ABSTRACTLimnological studies undertaken on Chautauqua Lake, N.Y., between 1974 and 1983 indicated that lake water quality can be assessed by monitoring Secchi disk transparency and phytoplankton chlorophyll a. These measurements of phytoplankton abundance were observed to vary from year to year depending on the amount of precipitation; high chlorophyll a and poor transparency correlated with high precipitation, presumably due to increased loading of nutrients to the lake. Comparison of the data collected between 1974 and 1983 with comparable measurements made in the 1930s revealed that Chautauqua Lake water quality has not been substantially altered over the last 50 years. Transparency and the occurrence of blue-green algae were similar, although diatoms may have been more abundant in the 1930s. The results of these limnological studies reinforce the concept that long-term monitoring of algal abundance and species composition can be an appropriate means of detecting changes in lake water quality.