Eutrophication in Zhoushan sea area:Spatiotemporal distribution and its relations with environmental factors

Based on the survey data of Zhoushan sea area in spring(May)and summer(August)2008,and by using modified nutrient status index in combining with the assessment method of potential eutrophication(N/P mass ratio),the eutrophication level of the sea area in spring and summer was assessed,with the spatiotemporal distribution of the eutrophication and the relationships between eutrophication index and environmental factors analyzed.Meanwhile,the assessment results of using nutrient status index and N/P mass ratio were compared.The results based on nutrient status index showed that the Zhoushan sea area had a lower eutrophication level,and the values of the eutrophication index at the stations surveyed were all lower than the thresholds of corresponding N/P mass ratios.The eutrophication level was higher in spring than in summer.The eutrophication index decreased from the alongshore to the open sea.In spring,the mean N/P mass ratio of the sea area was 17.35,being higher than Redfield value;in summer,this mass ratio was 56.34,with the characteristics of obvious imbalance in the nutrients ratio.The results based on N/P mass ratio indicated that spring could be the season of N-limited potential eutrophication,while summer could be the season of P-limited potential eutrophication.No significant correlations were observed between the eutrophication index and the water salinity,dissolved oxygen,and pH.The spatiotemporal distribution of the eutrophication in the sea area was mainly co-effected by the ground surface runoff,phytoplankton,and ocean current of open sea.