| Eutrophication and associated algal blooms are main environmental problems confronting reservoirs in China,threatening the safety of drinking water supply and the construction of ecological civilization.Large reservoirs generally exhibit with high water depth,long water retention time,low nutrients content and large heterogeneity and complexity of spatial pattern.Therefore,the variations of exogenous nutrients input and hydrodynamic characteristics,which induced by the hydrometeorological conditions,may play an important role in water quality and phytoplankton successions of reservoirs.Here,Xin’anjiang Reservoir(Lake Qiandaohu)was chosen as case study.The algal community and environmental variables of most sites were monthly sampled,and high-frequency(once for three days)monitoring were conducted in jiekou.Moreover,the historical data of phytoplankton and automatic monitoring data of buoys were also collected.Our objectives are to 1)clarify the spatiotemporal dynamics of nutrients and phytoplankton,2)to elucidate the effect of hydrometeorological conditions(e.g.,temperature,rainfall and inflow)to eutrophication and associated algal bloom in Lake Qiandaohu and the reserviors with similar ecological background.The results are as follows.(1)The concentration of nutrients in Lake Qiandaohu exhibited a relative high spatial-temporal variations.The TN content ranged from 0.69 mg/L and 2.06 mg/L,with the average value of 1.12±0.26 mg/L,and the peak of TN appeared in winter and spring.The TP concentration ranged from 0.004 mg/L and 0.096 mg/L,the average value was 1.12±0.26 mg/L,and the peak of TN appeared in winter and spring.The TP concentration was between 0.004 and 0.096 mg/L,the average value was 0.030±0.021 mg/L,and the peak value of TP appeared in spring.In terms of spatial variations,the concentration both of TN and TP decreased gradually from riverine zone to lacustrine zone.Better correlation relationship between cumulative inflow and nutrients were found than rainfall,and greater effect were found in phosphorus than nitrogen.The response time of different zone to the inflow was discrepant,the riverine zone responds the fastest,followed by the transition zone,and the lacustrine zone was the slowest,implying the lag effect of the nutrients diffusion in the reservoir.The content of TN and TP in water were also influenced by temperature,because this factor controls the growth of phytoplankton which affects the granular nutrients.However,this response of different zone is different.Better relationship between cumulative temperature and nitrogen and phosphorus was occurs in transition zone,rather than riverine zone or lacustrine zone.(2)The results showed that from 2002 to 2017,seven phyla including 93 genera of phytoplankton were existed in Lake Qiandaohu,which were mainly composed of Bacillariophyta,Chlorophyta,Cyanophyta and Cryptophyta.In the past 16 years,Annual abundance and community structure of phytoplankton experienced four stages:before 2008,the phytoplankton cell abundance maintain low lever and cyanobacteria were not the main group.In 2009-2012,the cell abundance of phytoplankton was high lever and cyanobacteria became the main group.The cell abundance of phytoplankton and proportion of Cyanophyta decreased in 2013-2015.Phytoplankton cell abundance increased in 2016-2017 and cyanobacteria became the main group again.When the phytoplankton changed,the dominant genus also changed.The phytoplankton dominant genus was Cyclotella,Cryptomonas,and Chroomonas in 2002 to 2008,then changed to Oscillatoria,Chlorella,Cyclotella,and Chroomonas during 2009-2012,and turned Anabaena,Aphanizomenon,Cyclotella,Synedra,Melosira,Scenedesmus,and Chroomonas during 2013-2017 at last.RDA analysis showed that hydro-meteorological factors such as temperature,wind speed,water level and inflow,and water quality factors such as total nitrogen,conductivity,nitrogen-phosphorus ratio and transparency were significantly related to phytoplankton community structure.However,the main influencing factors varied for different site.For example,Jiekou was mainly affected by hydro-meteorological conditions,while Xiaojinshan and Santan islands were both affected by hydrometeorology and nutrients.(3)Based on high-frequency data,corresponded with the seasonal dynamics of inflow and rainfall,the nutrients concentrations in the Jiekou were also observed with obvious seasonal changes.The nutrients concentration in March-June is twice as high as that in other months The significant positive correlation between hydrologic conditions(e.g.,rainfall,inflow)and nutrients(nitrogen and phosphorus concentrations)(P<0.01)were found.In terms of thermal factor,no significant relationship between temperature and TN were found,but extremely significant positive correlation with TP were observed.During 2017 to 2018 the phytoplankton at Jiekou was dominated by Bacillariophyta,Chlorophyta,Cyanophyta and Cryptophyta,with the proportion of49%,21%,17%and 11%,respectively.Aphanizomenon,Pediastrum,Cyclotella,Synedra,Melosira and Cryptomonas were the main dominating genus.The seasonal dynamics of phytoplankton biomass and composition varied significantly between 2017 and 2018.And the influencing factors of phytoplankton community structure among the four seasons were different.The influencing factors of phytoplankton community structure in spring were cumulative rainfall,cumulative inflow,water level and DOC;the influencing factors during summer were cumulative inflow,water level,DOC,TN,TP and SD,the influencing factors in autumn were accumulated temperature,water level,DOC,TN,TP and TN:TP.During winter,the influencing factors were accumulated temperature,water level and DOC.The thermocline and oxygenline of Jiekou were formed in April and disappeared in October,and the rainfall and inflow have an effect on the stability of the stratification.When the inflow is close to 500×10~5 m~3 or the rainfall exceeds 80 mm,the stratification will be completely broken.In summary,hydro-meteorological conditions exert significant and complicated effects on the aquatic environment in large reservoirs.And the effects of physical diffusion and sedimentation,chemical changes and biological growth accumulation make the responses of water environment indicators to the hydro-meteorological processes have great spatial and temporal heterogeneity.This non-synchronized ecological characteristic should be fully considered in the management of reservoir water quality.The abundance of cyanobacteria in Lake Qiandaohu has increased significantly in recent years,and cyanobacteria of Jiekou were also dominant in winter.The reservoir were faced with the risk of cyanobacteria blooms.High-frequency monitoring can better capture the changes of algae.To better clarify the influence mechanism of hydro-meteorological conditions(e.g.,rainfall,inflow,temperature)on reservoir eutrophication and algal blooms,high frequency monitoring system should establish as soon as possible. |
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