Oceans worldwide are under rapid changes due to climate perturbations; understanding the consequences of this phenomenon is a current major issue to keep environmental quality and ecosystem functioning. Climate change related effects, such as ocean acidification, temperature rising and alteration in nutrient concentrations, have direct influence on natural marine ecosystems and their energy fluxes. Phytoplankton communities are responsible for about 50% of the total primary production on Earth and play a key role in pelagic food webs. Given their importance, disturbances in this group might heavily impact the whole functioning of marine systems. Hence, this project aims to evaluate the effects of climate change on phytoplankton communities considering predicted shifts in abiotic parameters in the North Sea. Two mesocosm experiments will manipulate multiple stressors (pCO2, temperature and nutrients) to assess variations in the phytoplankton biomass, species composition, biodiversity, primary production and chemical food quality. The experiments will take place at the beginning of spring and end of summer, when the main natural phytoplankton blooms occur. Parallel bioassay experiments will provide additional information about limiting parameters in the system (nutrient, temperature, light).