Oligotrophication of Lake Zürich as reflected in Secchi depthmeasurements

Abstract

Secchi depths recorded in Lake Zürich are determined mainly by concentrations of autochtonous material such asplankton and other material related to biomass production. The surface contrast transmittance factor has a negligibleeffect on a Secchi depth of up to 7 m and is of minor importance up to 13 m. Alterations in transparency values after1970 as a result of the reduced nutrient input were most pronounced in the autumn-winter period (October 28 to February19) as well as in the clear water phase (April 29 to July 29). After 1970, almost all buildings in the Lake Zürich drainagearea were connected to a sewage treatment plant with a three stage purification. In the autumn-winter period, the Secchidepth is related to the development of Oscillatoria rubescens filaments and is not correlated with changes in trophicstate. 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 Secchitechnique is simple but seems to be valuable for recognizing long-term changes in the optical properties of a water body.