Dissolved and particle-bound geosmin in a mesotrophic lake (lake Zürich): spatial and seasonal distribution and the effect of grazers

Depth profiles of geosmin, of which a dissolved and particle-bound fraction was distinguished, were analyzed monthly in a seasonal study in a stratified mesotrophic prealpine lake (Lake Zurich). Remarkable seasonal and spatial differences in the total amounts and ratios between both fractions were observed. Equally low concentrations of dissolved and particle-bound geosmin were found in the winter season during turnover conditions (maximum total geosmin 3.1 ng l −1 ). In the clear-water period the dissolved fraction of geosmin increased dramatically (93% of the total) whereas the particle-bound geosmin showed only minor changes. In the autumn and first part of the winter surface films contributed essentially to the particle-bound fraction of geosmin exhibiting maximum concentrations of 21 ng l −1 . Possible producers of geosmin in Lake Zurich ( Aphanizomenon , Oscillatoria , Planktothrix ) were cultivated and their content of geosmin was determined. Feeding experiments with Daphnia and Simocephalus simulating the phenomenon of the clear-water period clearly demonstrated that grazing can be regarded as a major mechanism for the liberation of geosmin from particles. The mean degradation rate in a microcosm experiment caused by pelagic bacteria was 3.1 ng l −1 d −1 ; this rate was high enough to explain the concentration differences between consecutive sampling dates.