Interactive Effects of High CO2 and Temperature on Growth and Elemental Composition of the Western and Eastern Subarctic North Pacific Phytoplankton Communities

Increasing atmospheric CO₂ could increase ocean acidity via physical and chemical interactions between atmosphere and ocean surface. Associated with the CO₂ induced global warming, interactive effects of high CO₂ and temperature could be important factors modulating ecosystem processes. Here, we conducted two manipulation experiments using the western and eastern subarctic North Pacific phytoplankton communities, structure of which are different from each other, aboard R/V Mirai during summer 2014. The three CO₂ levels were set at present (in situ condition), 500 μatm CO₂, and ~800 μatm CO₂, and two temperature series (in situ and +4°C) were established. Net specific growth rate increased as increased temperature only in the western experiment, whereas maximum chlorophyll-a concentrations decreased in the high-temperature conditions of the eastern experiment. In both experiments, the high CO₂ conditions slightly but insignificantly decreased the net specific growth rate. The growth of pico-sized Synechococcus increased under the high temperature and CO₂ conditions in both experiments, whereas cell yields of nano-sized phytoplankton decreased under the high-temperature conditions in the eastern experiment. Particulate organic carbon to particulate nitrogen (C:N) ratio increased with increasing temperature and CO₂ only after the nutrient depletions in the western experiment. Overall, increasing temperature could positively affect the growth of western subarctic phytoplankton, specifically Synechococcus. In the eastern experiment, maximum biomass and larger cell phytoplankton decreased and Synechococcus increased as increased temperature. Both of these results in the high-temperature conditions could have negative impacts on the transfer efficiency of primary production to the higher trophic levels and the efficiency of the biological pump. The effect of high-CO₂ was not dramatic as compared with temperature effect, but high-CO₂ condition substantially modify the dynamics of phytoplankton communities. Our study suggests that the possible environmental perturbations in the future could alter ecosystem processes in the subarctic North Pacific via changing phytoplankton dynamics.