicochemical parameters were measured in the Beibu Gulf. The results showed that the lowest salinity was observed in the Maowei Sea and Qinzhou Bay during both wet and dry seasons, suggested that the significant influence of land-based fresh water in these areas. Higher nutrient concentrations occurred in the wet season, whereas lower concentrations occurred in the dry season. High nutrient concentrations were observed in the coastal bay along the coast of Guangxi Province, and the concentration in the northeastern part was higher than that in the northwestern part. The heavy land-source discharge during the wet season was responsible for the high nutrient load in the coastal bay during that season. In addition to the terrestrial inputs, strong decomposition may have partly contributed to the high nutrient load in the Maowei Sea and Qinzhou Bay. However, high Chl-a levels occurred in Qinzhou Bay, the offshore area of the Dafeng River Estuary and Tieshan Bay, indicating that excessive consumption by phytoplankton may have been responsible for the low PO43– concentration in those areas. Generally, of the areas, the Maowei Sea and Qinzhou Bay experienced the highest eutrophication levels, which were at moderate eutrophication levels.