Activity of alk. phosphatase (APA) was studied in 6 points of the downstream part of the Odra river and the Szczecin Lagoon by detn. of the contents of chlorophyll_a, reactive P and inorg. N in the water samples. The highest APA concn. was found in waters where the growth of phytoplankton was limited by P APA concn. was increased with an increase of content of chlorophyll_a and with a decrease of reactive P content Phosphatase-catalyzed hydrolysis of Na(2) p-nitrophenylphosphate was carried out at 25 degrees C for 48 h to det. the max. rate and the Michaelis-Menten constant of the reaction.