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

The assessment of direct effects of anthropogenic CO 2 increase on the marine biota has received relatively little attention compared to the intense research on CO 2 -related responses of the terrestrial biosphere. Yet, due to the rapid air'sea gas exchange, the observed past and predicted future rise in atmospheric CO 2 causes a corresponding increase in seawater CO 2 concentrations, [CO 2 ], in upper ocean waters. Increasing [CO 2 ] leads to considerable changes in the surface ocean carbonate system, resulting in decreases in pH and the carbonate concentration, [CO 2- 3 ]. These changes can be shown to have strong impacts on the marine biota. Here we will distinguish between CO 2 -related responses of the marine biota which (a) potentially affect the ocean's biological carbon pumps and (b) are relevant to the interpretation of diagnostic tools (proxies) used to assess climate change on geological times scales. With regard to the former, three direct effects of increasing [CO 2 ] on marine plankton have been recognized: enhanced phytoplankton growth rate, changing elemental composition of primary produced organic matter, and reduced biogenic calcification. Although quantitative estimates of their impacts on the oceanic carbon cycle are not yet feasible, all three effects increase the ocean's capacity to take up and store atmospheric CO 2 and hence, can serve as negative feedbacks to anthropogenic CO 2 increase. With respect to proxies used in palaeo-reconstructions, CO 2 -sensitivity is found in carbon isotope fractionation by phytoplankton and foraminifera. While CO 2 - dependent isotope fractionation by phytoplankton may be of potential use in reconstructing surface ocean pCO 2 at ancient times, CO 2 -related effects on the isotopic composition of foraminiferal shells confounds the use of the difference in isotopic signals between planktonic and benthic shells as a measure for the strength of marine primary production. The latter effect also offers an alternative explanation for the large negative swings in δ 13 C of foraminiferal calcite between glacial and interglacial periods. Changes in [CO 2 3 ] affect the δ 18 O in foraminiferal shells. Taking this into account brings sea surface temperature estimates for the glacial tropics closer to those obtained from other geochemical proxies.