Effects Of Hydrological And Meteorological Conditions On The Dynamics Of Phytoplankton Community Structure In Shahe Reservoir,Tianmu Lake

Phytoplankton is the primary producer and the basic link of the whole production process of water ecosystem.The change of their species and quantity is closely related to the environmental factors and directly respond to the changes of water ecological environment.Phytoplankton proliferation can deteriorate water quality and environmental problems in lakes and reservoirs,and it becomes more unpredictable under the circumstance of hydrological and meteorological conditions change.In order to reveal the effects of hydrological and meteorological conditions on phytoplankton in reservoirs,phytoplankton community structure and the associated environmental factors from 2008 to2016 from Shahe Reservoir in Tianmuhu were analyzed,the spatial and temporal variations of phytoplankton and the associated driving factors in the reservoir were investigated.The main results as following:1.Phytoplankton community structure varies greatly between seasons:the highest biomass of phytoplankton was recorded in the summer and the lowest in the winter.Synedra,Cyclotella,Cryptomonas,and Achnanthes were the dominant genera in the spring;Synedra,Cryptomonas,Raphidiopsis,and Phormidium were the dominant genera in the summer;Cryptomonas,Synedra,Raphidiopsis,and Aphanizomenon were the dominant genera in the autumn;and Cryptomonas,Synedra,Achnanthes,and Cyclotella were the dominant genera in the winter.Synedra and Cryptomonas were the dominant genera across the whole year.The temporal variations of phytoplankton are mainly controlled by water temperature.2.In spring,diatom and its dominant genera were predominately driven by temperature,and the relationship between hydrology and nutrient was insignificant.Distinct relationships were recorded between different diatom genera and temperature over the past nine years.The optimum growth temperature of diatom and Synedra were both 27°C,Cyclotella and Achnanthes were both 19°C.Low temperature was suitable for the growth of Melosira and its growth was compromised with increasing temperature.The total biomass of diatom,and the biomass of Synedra and Achnanthes were significantly and positively correlated with rainfall?P<0.05?.The biomass of Cyclotella were significantly and positively correlated with total phosphorus,rainfall and water level?P<0.05?,and significantly and negatively correlated with water exchange rate?P<0.05?.The biomass of Melosira was significantly and positively correlated with total phosphorus.The annual peak biomass of diatom can be predicted by a multiple regression model with independent variables including rainfall intensity,dissolved total phosphorus,and the accumulated temperature in winter and spring.3.Differences of water quality and phytoplankton community structure distribute in different areas of the resevoir.The concentrations of total nitrogen?TN?,total phosphorous?TP?,Turbidity?Turb?,suspended solids?SS?,and chlorophyll-a?CHL?were the highest in the upstream tributaries and the lowest in downstream-linked reservoir.In contrast,the highest Secchi disk depth?SDD?was recorded in the Shahe Reservoir and that the lowest in the upstream tributaries.Significant differences for the water quality indices were recorded among the upstream tributaries,the transition region,and the downstream-linked reservoir area?ANOVA,P<0.05?.The biomass of phytoplankton was the highest in the upstream tributaries and the lowest in the reservoir.TPand SDD were the most important driving factors for the spatial variations of phytoplankton.4.Shahe Reservoir is a middle-deep reservoir,the formation and destruction of thermal stratification affects water quality and phytoplankton community structure.The thermal stratification was a typical subtropical one-cycle mixing model,which last from May to September.The thermal stratification appeared and disappeared when the surface water temperature were 21oC during later spring and 19oC during the middle autumn,respectively.The difference between the water temperature at the epilimnion and hypolimnion increases with increasing solar radiation.When air temperature above 30oC,increases the stability of the thermal stratification.Heavy storm reduces the temperature of surface water,and weakens the temperature stratification of the column above 5 m depths,but have limited impacts on stratification of hypolimnion deeper than 5 m.The thermal stratification greatly impacts the water quality of the lake.The hypoxia in the bottom water occurred by thermal stratification,leading to an increased NH₄⁺-N.The concentrations of dissolved oxygen,total phosphorus and suspended solids in the hypolimnion increased after the disappearance of thermal stratification.