On-going global warming and eutrophication are expected to promote cyanobacterial dominance worldwide. Although increased lake temperature and nutrients are well-established drivers of blooms, the mechanisms that determine cyanobacterial biomass are complex, with potentially direct, indirect and interactive effects. Cyanobacteria can produce toxins that constitute a considerable risk for animal and human health. Such global range phenomena should be studied at a wide spatial scale, to directly compare phytoplankton response in different lake types across contrasting climatic zones. During this dissertation, the European Multi-lake survey (EMLS) was organised in order to sample lakes across Europe and disentangle the effect of environmental stressors on potentially toxic cyanobacterial blooms. The results demonstrated that the distribution of cyanobacterial toxins and the toxic potential in lakes was highly dependent on direct and indirect effects of temperature. Nutrients interacted synergistically with increased lake temperatures to promote cyanobacterial growth more than that of other phytoplankton taxa. Providing continental scale evidence is highly significant for the development of robust models that could predict cyanobacterial or algal response to environmental change.