Living in a changing world Rising CO₂ and its impact on lake phytoplankton

Phytoplankton convert CO₂ and sunlight into biomass, and form the base of freshwater and marine food webs. Cyanobacteria are one of the most ancient and ubiquitous phytoplankton groups. However, some cyanobacteria are toxic and their dense blooms are often deteriorating water quality in many lakes and reservoirs across the globe. Cyanobacterial blooms are predicted to increase with global warming, but how they are affected by rising atmospheric CO₂ levels has not yet been resolved. In this thesis, we try to improve the understanding of the impacts of rising atmospheric CO₂ on freshwater phytoplankton communities, with a particular emphasis on potentially toxic bloom-forming cyanobacteria. To tackle this main goal, we developed several distinctive approaches, including laboratory experiments with isolated cyanobacterial strains to measure their carbon uptake kinetics, competition experiments with mixtures of different strains and species, the application of mathematical models to predict the population dynamics and competitive interactions, a field campaign to investigate natural selection in the wild, as well as the analysis of large-scale variation in phytoplankton community composition in relation to temperature and dissolved inorganic carbon across ~1,000 lakes. Our results strongly show that future phytoplankton blooms are likely to differ significantly from present-day blooms, not only in terms of their intensity and frequency but also in terms of their physiological traits, genetic composition, and species composition.