Chair: Jean-Pierre Gattuso

K. G. Schulz^(1,2), R. G. J. Bellerby^(3,4), R. Bermúdez^(2,5), J. Büdenbender⁽²⁾, T. Boxhammer⁽²⁾, J. Czerny⁽²⁾, A. Engel⁽²⁾, S. Febiri⁽²⁾, A. Ludwig⁽²⁾, M. Meyerhöfer⁽²⁾, A. Larsen⁽⁶⁾, A. Paul⁽²⁾, M. Sswat⁽²⁾, and U. Riebesell⁽²⁾

¹ Centre for Coastal Biogeochemistry, School of Environment, Science and Engineering, Southern Cross University, P.O. Box 157, Lismore, NSW 2480, Australia
² GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
³ SKLEC-NIVA Centre for Marine and Coastal Research, East China Normal University, Zhongshan N. Road, 3663, Shanghai 200062,China
⁴ Norwegian Institute for Water Research, Regional Office Bergen, Thormøhlensgate 53 D, N-5006 Bergen, Norway
⁵ Facultad de Ingeniería Marítima, Ciencias Biológicas, Oceánicas y Recursos Naturales. Escuela Superior Politécnica del Litoral, ESPOL, Guayaquil – Ecuador
⁶ Uni Research Environment, Bergen, Norway

Background
At the base of the marine food-web phytoplankton is a key player in the global carbon cycle with feedbacks to the climate system. Rising levels of atmospheric carbon dioxide (CO₂), driven by a variety of human activities are changing climate and seawater carbonate chemistry by increasing CO₂ levels and decreasing pH, termed ocean acidification. How this might affect marine phytoplankton in terms of abundance and species composition is a pressing question as such changes can influence marine productivity, transfer to higher trophic levels and modify marine export production.

Methods
To study the effects of ocean acidification on marine phytoplankton and potential impacts on ecosystem functioning and biogeochemical element cycling, experiments are required that enclose natural plankton communities as a whole. Several such larger-scale studies have been realised in the last years by mooring mesocosms containing up to 75 m³ of seawater each in sheltered areas. Here we will focus on the 2011 KOSMOS (Kiel Off-Shore Mesocosms for Ocean Simulations) campaign in the Raunefjord, Norway, where plankton community composition, particulate and dissolved organic matter production and loss was followed for five weeks in a gradient of CO₂ levels ranging initially from 300 to 3000 μatm. Additionally, we will put the phytoplankton community composition resposnes into a perspective by comparing them with a number of other published experiments.

Findings
Direct and indirect effects of elevated CO₂ and associated carbonate chemistry speciation were found on several phytoplankton groups, with picoeukaryotes (chlorophytes, cryptophytes) and Synechococcus being positively affected and the coccolithophore Emiliania huxleyi and diatoms in certain phases of the experiment being negatively impacted. These CO₂ induced responses in phytoplankton community composition were reflected in net oxygen production and loss of particulate organic material into the sediment traps at 25m depth.

Conclusions
Ocean acidification can influence phytoplankton size and community structure with direct consequences for marine productivity and carbon export potential.