Variations Of Water Quality And Phytoplankton Community Structure Of Different Drinking Water Reservoirs In A City Of Southern China

The water supply of drinking water reservoirs is crucial for ensuring people’s livelihood,and citizen’s living standard depends on the quality of drinking water supply.However,with the frequent occurrence of water pollution and phytoplankton bloom in lakes and reservoirs in recent years,freshwater resources are facing the risk of water quality deterioration.As an effective risk warning means,environmental monitoring is of great significance for ensuring the safety of water supply and optimizing the management strategies for lakes and reservoirs.In this study,three different connected drinking water reservoirs in a city in southern China were chose as research objects,and the study aims were:to analyze the seasonal changes of water quality by monitoring the water physicochemical parameters;to study the seasonal succession patterns of phytoplankton community structure by monitoring algae ecology and classifying phytoplankton functional groups;to disentangle the influences of environmental factors on the succession of phytoplankton community structure by correlation analysis;to investigate the ecological relationships and environmental adaptability of phytoplankton functional groups by network analysis and multivariate analyses;to evaluate the water quality,nutritional and ecological status through the calculation of water quality index,trophic level index and Q index;to predicate the time trends of the key water chemical parameters affecting the algae ecology through time series analysis.These results will provide reliable data supports for reservoir management strategies and the study of phytoplankton control methods,and the main conclusions were:(1)Variations of thermal stratification and water quality:the three reservoirs were in natural mixing periods from October to February,and the stable stratification lasted from April to September.The dissolved oxygen,conductivity,nitrate nitrogen,total nitrogen,total phosphorus and permanganate index had significant differences among different reservoirs(P<0.05):the annual average values of dissolved oxygen,nitrate nitrogen,total nitrogen and total phosphorus were higher in reservoir A than in reservoir B and C,while the annual average values of conductivity and permanganate index were higher in reservoir C than in reservoir A and reservoir B.Due to the influences of water transfer,the pH,dissolved oxygen,conductivity,redox potential,nitrate nitrogen,ammonia nitrogen,total nitrogen,total phosphorus and permanganate index at surface points in the three reservoirs exhibited significant seasonal differences(P<0.05).(2)Seasonal variations of algae cell density and seasonal succession of phytoplankton functional groups:due to the effects of water transfer,the algae cell density and chlorophyll a at surface points of the three reservoirs exhibited significant seasonal trends(P<0.001),with their values being higher in spring and lower in autumn,and the average algae cell density was 1762.82×10~4 cells/L in spring and 982.91×10~4 cells/L in autumn,respectively.From spring to winter,the dominant functional groups at surface points in reservoir A were MP dominated by Navicula sp.and Achnanthes sp.,J dominated by Scenedesmus sp.,P dominated by Melosira sp.and P dominated by Melosira sp.,respectively,while the main functional groups of reservoir B and C were both SN dominated by Cylindrospermopsis sp.(3)The effects of environmental factors on phytoplankton succession and the ecological relations and environmental adaptability of functional groups:the mantel test results showed that water temperature,pH and nitrate nitrogen significantly influenced the phytoplankton abundance in the three reservoirs(P<0.05).Correlation heatmap showed that the abundance of the dominant functional group P in reservoir A was significantly influenced by water temperature,pH,conductivity and nitrate nitrogen,the abundance of the dominant functional group SN in reservoir B and C was significantly influenced by water temperature,pH,conductivity,nitrogen nutrients(P<0.05).Linear regression analysis demonstrated that the change of phytoplankton abundance in reservoir A and B can be best explained by the combinations of different water physicochemical parameters,while the change of algae abundance in reservoir C can be best explained by water physical factors.Network analysis illustrated that the interactions of phytoplankton species in the three reservoirs were positive.Niche breadth results found that the niche breadth of reservoir A(11.61)was significantly lower than that of reservoir B(26.67)and C(20.61)(P<0.05),indicating the high available resources for the growth of phytoplankton species and the uniform distribution of specialized algae species in reservoir A.(4)The evaluation of water condition and water quality prediction:the WQI calculation showed that the annual average values of reservoir A,B and C were 74.62,76.09 and 76.08respectively,representing the good water quality in the three reservoirs.The TLI calculation showed that the annual average values of reservoir A,B and C were 43.36,43.45 and 44.43respectively,representing the medium trophic level in the three reservoirs.The Q index calculation showed that the annual average values of reservoir A,B and C were 2.25,1.19and 1.37 respectively,indicating the medium ecological status in reservoir A and the tolerable status in reservoir B and C.Time series analysis based on Autoregressive Integrated Moving Average model was conducted for nitrate of reservoir A,nitrate and total nitrogen of reservoir B and nitrate of reservoir C,with the total prediction error ranging from 0.91%to 34.48%,indicating the accurate prediction of time trends of water physicochemical parameters still needs long-term monitoring data.