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CO 2 increases 14 C primary production in an Arctic plankton community

A. Engel,C. Borchard,J. Piontek,K. Schulz,U. Riebesell,R. Bellerby,R. Bellerby

Published 2012 in Biogeosciences

ABSTRACT

Responses to ocean acidification in plankton com- munities were studied during a CO2-enrichment experiment in the Arctic Ocean, accomplished from June to July 2010 in Kongsfjorden, Svalbard (78 56 0 2 00 N, 11 53 0 6 00 E). Enclosed in 9 mesocosms (volume: 43.9-47.6 m 3 ), plankton was ex- posed to CO2 concentrations, ranging from glacial to pro- jected mid-next-century levels. Fertilization with inorganic nutrients at day 13 of the experiment supported the accumu- lation of phytoplankton biomass, as indicated by two periods of high chl a concentration. This study tested for CO2 sensitivities in primary produc- tion (PP) of particulate organic carbon (PPPOC) and of dis- solved organic carbon (PPDOC). Therefore, 14 C-bottle incu- bations (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. Compari- son between mesocosms revealed significantly higher PP POC at elevated compared to low pCO2 after nutrient addition. PPDOC was significantly higher in CO 2-enriched mesocosms before as well as after nutrient addition, suggesting that CO 2 had a direct influence on DOC production. DOC concentra- tions inside the mesocosms increased before nutrient addi- tion and more in high CO2 mesocosms. After addition of nutrients, however, further DOC accumulation was negligi- ble and not significantly different between treatments, indi- cating 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 PPDOC provided on average sufficient carbon for synthesis of bacte- rial biomass. During the later course of the bloom, the re- sponse of 14 C-based PP rates to CO2 enrichment differed from net community production (NCP) rates that were also determined during this mesocosm campaign. 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.

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