Extreme events in the marine environment: The role of species-specific traits and adaptive strategies in harmful dinoflagellate bloom formation

In this thesis the competitive ability of two different potentially harmful dinoflagellates, Lingulodinium polyedrum and Alexandrium catenella, was investigated under different nutrient conditions and grazing control by zooplankton consumers. I particularly focused on the species-specific adaptive strategies mixotrophy and allelopathy. My results indicated that both strategies can be important for reducing competitors either through phagotrophic feeding or by lytic activity. Beside these benefits, both strategies also affected the cellular nutrient concentrations of the dinoflagellates, which indicated initial metabolic ‘costs’ for switching nutritional modes from photosynthesis to phagotrophic feeding, before having a benefit from ingested prey. Furthermore, the competitive success of L. polyedrum was potentially facilitated by selective zooplankton grazing on smaller prey in a more complex natural plankton community. My study emphasizes that bloom forming dinoflagellates may use very different strategies to become dominant in a plankton community, entailing different trophic interactions leading to altered food web dynamics.