Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 5 3 2008 10.5194/bg-5-847-2008 http://www.biogeosciences.net/5/847/2008/ http://www.biogeosciences.net/5/847/2008/bg-5-847-2008.html http://www.biogeosciences.net/5/847/2008/bg-5-847-2008.pdf 847 864 2008-05-19 Climate-mediated changes to mixed-layer properties in the Southern Ocean: assessing the phytoplankton response P. W. Boyd pboyd@alkali.otago.ac.nz S. C. Doney R. Strzepek J. Dusenberry K. Lindsay I. Fung NIWA Centre for Chemical and Physical Oceanography, Dept. of Chemistry, University of Otago, Dunedin, New Zealand Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Department of Chemistry, University of Otago, Dunedin, New Zealand Climate and Global Dynamics, National Center for Atmospheric Research Boulder, CO 80307, USA Department of Earth & Planetary Science, Berkeley Institute of the Environment, Berkeley, CA 94720-1250, USA Concurrent changes in ocean chemical and physical properties influence phytoplankton dynamics via alterations in carbonate chemistry, nutrient and trace metal inventories and upper ocean light environment. Using a fully coupled, global carbon-climate model (Climate System Model 1.4-carbon), we quantify anthropogenic climate change relative to the background natural interannual variability for the Southern Ocean over the period 2000 and 2100. Model results are interpreted using our understanding of the environmental control of phytoplankton growth rates – leading to two major findings. Firstly, comparison with results from phytoplankton perturbation experiments, in which environmental properties have been altered for key species (e.g., bloom formers), indicates that the predicted rates of change in oceanic properties over the next few decades are too subtle to be represented experimentally at present. Secondly, the rate of secular climate change will not exceed background natural variability, on seasonal to interannual time-scales, for at least several decades – which may not provide the prevailing conditions of change, i.e. constancy, needed for phytoplankton adaptation. Taken together, the relatively subtle environmental changes, due to climate change, may result in adaptation by resident phytoplankton, but not for several decades due to the confounding effects of climate variability. 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