SEASONAL-CHANGES IN ALKALINE-PHOSPHATASE ACTIVITY OF BACTERIA AND MICROALGAE IN LAKE PAVIN (MASSIF-CENTRAL, FRANCE)

Seasonal changes in alkaline phosphatase activity of bacteria and micr oalgae were studied in the dimictic Lake Pavin (Massif Central, France ), to test whether this activity is primarily algal or bacterial and w hether the APA presents seasonal variations coupled with abiotic and b iotic variables. Samples were collected at different depths from May 1 992 to May 1993. The specific phosphatase activities were analysed spe ctrophotometrically with p-Nitrophenyl Phosphate (p-NPP) as substrate and were related to the protein concentrations. No correlation was fou nd between alkaline phosphatase activity and soluble reactive phosphor us (SRP) concentrations across an annual cycle. The specific activitie s of the small class (0.2-1.2 mu m) were the highest and the contribut ion of this picoplanktonic size class (0.2-1.2 mu m) increased with de pth. In addition, the linear correlations between alkaline phosphatase activity and protein concentration seemed to indicate that most of th ese enzymes are constitutive. However, it cannot be excluded that the high phosphorus concentrations repress APA. Finally, the measure of AP A does not seem to be a valid quantitative test of the deficiency of p hosphorus for aquatic microorganisms.