Synergistic effects of pCO₂ and iron availability on nutrient consumption ratio of the Bering Sea phytoplankton community

Abstract. Little is known concerning the effect of CO₂ on phytoplankton ecophysiological processes under nutrient and trace element-limited conditions, because most CO₂ manipulation experiments have been conducted under elements-replete conditions. To investigate the effects of CO₂ and iron availability on phytoplankton ecophysiology, we conducted an experiment in September 2009 using a phytoplankton community in the iron limited, high-nutrient, low-chlorophyll (HNLC) region of the Bering Sea basin . Carbonate chemistry was controlled by the bubbling of the several levels of CO₂ concentration (180, 380, 600, and 1000 ppm) controlled air, and two iron conditions were established, one with and one without the addition of inorganic iron. We demonstrated that in the iron-limited control conditions, the specific growth rate and the maximum photochemical quantum efficiency (F_v/F_m) of photosystem (PS) II decreased with increasing CO₂ levels, suggesting a further decrease in iron bioavailability under the high-CO₂ conditions. In addition, biogenic silica to particulate nitrogen and biogenic silica to particulate organic carbon ratios increased from 2.65 to 3.75 and 0.39 to 0.50, respectively, with an increase in the CO₂ level in the iron-limited controls. By contrast, the specific growth rate, F_v/F_m values and elemental compositions in the iron-added treatments did not change in response to the CO₂ variations, indicating that the addition of iron canceled out the effect of the modulation of iron bioavailability due to the change in carbonate chemistry. Our results suggest that high-CO₂ conditions can alter the biogeochemical cycling of nutrients through decreasing iron bioavailability in the iron-limited HNLC regions in the future.