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 Zürich). 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 Zürich (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.