The growth, sinking and remineralisation of phytoplankton constitute the soft-tissue carbon pump. Here we derive the expression for the strength of this carbon pump in terms of the nutrient utilisation and the carbon:nutrient ratio of plankton. Furthermore, we aim to determine how the physical environment affects this carbon pump. Thereto, we have used a plankton physiological model with a novel representation of the effect of temperature and light on plankton stoichiometry; it predicts that the effect of temperature on the carbon:nutrient ratio depends on the light conditions. Using this model combined with a mechanistic model for the mixed layer, we have studied the biological carbon pump at the BATS site and at various locations along a meridional transect through the Atlantic Ocean. Our results suggest that the soft-tissue carbon pump is stronger at mid-latitudes than in the (sub-)tropics and near the poles as a consequence of latitudinal variations in algal stoichiometry.