Spatial Variability of Bio-Optical Characteristics in the Upper Water Layer of the Middle Volga in September 2023 Based on Lidar Data

The remote sensing of inland waters has begun to rapidly develop with the recent setting into operation of high- and medium-resolution satellite constellation Sentinel, as well as the introduction of machine learning to data processing and analysis. However, the collection of large arrays of in situ data remains a weakness due to the significant diversity of inland waters, seasonal features, and the strong influence of meteorological conditions and hydrology. As a possible solution, this paper considers the collection and investigation of bio-optical characteristics of the upper water layer with the use of UFL-9 fluorescent shipborne LiDAR with meter resolution in a 1000 km section of the middle Volga from Nizhny Novgorod to Bolgar (close to Ul’yanovsk) as an example. Continuous measurements were performed on September 9–17, 2023, from a moving boat. Based on the obtained LiDAR data, chlorophyll “a”, suspended matter, and the absorption coefficient at 440 nm as an indicator of colored dissolved organic matter were mapped and described. The ranges of their variability and effects of inflows and meteorological conditions are analyzed. The results expand previous knowledge on the bio-optical characteristics of middle Volga and its variability on different spatial and temporal scales and contribute to the solution of the problem of developing bio-optical models for the Cheboksary and Kuibyshev reservoirs.