Simulating The Seasonal Succession And Long-term Changes Of Harmful Algal Blooms In The Coastal Waters Adjacent To The Changjiang River Estuary

The sea area adjacent to the Changjiang River estuary is the most notable region for harmful algal blooms(HABs)in China,the frequency,scale and duration of HABs increased dramatically.Since the beginning of the 21 st century,large-scale blooms of dinoflagellates(ig.Prorocentrum donghaiense and Karenia mikimotoi)started to appear in this region,and the mechanisms for the occurrence of dinoflagellate blooms kept an intriguing issue.According to the previous studies,succession of different algal blooms was observed in this area.Diatom blooms usually occur in early spring,and dinoflagellates take the place of diatoms to form blooms in late spring and early summer.The seasonal succession processes reflect the differences of the bloom causative species in competitive and adaptive strategies.With the long-term changes of environment,the relative intensity of diatom bloom and dinoflagellate bloom changes accordingly,and finally leads to the large-scale dinoflagellate blooms.Therefore,a better understanding on the succession mechanism of different microalgal blooms would give help to elucidate the mechanisms for the occurrence of large-scale dinoflagellate blooms in this region.The formation of HABs is closely related to the environmental factors including temperature,light intensity,salinity and nutrients.Affected by the Changjiang diluted water(CDW)and Taiwan warm current(TWC,particularly the branches of Kuroshio),the environmental conditions in the coastal waters adjacent to the Changjiang River estuary are quite complex.In order to obtain a better understanding of the mechanisms of HABs in this region,statistical analyses on the basis of survey data were performed to reveal the relationships between two types of blooms and environmental factors,and numerical models were developed to evaluate the effects of environmental factors on the seasonal succession and long-term changes of HABs.Based on the field observations conducted in the coastal waters adjacent to the Changjiang River estuary in 2005,statistical analyses were performed using principle component analysis(PCA),multiple regression analysis(MRA)and path analysis(PA).The results suggested that phosphate and silicate are major factors directly affecting diatom blooms,while dissolved inorganic nitrogen(DIN),temperature and turbidity are factors influencing dinoflagellate blooms.The CDW and TWC have different roles in affecting the two types of algal blooms.The CDW,which has a high concentration of nitrate and silicate,is essential for diatom blooms,while the intrusion of the offshore waters is critical for the maintenance of dinoflagellate blooms.Based on above viewpoints,nonlinear fitting methods were used to establish equations representing the quantitative relationships between phytoplankton biomass and inorganic nutrients,temperature and salinity during the diatom and dinoflagellate blooms.The optimum values for different algal blooms could provide references for further studies on numerical simulation and parameter optimization of harmful algal blooms.Besides,the relationships between water stratification and two types of blooms were examined.The results indicated that the blooms were mainly distributed above the thermocline and halocline.The CDW played an important role in affecting the distribution of diatom bloom in this region,while the dinoflagellate bloom was partially affected by the upwelling and intrusion of seawater from the open ocean.Based on the studies above,a simple zero-dimensional numerical model was established to simulate the succession of microalgal blooms.The model could well reproduce the succession from diatom blooms to dinoflagellate blooms.Using this model,analyses were performed under different scenarios to analyze the effects of temperature,light and nutrients on the succession of different microalgal blooms.It was found that phosphorous stress was the most critical factor controlling the succession from diatom blooms to dinoflagellate blooms,and nitrate played an important role in affecting the scale of dinoflagellate blooms.Considering the effects of nutrients transported by the Changjiang River and their long-term changes,the historical patterns of HABs were retrieved using the numerical model.It can be seen that inter-decadal changes in nutrient supply from the Changjiang River play notable roles on the changes of HABs.Besides,a set of scenario analyses indicate that dinoflagellate blooms would be getting worse if the nutrient pollution were not controlled,but the intensity of dinoflagellate blooms could be reduced based on the regulation of nutrients discharged from the Changjiang River.In summary,a set of statistical methods were applied to find the factors controlling diatom blooms and dinoflagellate blooms,and evaluate the effects of each environmental factor.Moreover,a numerical model was developed to simulate the seasonal succession from diatom blooms to dinoflagellate blooms,and to predict the trend of HABs in this region.The findings could further improve the current understandings on the mechanisms of large-scale dinoflagellate blooms in the East China Sea,and help to develop prevention strategies against HABs in this region.