To evaluate the influence of episodic extreme ocean acidification events in coastal regions, we deployed eight pelagic mesocosms for 53 days (Mai to July) 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 planktonic organisms inside and outside the mesocosms were measured over the course of the experiment in regular intervals. The data stems from 55µm and 500µm Apstein net hauls and subsequent microscopic analyses.