1000 µatm by the end of this century (RCP6.0 scenario; IPCC 2014). The dissolution of CO 2 in seawater alters its chemistry by increasing the dissolved CO 2 concentration and lowering pH (called 'ocean acidifi-cation', OA). Since the beginning of the industrial revolution, the ocean has absorbed about a third of the anthropogenic CO 2 emissions. Among the world oceans, the Southern Ocean (SO) sequesters a disproportionally large share of anthropogenic CO 2 , accounting for about 40% of the global oceanic uptake of anthropogenic CO 2 (Sabine et al. 2004, Landschützer et al. 2015). However, in this region the biological sequestration potential is constrained by iron (Fe) input (Martin et al. 1990, Boyd et al. 2007,

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The Western Antarctic Peninsula (WAP), one of the most productive regions of the Southern Ocean, is currently undergoing rapid environmental changes such as ocean acidification (OA) and increased daily irradiances from enhanced surface-water stratification. To assess the potential for future biological CO 2 sequestration of this region, we incubated a natural phytoplankton assemblage from Ryder Bay, WAP, under a range of pCO 2 levels (180 μatm, 450 μatm, and 1000 μatm) combined with either moderate or high natural solar radiation (MSR: 124 μmol photons m −2 s −1 and HSR: 435 μmol photons m −2 s −1 , respectively). The initial and final phytoplankton communities were numerically dominated by the prymnesiophyte Phaeocystis antarctica, with the single cells initially being predominant and solitary and colonial cells reaching similar high abundances by the end. Only when communities were grown under ambient pCO 2 in conjunction with HSR did the small diatom Fragilariopsis pseudonana outcompete P. antarctica at the end of the experiment. Such positive light-dependent growth response of the diatom was, however, dampened by OA. These changes in community composition were caused by an enhanced photosensitivity of diatoms, especially F. pseudonana, under OA and HSR, reducing thereby their competitiveness toward P. antarctica. Moreover, community primary production (PP) of all treatments yielded similar high rates at the start and the end of the experiment, but with the main contributors shifting from initially large to small cells toward the end. Even though community PP of Ryder Bay phytoplankton was insensitive to the changes in light and CO 2 availability, the observed size-dependent shift in productivity could, however, weaken the biological CO 2 sequestration potential of this region in the future.

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