To evaluate the influence of episodic extreme ocean acidification events in coastal regions, we deployed eight pelagic mesocosms for 53 days in Raunefjord, Norway, and enclosed 60 m³ of local seawater containing a natural plankton community under post-bloom conditions. Four mesocosms were manipulated to simulate extreme pCO2 levels of 2069 µatm while the other four served as untreated controls. To monitor the effects of extreme pCO2 conditions, a variety of physical, ecological, and biogeochemical parameters inside and outside the mesocosms were measured over the course of the experiment in regular intervals.