A com. lanthanum-modified bentonite sorbent was added (0.05 to 0.125 g/L) to water samples collected from Lake Goldap, PL. Phosphate content in both shaked and immobile water samples was detd. each day by calorimetry for 6 days. Addn. of the sorbent resulted in a decrease in phosphate concn. from 0.15 mg/L down to below 0.05 mg/L. The field test in the lake was carried out in 4 lagoons vol. 1 m(3) each where photosynthesis took place (sorbent addn. 50, 100 and 200 mg/m(2) of the bottom surface). The water was analyzed for phosphates and chlorophyll each 12 h for 3 days. The phosphate concn. was decreased down to below 0.01 mg/L. In the case of the highest dose, a temporary decrease in the chlorophyll concn. (from about 40 mu g/L down to 0 mu g/L after 60 h) was obsd.