We studied extracellular acid phosphatase activity (AcPA) of planktonic microorganisms, aluminium (Al) speciation, and phosphorus (P) cycling in three atmospherically acidified (pH of 4.5-5.1) mountain forest lakes: Certovo jezero (CT), Prasilske jezero (PR), and Plesne jezero (PL) in the Bohemian Forest (Sumava, Bohmerwald). Microorganisms dominated pelagic food webs of the lakes and crustacean zooplankton were important only in PR, with the lowest Al concentrations (193 mu g L-1) due to 3-4 times lower terrestrial input. The lakes differed substantially in Al speciation, i.e., in the proportion of ionic and particulate forms, with the highest proportion of ionic Al in the most acid CT (pH = 4.5). The P concentration in the inlet of PL (mean: 22.9 mu g L-1) was about five times higher than in CT and PR (3.9 and 5.1 mu g L-1, respectively). Average total biomass of planktonic microorganisms in PL (593 mu g C L-1) was, however, only similar to 2-times higher than in CT and PR (235 and 272 mu g C L-1, respectively). Enormous AcPA (means: 2.17-6.82 mu mol L-1 h(-1)) and high planktonic C : P ratios suggested severe P limitation of the plankton in all lakes. Comparing 1998 and 2003 seasons, we observed changes in water composition (pH and Al speciation) leading to a significant increase in phytoplankton biomass in the lakes. The increase in the seston C : P ratio during the same time, however, indicates a progressive P deficiency of the lakes. The terrestrial Al inputs, together with in-lake processes controlling the formation of particulate Al, reduced P availability for planktonic microorganisms and were responsible for the differences in AcPA. At pH