Variation of organic pollution in the last twenty years in the Qinzhou bay and its potential ecological impacts

Based on the data obtained from 1990 to 2010,concentrations of dissolved oxygen,chemical oxygen demand,inorganic nitrogen,and reactive phosphate in Qinzhou Bay in the past 20 years(1990—2010) were recorded and analyzed to demonstrate the long-term variation of organic pollution level in the bay.Organic pollution index(OPI) was employed to study the organic pollution status,and the potential ecological risk originated from such pollution in the study bay was also been discussed.The result showed that OPI was valuable during last twenty years in the Qinzhou bay.OPI ranged from-0.55 to 5.49 in the inner bay,and ranged from-0.44 to 3.96 in the outer bay,showing a wide variation pattern.Based on the results,the outer bay can be considered as clean or moderately polluted,and the inner bay was moderately polluted as well.Generally,OPI in the outer bay decreased from 1990 to 1999,but increased from 1999 to 2003,and then remained undulatory afterwards.Variation of OPI in the inner bay from 1990 to 2003 was similar to that in the outer bay.The OPI in the inner bay however kept increasing in recent years(2003—2010),and organic pollution status in recent years was serious,which indicated a degradation pattern of the inner bay.Inorganic nitrogen was the most important factor that contributed the variation of OPI,and the effects of reactive phosphate was getting more and more significant in recent years from 2003 to 2010.River input(including the input of the Maolingjiang River and the Qinjiang River),phytoplankton consumption,water exchange as well as the interaction of these factors were the main reasons for the variation of organic pollution in the Qinzhou bay.The difference organic pollution status in recent years between the inner and the outer bay may be due to the change of such reasons.The variations of organic pollution bay resulted in the variation of eutrophication in the study.And the increasing OPI in recent years explained the increasing phytoplankton biomass from 2003 to 2010 in the bay.Although no significant positive relationship was found between organic pollution and phytoplankton biomass,the increasing organic pollution in the inner bay may trigger some ecological impact,such as algae bloom.The increasing reactive phosphate may finally breach the phosphate limitation in the bay and cause a series of deleterious effects to the aquaculture industry.Organic pollution in the inner bay might also influenced the aquatic product in the study bay.