| Lake systems,serve as special natural complex on the surface,are important part of the terrestrial hydrosphere and ecosystem,and provide important water resource and environmental foundation for regional economic development.The ecological environmental function of lakes has been deteriorated rapidly with the continuous enhancement of human activities and enhanced watershed development.And heavy metal pollution has become an important factor threatening the water quality and ecological health of lakes and reservoirs.The survey showed that about 23.2% of the key lakes and reservoirs in China were polluted by heavy metals.Heavy metal pollution is also one of the main environmental problems in Yunnan plateau lakes and reservoirs,and arsenic is the main pollution index.However,to date,little is known about the long-term ecological effect of arsenic pollution.From June 2017 to March 2018,we conducted a comparative study of phytoplankton through field work and indoor analysis in Yangzong Lake and Datun Lake,which have different histories of eutrophication and arsenic contamination.We identified the spatial-temporal distribution patterns of phytoplankton communities,dominant species and biomass,and further explored the driving factors through ANOVA,ANOSIM,Pearson,RDA,and variance analysis in these two lakes.We performed laboratory simulation experiments to study the response of phytoplankton communities to the effect of arsenic.Finally,the long-term ecological effects of arsenic pollution were discussed.The main results are as follows:The main results are as follows:(1)A total of 7 phyla and 44 genera of phytoplankton were detected in Yangzong Lake.Chlorophyta had the most genera(18 genera),followed by Cyanophyta(11genera)and Bacillariophyta(9 genera).The number of Cryptophyta,Pyrrophyta,Chrysophyta and Euglenophyta was small.A total of 7 phyla and 45 genera of phytoplankton were identified in Datun Lake.Chlorophyta had the most genera(22genera),followed by Cyanophyta(9 genera)and Bacillariophyta(8 genera).The number of Cryptophyta,Pyrrophyta,Chrysophyta and Euglenophyta was small.(2)There were significant seasonal changes in phytoplankton community structure(genus level)in Yangzong Lake and Datun Lake,but there were not significant spatial differences.Cyanophyta was dominant in both lakes,and the biomass percentage of Cyanophyte in Datun Lake(90.73%-99.13%)was greater than that of Yangzong Lake(53.82%-93.92%).The dominant species were Planktothrix,Aphanizomenon,Microcystis of Cyanophyta,and Ceratium of the Pyrrophyta in Yangzong Lake with Aphanizomenon dominant throughout the year,while Aphanizomenon and Planktothrix in Datun Lake with the sum of relative percentages over 88.4%.(3)There were significant time variations in phytoplankton biomass in Yangzong and Datun Lake,while no significant changes were observed over space.The biomass of Datun Lake(83.05 mg/L-198.83 mg/L)was much higher than that of Yangzong Lake(2.14 mg/L-30.37 mg/L).The biomass change of phytoplankton and Cyanophyta had the same trend in both lakes.The maximum average of phytoplankton and Cyanophyta biomass was highest in September and June in Yangzong and Datun Lake,respectively.(4)The Pearson correlation analysis,Redundancy analysis and Variation partitioning showed that the phytoplankton biomass was significantly positively correlated with water temperature and precipitation,and negatively correlated with total nitrogen,transparency and dissolved oxygen in Yangzong Lake.Water temperature,arsenic and nutrients were significant environmental factors driving the changes of the phytoplankton community in the Yangzong Lake,and independently explained 9.46%,11.77% and 10.79% of community changes.Nutrients and water temperature,water temperature and arsenic,arsenic and nutrients jointly explained5.02%,2.57% and 2.25% of community changes,respectively.The biomass of phytoplankton was significantly positively related to soluble orthophosphate,arsenic and precipitation,while significantly negatively related to transparency in Datun Lake.Nutrients and arsenic were the main environmental factors that drive changes in the phytoplankton community in Datun Lake.Nutrients and water temperature explained17.61% and 3.76% of the phytoplankton community variations in the Datun Lake independently,and had strong interaction with arsenic,reflecting that arsenic was not the main controlling factor.And the process of artificial arsenic removal could affect the driving strength of arsenic on the phytoplankton community variations in Yangzong Lake.(5)The laboratory simulation experiments demonstrated that arsenic showed low-promoting and high-inhibiting effects in mesotrophic level on 7th day,and arsenic at low level resulted in a significant increase in the Cyanophyta biomass percentage.Both arsenic at low level and high level promoted the phytoplankton growth in eutrophic condition on 28 th day,indicating that phosphorus could significantly reduce the toxicity of high arsenic and even exhibit a synergistic promoting effect with arsenic.Therefore,under the background of the water body eutrophication,low-As promotion and high-As inhibition effect and the combined effects of phosphorus and arsenic on phytoplankton should be considered in discussion of the long-term ecological effect of arsenic and restoration of polluted lakes. |
