Impacts of elevated CO₂ on phytoplankton community composition and organic carbon dynamics in nutrient-depleted Okhotsk Sea surface waters

Abstract. Impacts of the increasing CO₂ in seawater (i.e. ocean acidification) on phytoplankton physiology may have various and potentially adverse effects on phytoplankton dynamics and the carbon cycle. We conducted a CO₂ manipulation experiment in the Sea of Okhotsk in summer 2006 to investigate the response of the phytoplankton assemblage and dynamics of organic carbon. During the 14-day incubation of nutrient-depleted surface water with a natural phytoplankton assemblage under 150, 280, 480, and 590 μatm pCO₂, the relative abundance of fucoxanthin-containing phytoplankton such as diatoms and prymnesiophytes decreased with increasing pCO₂. The amount of DOC accumulation also decreased with increasing pCO₂, while differences in POC accumulation between the treatments were small and did not show a clear trend with the pCO₂. Change in the phytoplankton community composition under different CO₂ conditions will alter the organic carbon dynamics as found in the present experiment. Compared to results in the literature from nutrient-replete conditions indicating a potential enhancement of phytoplankton production with elevated CO₂, the present results indicated a different physiological response of phytoplankton under nutrient-depleted conditions. These results indicate that the continuing increase in atmospheric CO₂ can significantly affect the structure of marine ecosystems and carbon cycle in nutrient-depleted subpolar surface waters.