The Effects of Temperature on the Growth and Competition of Phytoplankton

With aquatic environments changing in the Anthropocene at unprecedented rates, phytoplankton are under enormous abiotic pressure to cope with environmental conditions they did not encounter in the past. In this thesis, I investigated the growth and competitive response of algae to changes in temperature and nitrogen concentrations across high resolution temperature gradients. The first three data chapters of this thesis concentrated on the response of isolated algae species to changes in their thermal environment. Focus lay on whether thermal adaptations could occur when culturing algae for extended periods of time at elevated temperatures. My results showed that warming does not necessarily lead to thermal adaptation and that the responses will not be as simple as a mere shift in thermal performance curves towards warmer temperatures. For the last two data chapters I focused on mixed populations of the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana and how they compete across stable and fluctuating temperatures under high and low nitrate concentrations. I could show that under stable thermal conditions, the outcome of competition could be well predicted; both in nutrient-replete and nitrogen-limited conditions, but those predictions became less reliable under fluctuating temperatures. This thesis provides further insight into adaptation and competition in phytoplankton communities faced with environmental change. The gained knowledge can provide a better understanding of some of the uncertainties and limitations surrounding the prediction of future phytoplankton community composition from single species responses and teaches us to be cautious when it comes to extrapolating experimental results from the laboratory to natural environments.