On the Development of a Regional Algorithm for Determining the Thickness of the Euphotic Layer of Lake Teletskoye Based on Measurement Data of Optical Characteristics Profiles in August 2023

The thickness of the euphotic layer Zeu is one of the key parameters of an aquatic ecosystem, in particular, a lake. For various reasons, namely: variability of illumination during the measurement of one profile, the influence of the vessel position relative to the incident sunlight and, finally, measurements at low solar altitudes, the use of direct measurements of the photosynthetically active radiation profile is limited. Therefore, the development of regional algorithms for determining Zeu, free from the above-mentioned shortcomings, is a promising approach. In this paper, various options for such an approach are analyzed, namely: two methods for reconstructing the lower boundary of the euphotic zone based on measurements of three types of profiles (the beam attenuation coefficient in the long-wavelength region of the spectrum, organic matter fluorescence and chlorophyll-a fluorescence) in the entire euphotic and upper quasi-homogeneous layers, respectively, are proposed and tested. With the overall variability of the euphotic layer thickness from 8 to 16 m, both methods give approximately the same result of its reconstruction with a relative error of 10–15% when using the profiles of the beam attenuation coefficient and organic matter fluorescence (or their combination with other profiles). The worst result with a relative error of 50–100% was recorded when using the chlorophyll-a fluorescence profiles.