The article describes results of seasonal measurements of spectral transparency of water in the floodplain lake Lapa (the basin of the Upper Ob River, Altai Krai) in the wavelength range 400-800 nm in the autumn, winter and spring, 2011-2012. It was discovered that the spectral transparency depends on concentration of chlorophyll "a " defined by the standard spectrophotometric method. The researchers analyze the experimental data with respect to the composition of the dispersed particles of algae phytoplankton.

Key worlds: spectral transparency, light attenuation, phytoplankton, chlorophyll, eutrophication.