Biological activity gives rise to a difference in carbon concentration between the ocean surface and the deep waters. This difference is determined by the carbon:nutrient ratio of the sinking organic material and it is crucial in determining the distribution of CO2 between the atmosphere and the ocean. For this reason, it is interesting to determine whether the physical environment affects the carbon:nitrogen ratio of phytoplankton. Using a model with a novel representation of the effect of temperature on phytoplankton stoichiometry, we have investigated the influence of mixed-layer depth and water temperature on the elemental composition of an algal community. In the light-limited regime, the carbon:nutrient ratio turns out to decrease with increasing mixed-layer depth and temperature. Hence our model suggests the existence of a positive feedback between temperature and atmospheric CO2 content through the stoichiometry of phytoplankton. This feedback may have contributed to the glacial/interglacial cycles in the atmospheric CO2 concentration.