Phytoplankton communities are recognized as important indicators for environmental changes since they usually respond rapidly to a changing environment in aquatic systems. To evaluate how changes in the climate and atmospheric deposition might impact phytoplankton communities, we examined a complete 14-year time series of different physical, chemical and biological variables from 13 nutrient-poor Swedish reference lakes along a latitudinal gradient. We found numerous significant changes in physical and chemical lake conditions that were highly coherent among the lakes, in particular for sulfate concentrations, surface water temperatures and intensity of thermal stratification. The phytoplankton biomass and species richness of six phytoplankton groups, measured at the same occasions as the water chemical variables, showed only few and weak significant changes over time. The only coherent significant change over Sweden, occurring in seven lakes, was observed in the species richness of chlorophytes. We claim that the response of the phytoplankton biomass and species richness to strong physical and chemical changes is comparatively weak and lake-specific. While many studies deal with the loss of species as a response to global change, we were able to show that some species can clearly profit from global changes. Analyzing long-term data and carrying out laboratory experiments we were able to detect a direct temperature effect on the growth rates of the nuisance alga Gonyostomum semen, which could explain the increases in the occurrence and abundance of this species over Sweden. From the results we predict that Gonyostomum semen will further increase in its distribution and abundance along with increasing temperatures. The findings of this thesis give new insights into the impact of global changes on phytoplankton community structures in nutrient-poor lakes.
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