Dinoflagellates Bloom Dynamic Change And Its Correlation With Environmental Factors In Ruxi River

After the impoundment of the Three Gorges reservoir,a part of the tributaries have formed a long or short backwater characteristics with the rise of water level in the reservoir area:a slowing speed of water flow,a decrease of water dilution and self purification ability,a reduce of water environment endurance,aggravation of water eutrophication and a deteriorating water quality,these changes lead to a rapidly reproduction of phytoplankton,even the occurrence of bloom.Ruxi river is one of the most important tributaries,located in the core zone of the Three Gorges Reservoir area.Therefore its water quality is directly related to the Three Gorges reservoir water quality and safety.After the impoundment of the Three Gorges reservoir,Ruxi river has developed to 15 km of the backwater area,and the backwater area of water is eutrophication.On March,2015,the occurrence of water blooms in the Ruxi River backwater area is monitored and its dominant species is identified as Peridiniopsis bloom.Dinoflagellate is an important component of marine and freshwater phytoplankton,as well as one of the main red tide and water bloom.,Because they can move via flagellate,dinoflagellate is thought as a more competitive in contrast to other algae that can’t move.In recent years,with the increase of economic development and environmental pollution,especially the aggravation of water eutrophication,the outbreak of freshwater water bloom is enhanced at the extent,frequency and intensity.It is very important to study the freshwater algae blooms,since algae blooms have seriously affected the safety of water environment and the health of aquatic ecosystem.However,there are few studies on freshwater dinoflagellate bloom.The eutrophication of water body provides a suitable environment for the occurrence of water bloom,while the outbreak and regression of water bloom are affected by various environment factors:water temperature,light,flow rate,upwelling,and so on.In order to investigate the dynamic change of phytoplankton community,and its main environmental factors during the process of dinoflagellate bloom in the Ruxi river,a field monitoring of dinoflagellate bloom in the backwater area of Ruxi River and a canonical correspondence analysis,as well as Pearson correlation analysis were applied in this study,.The main results are as follows:Firstly,during the period of dinoflagellate bloom,6 phytoplankton species,belonging to 62genera were found.Among them,there are many kinds of Chlorophyta,Bacillariophyta and Cyanophyta.The average density of algae bloom during the total reached 4.68×10⁸ cells/L,a significantly higher value compared with the average density of algae Ruxi river.During the bloom,the cell number of Peridiniopsis is absolutely dominant.However,with the change of environmental factors,Peridiniopsis was gradually replaced by green algae,diatoms and cyanobacteria.The diversity and evenness index were found to have moderate or severe pollution during the bloom.Secondly,during the bloom,with the rising of temperature,water temperature is increased to15℃,being more suitable for the growth and proliferation of many dinoflagellate.On March 26,water conductivity,pH and total nitrogen have decreased significantly compared with those on March 16,whereas the redox potential had a significantly increased.The highest value of chlorophyll a concentration showed a trend from the upstream to the downstream among the three sampling.The analysis of eutrophication showed that the water quality is ranged from moderate to severe eutrophication,and the water quality is poor.Thirdly,CCA ordination analysis found that dinoflagellate growth was positively correlated with total nitrogen,but a negative correlation with water temperature and oxidation reduction potential.Additionally,green algae,diatoms and cyanobacteria was the opposite,indicating that total nitrogen,temperature and oxidation reduction potential may influence the intention to dinoflagellate process.Chlorophyll a concentration showed a positively correlation with water temperature and redox potential,and negatively correlated with electrical conductivity.Lastly,dinoflagellate can move up and down since it has flagellate.In the present study,Peridiniopsis showed an obvious cycle of diel vertical migration characteristic in water depth of more than 10 m.05:00-15:00,they tended to surface of water body for migration and accumulation;17:00-03:00,they came from the surface and gradually tended to the lower part of the migration.The day and night vertical transport characteristics in Peridiniopsis leaded to a continuous change of the apparent color during the period of the algae bloom.The Chl a concentration and light,dissolved oxygen,turbidity,COD_Mnand TP concentration has a significant is related,and conductivity,redox potential and total nitrogen concentration has a significant negative correlation,suggesting that these environmental factor affected the vertical migration process of Peridiniopsis.However,total chlorophyll a did not greatly change in the same water column,indicating that surface water color suddenly deepened may be not a continuous rapid growth in a short period of time,but the algal groups,dispersed in certain layer,floated,aggregated,or migrated to the surface under the suitable conditions.Due to large variation of chlorophyll a concentration varied at each layer,chlorophyll a concentration at a single layer is not enough to evaluate the degree of algal bloom.Suggesting that standing crop of algae in total water column may be used as one of the indicators to judge the severity of the algal bloom.