In water ecosystems, enzymes play a key role in the processes of nutrient cycling and energy transformation. The origin, characteristics and ecological significance of the phosphatases in lakes, as an example, are reviewed in this paper. The enzymes, extracellular and dissolved, are mainly from bacteria, phytoplankton and zooplankton. They show stability and suitability towards the variations in pH value, temperature and other physical and chemical factors. Planktons seem to compensate for their phosphorus deficiency not only by an increase in enzyme production but also by an improved ability to use low substrate concentrations, which implies the potential significance of enzymes in environmental monitoring. Some ecological relationships might be explained from the points of view of enzymatic reactions. In short, the special and integrated functions of the enzyme in natural waters, in terms of nutrient supplying, nutrient status indicating and cycle mediating, have made it an essential part of the ecological enzymology.