Impacts of elevated CO2 on particulate and dissolved organic matter production: microcosm experiments using iron-deficient plankton communities in open subarctic waters

Response of phytoplankton to increasing CO 2 in seawater in terms of physiology and ecology is key to predicting changes in marine ecosystems. However, responses of natural plankton communities especially in the open ocean to higher CO 2 levels have not been fully examined. We conducted CO 2 manipulation experiments in the Bering Sea and the central subarctic Pacific, known as high nutrient and low chlorophyll regions, in summer 2007 to investigate the response of organic matter production in iron-deficient plankton communities to CO 2 increases. During the 14-day incubations of surface waters with natural plankton assemblages in microcosms under multiple pCO 2 levels, the dynamics of particulate organic carbon (POC) and nitrogen (PN), and dissolved organic carbon (DOC) and phosphorus (DOP) were examined with the plankton community compositions. In the Bering site, net production of POC, PN, and DOP relative to net chlorophyll- a production decreased with increasing pCO 2 . While net produced POC:PN did not show any CO 2 -related variations, net produced DOC:DOP increased with increasing pCO 2 . On the other hand, no apparent trends for these parameters were observed in the Pacific site. The contrasting results observed were probably due to the different plankton community compositions between the two sites, with plankton biomass dominated by large-sized diatoms in the Bering Sea versus ultra-eukaryotes in the Pacific Ocean. We conclude that the quantity and quality of the production of particulate and dissolved organic matter may be altered under future elevated CO 2 environments in some iron-deficient ecosystems, while the impacts may be negligible in some systems.