A ¹³C labelling study on carbon fluxes in Arctic plankton communities under elevated CO₂ levels

Abstract. The effect of CO₂ on carbon fluxes (production, consumption, and export) in Arctic plankton communities was investigated during the 2010 EPOCA (European project on Ocean Acidification) mesocosm study off Ny Ålesund, Svalbard. ¹³C labelled bicarbonate was added to nine mesocosms with a range in pCO₂ (185 to 1420 μatm) to follow the transfer of carbon from dissolved inorganic carbon (DIC) into phytoplankton, bacterial and zooplankton consumers, and export. A nutrient–phytoplankton–zooplankton–detritus model amended with ¹³C dynamics was constructed and fitted to the data to quantify uptake rates and carbon fluxes in the plankton community. The plankton community structure was characteristic for a post-bloom situation and retention food web and showed high bacterial production (∼31% of primary production), high abundance of mixotrophic phytoplankton, low mesozooplankton grazing (∼6% of primary production) and low export (∼7% of primary production). Zooplankton grazing and export of detritus were sensitive to CO₂: grazing decreased and export increased with increasing pCO₂. Nutrient addition halfway through the experiment increased the export, but not the production rates. Although mixotrophs showed initially higher production rates with increasing CO₂, the overall production of POC (particulate organic carbon) after nutrient addition decreased with increasing CO₂. Interestingly, and contrary to the low nutrient situation, much more material settled down in the sediment traps at low CO₂. The observed CO₂ related effects potentially alter future organic carbon flows and export, with possible consequences for the efficiency of the biological pump.