Abstract
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.
Highlights
Other processes that contributed to the initial label decrease were the subsequent CO2 additions, which diluted the 13CDIC pool and respiration of unlabelled organic material
While most of the CO2 enrichment mesocosm experiments involved inorganic nutrient addition and focussed on production and export food chains, this study investigated ocean acidification in a nutrient regenerating food chain, at least during phase 1 of the experiment
Our findings suggest that CO2 effects on some processes are stepwise rather than gradual, which can be of interest for future research
Summary
We aimed to increase our understanding of CO2 effects on primary production, community respiration, and export in Arctic communities by looking at individual uptake and loss rates and by quantifying the interactions between food web compartments with a food web model
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