Evaluation of eutrophication and impact of river runoff along the coastal zone of Yancheng City, Jiangsu Province

Based on in-situ investigation of water nutrients and chemical oxygen demand （COD） along the coast of Yancheng City, Jiangsu Province in 2018, nutrient concentrations, compositions, and spatial distributions were analyzed using single factor pollution index, nutrient ratios, and nutrient limitation. The related eutrophication status was evaluated using eutrophication state index. Meantime, the fluxes of dissolved inorganic nitrogen （DIN）, dissolved inorganic phosphorus （DIP）, and COD discharged into coast from four local rivers were calculated, and their impacts on the coast were discussed. Results show that DIN and DIP contents in 90% and 71% of sampling stations, respectively, were higher than the Class III seawater quality standard of China, showing apparent DIN and DIP pollutions. Comparatively, COD pollution was not significant because COD contents in 78% of sampling stations were below the Class I seawater quality standard. Spatial distribution patterns of water nutrient and COD were similar, and their contents decreased from nearshore to offshore in general, but increased sharply in some estuaries and ports. The eutrophication in the study area was very serious, because sampling stations with eutrophication state index （E）≥1 accounted for 98%, and stations with severe eutrophication （E>10） accounted for about 30%. Water N/P ratios were about 34 on average, much higher than the Redfield ratio, implying nutrient composition was seriously unbalanced, and algal growth might be potentially limited by phosphorus. Annual fluxes of DIN, DIP, and COD from four inflowing rivers into the coast were estimated to be 12 901, 484, and 33 077 t, respectively, which were the main pollutant sources of coastal waters. In addition, their fluxes in the wet season were about twice higher than those in the dry season, and N/P ratios of river runoff in the wet season were 24.7 in average, suggesting the serious N pollution, which was possibly the major reason for the imbalance of nitrogen and phosphorus in the coastal waters.