Phytoplankton production during the austral summer in the Southern Ocean is known to be limited by iron and light. Distributions of satellite-detected chlorophyll-a (Chl-a) show very complex and time-variable patterns that are hard to explain. We analysed covariance between satellite-detected and modelled variables and show that this covariance in time between the mixed layer depth (MLD), sea surface temperature (SST) and Chl-a can be used to map areas where different factors control phytoplankton production. Statistically significant spatial patterns in the covariance between MLD, SST and Chl-a show that the physical factors controlling phytoplankton production in the Southern Ocean change in a predictable manner. Well-defined areas exist where phytoplankton is light-limited in the summer due to insufficient stratification and where phytoplankton is clearly limited by nutrients (probably iron). The boundary between light limitation and nutrient limitation can be sharp and may be associated with the main hydrographic fronts (e.g. the Subantarctic Front).