KOSMOS Bergen 2015 mesocosm study: Hydrozoa, C. harengus, and Copepoda abundances and biomasses

To evaluate the influence of ocean acidification on predatory plankton, e.g. Hydrozoa and fish larvae as well as their interaction in complex natural communities, 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 investigate the interaction between Hydrozoa and fish larvae influenced by OA ee studied OA-induced changes at the top of the food web by following ≈2000 larvae of Atlantic herring (Clupea harengus) hatched inside each mesocosm during the first week of the experiment, and a Hydrozoa population that had already established inside the mesocosms. Organisms of both taxa 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, subsequent microscopic analyses as well as carbon to nitrogen measurements. Under OA, we detected 20% higher abundance of hydromedusae staged jellyfish, but 25% lower biomass. At the same time, survival rates of Atlantic herring larvae were higher under OA (control pCO2: 0.1%, high pCO2: 1.7%) in the final phase of the study.