Secchi depths recorded in Lake Zürich are determined mainly by concentrations of autochtonous material such as plankton and other material related to biomass production. The surface contrast transmittance factor has a negligible effect on a Secchi depth of up to 7 m and is of minor importance up to 13 m. Alterations in transparency values after 1970 as a result of the reduced nutrient input were most pronounced in the autumn-winter period (October 28 to February 19) as well as in the clear water phase (April 29 to July 29). After 1970, almost all buildings in the Lake Zürich drainage area were connected to a sewage treatment plant with a three stage purification. In the autumn-winter period, the Secchi depth is related to the development of Oscillatoria rubescens filaments and is not correlated with changes in trophic state. The restoration measures led to a statistically significant increase in the Secchi depth during the clear water phase, and additionally to a prolongation of the period with the highest tranparency values (May 20 to July 4). The Secchi technique is simple but seems to be valuable for recognizing long-term changes in the optical properties of a water body.