Do distinct water chemistry, reservoir age and disturbance make any difference on phosphatase activity

Alkaline phosphatase activity was assessed concomitantly with total phosphorus, orthophosphate and phosphomonoester con-centrations in two meso-eutrophic reservoirs with distinct age and subjected to different kinds of environmental influence. Differ-ences in conductivity, temperature and pH were found. However, during the study period alkaline phosphatase activity was similar in both reservoirs. Water colour was higher in S. Serrada Reservoir. This fact can be related to (1) reservoir age (2) high internal dis-turbance (3) large imputs of allochthonous detritus, resulting from the combined effect of grazing, fire and catchment slope. Despite the high water colour recorded in S. Serrada, alkaline phosphatase activity was apparently not inactivated by humic substances. Be-sides, the obtained results demonstrated that hydrolysis of phosphomonoesters by alkaline phosphatase was not important for ortho-phosphate regeneration in these reservoirs. Probably orthophosphate was always available to biota. In fact, in the experiments based on Selenastrum capricornutum Printz algal test, similar phytoplankton growth responses were obtained for different phosphorus concentrations. Thus, these results seem to indicate that phosphorus was not a limiting nutrient in either reservoir. Although phos-phatase activity was significantly correlated with some phytoplankton genera in both reservoirs, no significant correlations were found between enzyme activity and chlorophyll-a. Significant correlations between phosphatase activity and crustacean zooplankton were only recorded in S. Serrada. In spite of these results there was some indication that the main source of phosphatase might have been bacteria involved in decomposition processes instead of phyto- and zooplankton taxa. Key-words: Alkaline phosphatase; orthophosphate regeneration; phytoplankton; zooplankton; reservoir