Prior to the early 1960's, Kezar Lake was an attractive and popular recreational area. The fact that Kezar Lake was chosen as the site of Wadleigh State Park in 1934 reflects the aesthetic value the lake once enjoyed.

As a result of the construction of the New London Waste Water Treatment Plant in 1931, water quality deteriorated in Kezar Lake. Secondary effluent from the treatment plant contributed high concentrations of phosphorus to Lion Brook, much of which became entrapped in the sediment and biomass of Kezar Lake. As a result of this phosphorus input, massive blooms of obnoxious blue-green algae began in 1961 and deteriorated the water quality in the lake.

Remedial attempt to improve the recreational quality of Kezar Lake has been undertaken by the New Hampshire Water Supply and Pollution Control Commission. The use of copper sulfate as an algicide, as an acceptable method which was utilized with considerable success for several years, was the first attempt to improve the aesthetic lake conditions. Although this method does not reverse the eutrophication trends, it temporarily alleviates nuisance algal blooms and provides an acceptable recreational season.

In 1968, the New Hampshire Water Supply and Pollution Control Commission attempted a new method to remedy the poor aesthetic conditions of the lake. This new approach was to completely mix the lake by releasing compressed air from the deep spot of the lake. Mixing of large bodies of water was shown to alter the ecology in some way which may affect algal growth. Although destratification of Kezar Lake by mixing was initially considered a success, the procedure was abandoned in 1974, when water clarity again declined, marking the end to the effectiveness of destratification.

In 1970, tertiary treatment for phosphorus removal was added to the New London Treatment Plant, resulting in an appreciable decrease of phosphorus input to Lion Brook. In January of 1981, effluent from the plant was diverted outside the watershed to the Sunapee Wastewater Plant.

In 1981, biologists from the New Hampshire Water Supply and Pollution Control Commission began an exhaustive three-year study of Kezar Lake to identify the lake's hydrologic and nutrient budgets. As a result of these budgets, biologists were able to recommend various lake restorative techniques to limit the phytoplankton's main source of food, phosphorus (Q).