Eutrophication Status And Evaluation Of Coastal Waters In Zhejiang Province

Based on the monitoring data of nutrients and temperature and salinity of the three voyages in the spring,summer and autumn of Zhejiang coastal waters in 2018,this paper uses three methods to analyze and evaluate the single factor index,eutrophication index and potential eutrophication evaluation.The nutrient content and distribution,main environmental distribution characteristics and eutrophication index temporal and spatial changes in the seawater in the coastal waters of Zhejiang Province are analyzed.The results show that:The annual average value of inorganic nitrogen is 0.526 mg/L,spring>autumn>summer,showing a trend of decreasing from west to eastern sea;the annual average value of active phosphate content is 0.023mg/L,autumn>spring>summer,the horizontal distribution is basically the same as inorganic nitrogen,The chemical oxygen demand is relatively low throughout the year,with an annual average value of0.80mg/L,and the average value meets the first-class water quality standards.NO₃-N is the main form of DIN.The proportion of inorganic nitrogen is 95.3%;the proportion of NO₂-N and NH₄-N in DIN is 3.0%and 1.7%respectively.In the coastal waters of Zhejiang,nitrogen and phosphorus nutrients vary greatly in different seasons,and their N/P molar ratios also vary greatly,with an average of 97.5.In spring,the N/P molar ratio was 119.5,and 100%of stations exceeded 30;in summer,the N/P molar ratio was 76.1,and 82.1%of stations exceeded 30;in autumn,the N/P molar ratio was 96.9,and 69.6%of stations exceeded 30.Active phosphate has become a limiting factor affecting phytoplankton.In the whole year,48.2%of the stations were in a state of eutrophication,with eutrophication degree;spring>autumn>summer.Eutrophication stations accounted for49.2%of the total monitoring stations.According to the PEAM evaluation results,the ratio of stations I was 5.9%,the ratio of IV_p stations was 3.6%,the ratio of V_p stations was28.8%,and the ratio of VI_p stations was 23.2%.The proportion of stations restricted by phosphorus has reached more than half.The degree of eutrophication in the coastal waters of Zhejiang ranks second in the country.The statistical results of the relationship between nutrients and environmental factors show that inorganic nitrogen and active phosphate are greatly affected by salinity,and land-based pollution and surface runoff are the main factors affecting nitrogen and phosphorus.Over the past 20 years in the coastal waters of Zhejiang,the overall eutrophication has been improved,and the water quality of the seas is showing a trend of improvement.However,the degree of eutrophication in Hangzhou Bay,Xiangshan Port,Sanmen Bay and Yueqing Bay is still increasing.Too much nutrient salt in the water body is the root cause of the problem of seawater eutrophication.Controlling the discharge of nutrients from the source and absorbing nutrients in seawater through ecological restoration methods are important means to reduce the concentration and load of nutrients in water bodies.