Blue-Green Algae in a Greenhouse Century? New Insights from Field Data on Climate Change Impacts on Cyanobacteria Abundance

Climate warming is likely to impact phytoplankton communities by providing a habitat in which cyanobacteria have competitive advantage over other phytoplankton taxa. We used extreme hot weather periods to investigate the potential impact of climate change on cyanobacteria abundance in three large and deep peri-alpine lakes, Lakes Geneva, Annecy, and Bourget. Between 2000 and 2011, there were four extreme warm weather periods: spring and summer 2003, autumn 2006 and winter 2007. We found that the consequences of extreme air temperatures on cyanobacteria abundance and phytoplankton composition depend on the time of year in which the extreme temperatures occur. In all three lakes studied, a warm summer did not clearly promote cyanobacteria blooms, whereas a warm autumn promoted cyanobacteria growth in the mesotrophic Lakes Geneva and Bourget, but not in the oligotrophic Lake Annecy. A warm winter was associated with high cyanobacteria abundance and a high contribution of cyanobacteria to total phytoplankton biomass. Our results reinforce the idea that lakes have an ecological memory by showing that a warm winter can influence subsequent seasonal succession in the cyanobacteria community. In both mesotrophic lakes studied, cyanobacteria abundance was strongly influenced by phosphorus concentrations and winter air temperatures. We conclude that although extreme hot weather periods can be used to analyze various aspects of the impacts of climate change, they are of limited value in forecasting the structure of phytoplankton communities in a warmer future.