Alkaline phosphatase expression by phytoplankton from two sites in Winyah Bay, SC, USA was investigated using nutrient-addition bioassays and cell-specific enzyme-labeled fluorescence (ELF) measurements. Our aim was to determine whether expression was group- or species-specific within the phytoplankton community. Diatoms dominated the riverine site in May, the coastal site in July, and both sites in August. Phytoplankton growth was limited by nitrogen (N) availability at the coastal site in May and the riverine site in August, but phosphate limitation was not observed. Alkaline phosphatase expression ranged from ∼30% of cells enumerated to less than 1% and was significantly reduced by inorganic phosphorus (P; 10 μM P) additions. Expression was restricted to species with low abundance, and there were no shifts in community composition consistent with alkaline phosphatase expression. Lack of phosphate limitation at higher-than-Redfield N/P ratios (up to 40:1), however, points to a potentially wider role of dissolved organic phosphorus in nutrition of Winyah Bay phytoplankton than indicated by the ELF assay.