Seasonal changes in nutrient concentrations, phytoplankton biomass and nutritional state were followed in two contrasting Antarctic lakes. Oligotrophic Sombre Lake receives little phosphate from its catchment and this limits the standing crop of phytoplankton which can develop in spring and summer. This situation appears to be typical of unenriched Maritime Antarctic lakes. Large numbers of seals and seabirds in the catchment of nearby Heywood Lake increase its phosphate loading and allow a much denser growth of phytoplankton. The N:P ratio is low and nitrogen rather than phosphate is the most limiting nutrient. Despite limitation of standing crop, photosynthetic rates in both lakes are relatively high and recycling of nutrients within the lake may be rapid.