Seasonal variation of the sound-scattering zooplankton vertical distribution in the oxygen-deficient waters of the NE Black Sea

Abstract. At the northeastern Black Sea research site, observations from 2010–2020 allowed us to study dynamics and evolution of vertical distribution mesozooplankton in oxygen-deficient conditions via analysis sound-scattering layers associated with dominant zooplankton aggregations. The data were obtained profiler mooring net sampling. was equipped an acoustic Doppler current meter, a conductivity–temperature–depth probe, fast sensors for concentration dissolved oxygen [O2]. instrument conducted ultrasound (2 MHz) backscatter measurements at three angles while being carried by through oxic zone. For lower part oxycline hypoxic zone, normalized beams (directional ratios, R) indicated aggregations, which defined taxonomic quantitative characteristics based on stratified sampling site. time series ? 14 000 R profiles as function [O2] depths where < 200 µm analyzed determine month-to-month variations layers. From spring early autumn, there two maxima corresponding (1) daytime mainly formed diel-vertical-migrating copepods Calanus euxinus Pseudocalanus elongatus chaetognaths Parasagitta setosa, usually = 15–100 µm, (2) persistent monospecific layer diapausing fifth copepodite stages C. suboxic zone 3 10 µm. late autumn winter, no deep observed. end basically uniform assessment seasonal variability is important understanding biogeochemical processes waters.