Phosphorus deficiency was analysed in the oxic-anoxic gradient of the karstic sulphurous lakes Vilar and Sisó during the stratification period. The distribution of planktonic photosynthetic populations along a vertical gradient coincided with an increase in alkaline phosphatase activity (APA). A multiple stepwise correlation analysis of data yielded a positive correlation of APA with planktonic phototrophic populations. MUF-P hydrolysis saturation curves were used to estimate the enzyme kinetics. High-affinity phosphatases (i.e. low K(M) saturation constant) coincided with the oxic-anoxic gradient and progressively declined through both the epi- and the hypolimnion. Changes in the K(M) values are likely due to phosphate inhibition and the contribution of different planktonic populations in the induction of alkaline phosphatases. Extremely low organic phosphorus turnover times (as short as 0.37 h) were also estimated in the gradient zone, indicating a high dependence of the bacterial populations on organic phosphate esters. Phosphatase saturation kinetics revealed K(M) values from 0.53 to 8.45 microM MUF-P, perfectly matching those found in the isolates Thiocapsa sp. UdG3513, Chlorobium limicola UdG6050 and UdG6055 and Chlorobium phaeobacteroides CL1401. The results obtained indicate that a relevant adaptation of sulphur phototrophic bacteria may occasionally face periods of phosphate limitation despite thriving in nutrient-rich anoxic waters.