Response Mechanisms Of Lake Plankton Community To Environment And Biodiversity

The relationship between biodiversity and ecosystem functioning is a central issue in ecology.Recently,more and more lakes face eutrophication caused by human activities and global climate warming.As the most important primary producer and consumer in aquatic ecosystems,phytoplankton and zooplankton are characterized by rapid growth rate,short generation time,and are sensitive to the variations of environmental factors.The extinction of plankton species may cause catastrophic consequences such as algae blooms and secondary extinctions.Ecologists have obtained conflicting conclusions while analyzing the influence of plankton species diversity on community productivity and stability.Also,the underlying mechanisms of these biodiversity effects are unclear.Besides,most previous studies to show diversity ecosystem functioning relationships in aquatic ecosystems mainly based on artificial plankton communities in laboratory,evidence from field lakes is still in lack.In this study,both phytoplankton and zooplankton were investigated seasonally from 2011 to 2014 in Lake Nansihu.We showed the temporal and spatial variations of community composition,density and biomasses of both phytoplankton and zooplankton.The responses of phytoplankton and zooplankton to environmental factors were analyzed using canonical correspondence analysis（CCA）.The effects of plankton species diversity on community productivity（meauserd by communty biomass）and adjusted community productivity（resource use efficiency,RUE）were also discussed.We then analyzed the influence of plankton species diversity on community temporal stability index（TSI）,and explored the underlying mechanisms involved.Besides,we analyzed the influence of main environmental factors on the TSI of phytoplankton based on a long term observation in Lake Qixinghu,a lake nearby Lake Nansihu.The main findings of the present study are shown in the following:1)A total of 138 phytoplankton species belonging to 78 genera and 8 phyla was identified in the lake during the study period.The phytoplankton community included 60 Chlorophyta species,33 Bacillariophyta species,20 Cyanophyta species,and 14 Euglenophyta species.Mean phytoplankton density ranged between 5.09×10⁵ ind./L and 6.95×10⁶ ind./L,and that of biomass varied from 0.44 mg/L to 5.46 mg/L in different seasons.Results of CCA indicated that water temperature（WT）,total nitrogen（TN）,total phosphorus（TP）and ammonia nitrogen（NH₄⁺-N）were the most significant variables that influenced phytoplankton.There were 78 zooplankton species in the lake,including 17 protozoa species,37 rotifera species,12 cladocera species and 12 copepods species.Mean zooplankton density ranged between 1474.1 ind./L and 5266.3 ind./L,and that of biomass varied from 0.72 mg/L to 3.13 mg/L in different seasons.Results of CCA revealed that WT,Secchi disk depth（SD）,nutrient concentrations were the most significant factors that influenced zooplankton.2)The relationship between phytoplankton species richness and community biomass varied in different seasons: in winter and spring,phytoplankton biomass increased with increasing species richness;while in summer,there was a negative relationship between the two variables.Thus,phytoplankton communities with larger number of coexisting species will generate higher biomass when the temperature was low,but lower biomass when the temperature was high.Zooplankton species richness had positive effects on community biomass in all the four seasons.The species richness of both rotifera and copepods had strong influence on their biomasses.Phytoplankton species diversity also had impact on zooplankton community biomass.In spring,there was a positive relationship between phytoplankton species richness and zooplankton biomass.Phytoplankton species richness influenced zooplankton biomass mainly through increasing the biomasses of protozoa and cladocera.In other seasons,the species richness of many phytoplankton taxa had a positive influence on the biomasses of many zooplankton taxa.The species richness of protozoa had a strong negative influence on the biomass of Cyanophyta.Therefore,increasing the species richness of protozoa may be a effective way to prevent the out break of Cyanophyta.Phytoplankton species richness was positively correlated with RUE,but negatively correlated with zooplankton RUE.Thus,phytoplankton communities with higher diversity are more efficient in resource use and more resistant to predation.In most seasons,zooplankton species richness was unimodally correlated with RUE,but had no relationship with phytoplankton RUE.With the increase of Cyanophyta biomass proportion,phytoplankton RUE continued to increase while that of zooplankton decreased.The concentration of TP also had strong negative influence on zooplankton RUE.3)There were significant differences among the mean TSI of different plankton taxa.Rotifera,cladocera and copepods were the most stable taxa,followed by protozoa,and the stability of the phytoplankton taxa was the lowest.There were complex relationships between species richness and temporal stability of different phytoplankton taxa: a unimodal relationship for Cyanophyta and Bacillariophyta;a strong concave relationship for Euglenophyta;and no apparent relationship for Chlorophyta and total phytoplankton.‘Portfolio effect’ was the dominant mechanism.Zooplankton species richness also had influence on its TSI.The species richness of protozoa,rotifera and total zooplankton had positive influence on their TSI.The influence of cladocera and copepods diversity on TSI was not significant.Zooplankton diversity influence TSI mainly through ‘overyielding’ and ‘portfolio effect’.The diversity of phytoplankton and zooplankton could also influence the TSI of each other.Phytoplankton species richness had a significant positive influence on the TSI of protozoa,rotifera and total zooplankton,while its influence on cladocera and copepods was not significant.‘Trophic overyielding’ and a weak ‘trophic portfolio effect’ were the main mechanisms involved.The species richness of zooplankton had a positive relationship with the TSI of Cyanophyta,a concave relationship with the TSI of Euglenophyta,and no relationship with the TSI of Chlorophyta,Bacillariophyta and total phytoplankton.Zooplankton diversity influence phytoplankton TSI mainly through weak ‘Trophic overyielding’,‘trophic portfolio effect’ and ‘trophic species asynchrony’.4)There were complex relationships between the mean environmental factors and phytoplankton TSI: a positive relationship for dissolved oxygen（DO）and p H,a negative relationship for TN and NH₄⁺-N,a unimodal relationship for TP,and no relationship for WT.Mean values of DO and p H mainly increased the stability of phytoplankton through increasing the average total biomass;while that of TN and NH₄⁺-N destabilized phytoplankton primarily through increasing the variability（standard deviations）of phytoplankton.There were also complex relationships between the variability of environmental factors（measured by the standard deviations of environmental factors in the long term observation）and phytoplankton TSI: a negative relationship for TN,a unimodal relationship for NH₄⁺-N and TP,and no relationship for WT,DO and p H.The variability of nutrient concentrations mainly affected phytoplankton TSI through influencing the variability of community biomass.Therefore,at a local scale,both the temporal mean and variability of nutrients will decrease the temporal stability of phytoplankton.Results in this study will be helpful in understanding the effects of plankton diversity on community biomass and stability in natural aquatic ecosystems,and will be helpful in guaranteeing the water quality and ecological security of Lake Nansihu.