High bulk extracellular phosphatase activity (PA) suggested severe phosphorus (P) deficiency in plankton of three acidified mountain lakes in the Bohemian Forest. Bioavailability of P substantially differed among the lakes due to differences in their P loading, as well as in concentrations of aluminum (Al) and its species, and was accompanied by species-specific responses of phytoplankton. We combined the fluorescently labeled enzyme activity (FLEA) assay with image cytometry to measure cell-specific PA in natural populations of three dinophyte species, occurring in all the lakes throughout May-September 2007. The mean cell-specific PA varied among the lakes within one order of magnitude: 188-1,831 fmol center dot cell-1 center dot h-1 for Gymnodinium uberrimum (G. F. Allman) Kof. et Swezy, 21-150 fmol center dot cell-1 center dot h-1 for Gymnodinium sp., and 22-365 fmol center dot cell-1 center dot h-1 for Peridinium umbonatum F. Stein. To better compare cell-specific PA among the species of different size, the values were normalized per unit of cell biovolume (amol center dot mu m-3 center dot h-1) for further statistical analysis. A step-forward selection identified concentrations of total and ionic Al together with pH as significant factors (P