Global Change - The rise in atmospheric CO2 levels, which is predicted to reach up to 1000 µatm within this century, will result in profound alterations of the marine environment (see figure). Firstly, as a greenhouse gas, rising CO2 causes ocean warming: average temperatures of the surface waters have already increased by 0.7°C and may rise by an additional 3°C until the end of this century, with some regions like the Arctic Ocean changing fastest (IPCC 2007). The warming and the concomitant freshening due to ice melts increase the stratification of the surface ocean, which not only reduces the nutrient supply from deeper waters but also alters the light regime phytoplankton encounter in the shallower upper mixed layer. However, there is a secondary problem arising from CO2: As much of the anthropogenic CO2 is taken up by the ocean, concentrations of CO2 and bicarbonate increase while the concentration of carbonate ions and the pH decrease, also known as 'Ocean Acidification'. Since the industrial revolution, the mean pH of the surface ocean has decreased by about 0.1 pH units and is expected to further decline by 0.3 units until the end of this century, which represents a ~150% increase in acidity. To sum up, several environmental parameters will be altered simultaneously, and all of these are major drivers controlling phytoplankton productivity and species composition.