North Sea phytoplankton under global change, a multiple driver mesocosm approach

Phytoplankton is a major contributor to the total primary production in the North Sea. Yet the structure and functioning of phytoplankton communities are threatened by environmental degradation. Ocean acidification, rising water temperature and changes in nutrient inputs into the North Sea are among the global change processes expected to have the most intense effect on phytoplankton communities in this region. Even though these impacts have been addressed by several studies, previous work have essentially focused on single stressor effects and single species responses. Here, we conducted a mesocosm experiment on North Sea natural phytoplankton communities during the autumn bloom of 2018. We manipulated multiple drivers simultaneously, according to RCP scenarios (Representative Concentration Pathway) developed by the Intergovernmental Panel on Climate Change (IPCC) combined with predicted shifts in N:P ratios: Ambient (ambient temperature and atmospheric ppm CO2; 16 N:P), RCP 6.0 (+1.5°C and 800 ppm CO2; 25 N:P ratio), and RCP 8.5 (+3°C and 1000 ppm CO2; 25 N:P ratio). Our results show that phytoplankton autum bloom duration will be shorter under the environmental conditions predicted for 2100 by both scenarios. During bloom, the treatment RCP 8.5 scenario had low-to-moderate primary production and different community composition in comparison to the ambient. The experiment showed that impacts are directly linked to the intensity of environmental change as well. Overall, our work indicates that important shifts in traits and processes within phytoplankton communities are to be expected in the future, if global change mitigation actions do not achieve the goals intended by the 2016 Paris agreement