Professor Andrzej Orłowski, a long-time employee of National Marine Fisheries Research Institute, passed away on October 27, 2020. He was an outstanding scientist in the field of hydroacoustics. Inventor of the method of use of multiple echo measurements to assess the type of the seabed. To this day, this method is called the Orłowski Method. Professor Orłowski was a member of the Physics Section of the SCOR, the NMFRI Scientific Council, the Polish Acoustical Society, ICES Fishery Acoustic Science and Technology Group, ICES Fish Technology and Fish Behavior Working Group, ICES SG Acoustic Seabed Classification and ICES SG Fish Avoidance of Research Vessels.

Biography and scientific achievements of Academician Leonid Maksimovich Brekhovskikh - Russian physicist, the founder of the scientific school of Ocean Acoustics, Doctor of Physics and Mathematics Sciences, Professor, Academician of the Russian Academy of Sciences.

The understanding the influence of biological processes on the characteristics of the signals backscattered by the sea floor is crucial in the development of the hydroacoustical benthic habitat classification techniques. The impact of the microphytobenthos photosynthesis on the acoustical backscattering properties of the Atlantic sandy sediments was previously demonstrated by Holliday et al. (2004) and Wildman and Huettel (2012). To account for the sensitivity of the hydroacoustical classification techniques to the backscattering properties of local marine sediments, it is important to understand the microphytobenthos photosynthesis impact for the Baltic Sea where the techniques are being actively developed now. This is the main motivation of the paper. In the paper the influence of the microphytobenthos photosynthesis on the characteristics of the echo signals reflected by sandy sediments in the typical Baltic temperature and the salinity conditions is discussed. The interdisciplinary multiday laboratory experiment was conducted to study the impact of benthic microalgal photosynthesis on the characteristics of the echo signal reflected by sandy sediments. Hydroacoustical data were collected under controlled constant light, temperature and salinity conditions. The oxygen content at different levels of the water column was simultaneously monitored.