Unicellular, eukaryotic organisms - known as protists - form the base of all aquatic food webs. Frequently, marine protists are divided into either phytoplankton or (proto)zooplankton. Phytoplankton use phototrophy to acquire their energy from light to fix carbon dioxide into organic carbon, while protozooplankton use phagotrophy to directly acquire organic carbon from their prey. Mixoplankton that employ mixotrophy, i.e. the combination of phototrophy and phagotrophy within one cell, are often neglected. However, many marine protists are mixoplankton and they are ubiquitous in the worlds’ oceans. In oligotrophic oceans, mixoplankton are the base of food webs and many harmful algal blooms are formed by mixoplankton. Yet, the concept of mixoplankton is slow to mature within coastal water management. This thesis hypothesizes that the whole protist community, including mixoplankton, needs to be taken into account to understand and predict the effect of anthropogenic pressures on coastal systems. This thesis is a cumulative summary of three papers that employ data analysis, model developments and modelling scenarios to test this hypothesis. As a study area the Southern North Sea was chosen as it is an exceptionally well sampled coastal sea that is forecast to be heavily modified in the future. In a first step, routine monitoring data from the Southern North Sea were analyzed. The data analysis showed that the relative occurrence of mixoplankton was highest in seasonally stratified, clear, dissolved inorganic nutrient depleted environments. In a second step, a mathematical model, called PROTIST, was developed with the aim to reproduce the trophic composition of protist communities across abiotic gradients. Not only was PROTIST capable of reproducing the trophic composition of protist communities in the Southern North Sea, a sensitivity analysis conducted on the model results also showed that the occurrence of mixoplankton in the Southern North Sea is driven mainly by the availability of dissolved inorganic phosphate and silica and not by the availability of light. In a third step, PROTIST was used in a 3D model scenario of the North Sea to research whether the planned intensification of seaweed aquaculture affects the composition of protist communities. Preliminary 3Dmodel results show that seaweed aquaculture in the Southern North Sea could decrease nutrient concentrations in winter and lead to an increase in mixoplankton biomass. Pooling the information gained from the different approaches, this thesis concludes that coastal zone management should take mixoplankton into account to understand and predict the effect of future anthropogenic pressures on coastalecosystems. / Doctorat en Sciences agronomiques et ingénierie biologique / info:eu-repo/semantics/nonPublished