Turbidity, as an optical property describing water clarity, is a measure of the degree to which water loses its transparency due to the presence of suspended solids, including phytoplankton and dissolved substances. Lake Strzeszyńskie was regarded as the clearest lake in Poznań (western Poland) for many years. In July 2011 first time in the history of lake the watering place was closed due to picocyanobacteria bloom and high water turbidity. The subject of paper was the preliminary assessment of changes the vertical propagation and availability of PAR, turbidity and content of OAS in the lake before and at the time of phytoplankton bloom. To comparison were taken sample from a peak of summer season of 2009 and 2011, respectively. Large changes in depths of photic zone in both periods were stated as well as reducing of euphotic zone and increase the darker disphotic zone. Picocyanophyceae bloom and high light scattering caused by high content of OAS in surface layer resulted in visible effect of the water opalescence.