Impacts of elevated CO2 on organic carbon dynamics in nutrient depleted Okhotsk Sea surface waters

Increasing CO2 in seawater (i.e. ocean acidification) may have various and potentially adverse effects on phytoplankton dynamics and hence the organic carbon dynamics. We conducted a CO2 manipulation experiment in the Sea of Okhotsk in summer 2006 to investigate the response of the organic carbon dynamics. During the 14-day incubation of nutrient depleted and 200 μatm in situ pCO2 surface water with a natural plankton assemblage under 150, 280, 480, and 590 μatm pCO2, the amount of net dissolved organic carbon accumulation was significantly lower at > 480 μatm pCO2 than at 150 μatm pCO2, while differences in net particulate organic carbon accumulation between the treatments were small and did not show a clear relationship with the pCO2. This is the first report to show a decreased net organic carbon production of natural plankton community under elevated pCO2. Phytoplankton pigment analysis suggests that the relative contribution of fucoxanthin-containing phytoplankton such as diatoms to the phytoplankton biomass was lower at > 280 μatm pCO2 than at 150 μatm pCO2. Different pCO2 conditions may alter the organic carbon dynamics through changes in plankton processes. We conclude that the continuing increase in atmospheric CO2 in a time scale from the last half century to the end of this century has potential to affect the carbon cycle in nutrient depleted subpolar surface waters.