Direct effects of CO2 concentration on growth and isotopic composition of marine plankton

The assessment of direct effects of anthropogenic CO2 increase on the marin e biota has received relatively little attention compared to the intense re search on CO2-related responses of the terrestrial biosphere. Yet, due to t he rapid air-sea gas exchange, the observed past and predicted future rise in atmospheric CO2 causes a corresponding increase in seawater CO2 concentr ations, [CO2], in upper ocean waters. Increasing [CO2] leads to considerabl e changes in the surface ocean carbonate system, resulting in decreases in pH and the carbonate concentration, [CO32-]. These changes can be shown to have strong impacts on the marine biota. Here we will distinguish between C O2-related responses of the marine biota which (a) potentially affect the o cean's biological carbon pumps and (b) are relevant to the interpretation o f diagnostic tools (proxies) used to assess climate change on geological ti mes scales. With regard to the former, three direct effects of increasing [ CO2] on marine plankton have been recognized: enhanced phytoplankton growth rate, changing elemental composition of primary produced organic matter, a nd reduced biogenic calcification. Although quantitative estimates of their impacts on the oceanic carbon cycle are not yet feasible, all three effect s increase the ocean's capacity to take up and store atmospheric CO2 and he nce, can serve as negative feedbacks to anthropogenic CO2 increase. With re spect to proxies used in paleo-reconstructions, CO2-sensitivity is found in carbon isotope fractionation by phytoplankton and foraminifera. While CO2- dependent isotope fractionation by phytoplankton may be of potential use in reconstructing surface ocean pCO(2) at ancient times, CO2-related effects on the isotopic composition of foraminiferal shells confounds the use of th e difference in isotopic signals between planktonic and benthic shells as a measure for the strength of marine primary production. The latter effect a lso offers an alternative explanation for the large negative swings in delt a(13)C of foraminiferal calcite between glacial and interglacial periods. C hanges in [CO32-] affect the delta(18)O in foraminiferal shells. Taking thi s into account brings sea surface temperature estimates for the glacial tro pics closer to those obtained from other geochemical proxies.