Evaluation of Water Suspension Effect on Spectral Light Attenuation in Lake Water

The paper presents the results of experimental data studies of many years on the spectral transparency of lake water (using the example of the surface layer of Lake Krasilovskoye) obtained in various 2012-2023 hydrological seasons. The calculated Napierian values of light attenuation coefficient in the 400-800 nm spectral range vary from 3.5 to 35.0 m-1 during the studied period, and the values of yellow substance absorption coefficients vary from 2.5 to 26.1 m-1. The values of chlorophyll-a concentration over an 11-year period lies within the range of 0.5-55.9 mg/m3, and yellow substance concentration — 4.4-60.1 g/m3. To assess the effect of water suspension on light attenuation, the percentage of its relative spectral contribution is calculated along with the other essential optically active components of lake water, like chlorophyll, yellow substance, and pure water. The spectral contribution of the suspension to the light attenuation at λ=430 nm is within the range of 4.4-71.3 %, 11.7-76.3 % at λ=550 nm, and 0.5-59.5 % at λ=670 nm. The optical microscopy method is used to determine the particle size distribution and number concentration of suspension particles. According to the conducted measurements (2013-2023), the weighted average radius of particles in water samples in the surface layer of the lake varies within the range of 0.5÷2.5 μm, while concentrations vary from 1.0∙106 cm-3 to 26.0∙106 cm-3. Particle size distributions are approximated by the Junge function with determination coefficient values from 0.89 up to 0.99.