CO2 increases 14C-primary production in an Arctic plankton community

Responses to ocean acidification in plankton communities were studied during a CO₂-enrichment experiment in the Arctic Ocean, accomplished from June to July 2010 in Kongsfjorden, Svalbard (78°56, 2' N, 11°53, 6' E). Enclosed in 9 mesocosms (volume: 43.9–47.6 m³), plankton was exposed to CO₂ concentrations, ranging from glacial to projected mid-next-century levels. Fertilization with inorganic nutrients at day 13 of the experiment supported the accumulation of phytoplankton biomass, as indicated by two periods of high Chl a concentration. This study tested for CO₂ sensitivities in primary production (PP) of particulate organic carbon (PP_POC) and of dissolved organic carbon (PP_DOC). Therefore, ¹⁴C-bottle incubations (24 h) of mesocosm samples were performed at 1 m depth receiving about 60% of incoming radiation. PP for all mesocosms averaged 8.06 ± 3.64 μmol C l⁻¹ d⁻¹ and was slightly higher than in the outside fjord system. Comparison between mesocosms revealed significantly higher PP_POC at elevated compared to low pCO₂ after nutrient addition. PP_DOC was significantly higher in CO₂ enriched mesocosms before as well as after nutrient addition, suggesting that CO₂ had a direct influence on DOC production. DOC concentrations inside the mesocosms increased before nutrient addition and more in high CO₂ mesocosms. After addition of nutrients, however, further DOC accumulation was negligible and not significantly different between treatments, indicating rapid utilization of freshly produced DOC. Bacterial biomass production (BP) was coupled to PP in all treatments, indicating that 3.5 ± 1.9% of PP, or 21.6 ± 12.5% of PP_DOC provided sufficient carbon for synthesis of bacterial biomass. The response of ¹⁴C-based PP rates to CO₂ enrichment was at odds with O₂-based net community production (NCP) rates that were also determined during this study, albeit at lower light level. We conclude that the enhanced release of labile DOC during autotrophic production at high CO₂ exceedingly stimulated activities of heterotrophic microorganisms. As a consequence, increased PP induced less NCP, as suggested earlier for carbon limited microbial systems in the Arctic.