<p>To investigate the annual cycle of sound-scattering layers in the Black Sea, a moored profiler equipped with an acoustic Doppler current meter, a conductivity-temperature-depth probe, and fast sensors for dissolved oxygen [O2] was employed. Approximately 13,350 multiparameter profiles from the near-surface layer down to the near-bottom layer were obtained at intervals of 1–2 h from 2013–2020. The acoustic system allowed for observations of ultrasound backscattering at 3 angles at 2 MHz frequency. Combinations of the volume strength data of the 3 acoustic beams (directional acoustic backscatter ratios, <i>R</i>) were found to be a useful tool for visualizing acoustic backscatter patterns associated with mesoplankton in the oxycline and hypoxic zone. The time series of <i>R</i> as a function of [O2] at depths where [O2] < 200 μM were analyzed to determine the annual cycle of sound-scattering mesoplankton aggregations. It was shown that from spring to early autumn, there are two sound-scattering maxima corresponding to the daytime aggregations of diel-vertical-migrating specimens usually at [O2] = 20–60 μM and the persistent layer of diapausing specimens at [O2] < 10 μM. During the rest of autumn until early winter, there is usually no persistent deep sound-scattering layer, while the maximum corresponding to the daytime mesoplankton aggregations shifted deeper to [O2] = 10–30 μM. During the rest of winter, the acoustic backscatter is basically uniform throughout the water column. The <i>R</i> graphs also indicate that the mesoplankton specimens tended to be oriented vertically in the lower part of the oxycline and hypoxic zone.</p>