SUMMARY1 The kinetics of surface phosphatase activity (PA) in the diatomSynedra acuswere studied at low substrate concentrations that are frequently encountered in freshwaters and pH variations typical of hardwaters, ranging from pH 7 to pH 9. Higher pH resulted in higher values for the half saturation constant (Km) and the maximal velocity (Vmax).2 The pH optimum of PA was shown to be a linear function of the logarithm of substrate concentration.3 Similar slopes of Michaelis‐Menten curves at different pHs in the range of low substrate concentrations indicate that the species is well adapted to hardwaters. The rate of release of phosphate from enzymaticaliy hydrolysable phosphorus was calculated from Michaelis‐Menten kinetics for lake‐water samples dominated byS. acus. Algal surface phosphatases were responsible for the hydrolysis of 1.54–1.64 nmmin−1although incubation was performed at a lake temperatur