Thermal responses of marine phytoplankton: Implications to their biogeography in the present and future oceans

Phytoplankton are ecologically significant as primary producers and as regulators of the biogeochemical cycle. However, some may form harmful algal blooms that are a global problem due to the production of toxins that pose a risk to public health, the environment, and our economy. Climate change poses a serious threat to phytoplankton communities. It is, therefore, crucial to advance our knowledge on how they respond to the changes in temperature that is projected to increase in the next decades. The main aim of this thesis is to investigate how temperature limits biogeography, growth, toxin production, and competition in marine phytoplankton. To achieve this aim, the thesis presents a series of chapters with independent objectives. In Chapter 2, I analysed a global dataset of species occurrence data to examine the global patterns in the realised thermal niche and geographic range of marine phytoplankton. In Chapter 3, I investigated the global patterns of thermal traits, thermal sensitivity, and exposure and vulnerability to warming in marine phytoplankton. In Chapter 4 and 5, I conducted laboratory experiments to examine the temperature dependence of growth and toxin production in marine dinoflagellates. In Chapter 6, I also conducted laboratory experiments to test the effect of increased temperature on growth and competition in marine phytoplankton using dinoflagellates as test organisms. The key results of this thesis are as follows: (1) the current distribution of marine phytoplankton is limited by temperature, (2) their thermal traits are contingent on their biogeography and phylogeny, (3) their growth and toxin production is affected by temperature, and (4) interspecific competition in dinoflagellates is altered by increasing temperature. The findings of this thesis advance our current predictive understanding of the ecological responses of marine phytoplankton to climate change.