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

Abstract. The effect of ocean acidification on the balance between gross community production (GCP) and community respiration (CR) (i.e., net community production, NCP) of plankton communities was investigated in summer 2010 in Kongsfjorden, west of Svalbard. Surface water, which was characterized by low concentrations of dissolved inorganic nutrients and chlorophyll a (a proxy of phytoplankton biomass), was enclosed in nine mesocosms and subjected to eight pCO 2 levels (two replicated controls and seven enhanced pCO 2 treatments) for one month. Nutrients were added to all mesocosms on day 13 of the experiment, and thereafter increase of chlorophyll a was provoked in all mesocosms. No clear trend in response to increasing pCO 2 was found in the daily values of NCP, CR, and GCP. For further analysis, these parameters were cumulated for the following three periods: phase 1 -end of CO 2 manipulation until nutrient addition (t4 to t13); phase 2 -nutrient addition until the second chlorophyll a minimum (t14 to t21); phase 3 -the second chlorophyll a minimum until the end of this study (t22 to t28). A significant response was detected as a decrease of NCP with increasing pCO 2 during phase 3. CR was relatively stable throughout the experiment in all mesocosms. As a result, the cumulative GCP significantly decreased with increasing pCO 2 during phase 3. After the nutrient addition, the ratios of cumulative NCP to cumulative consumption of NO 3 and PO 4 showed a significant decrease during phase 3 with increasing pCO 2 . The results suggest that elevated pCO 2 influenced cumulative NCP and stoichiometric C and nutrient coupling of the plankton community in a high-latitude fjord only for a limited period. However provided that there were some differences or weak correlations between NCP data based on different methods in the same experiment, this conclusion should be taken with caution.

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