Preprints
https://doi.org/10.5194/os-2020-106
https://doi.org/10.5194/os-2020-106

  08 Dec 2020

08 Dec 2020

Review status: a revised version of this preprint is currently under review for the journal OS.

Annual cycle of sound-scattering mesoplankton in the oxycline and hypoxic zone in the northeastern Black Sea

Alexander G. Ostrovskii, Vladimir A. Solovyev, and Dmitry A. Shvoev Alexander G. Ostrovskii et al.
  • Shirshov Institute of Oceanology, Russian Academy of Sciences, 36, Nahimovskiy prospekt, Moscow, 117997, Russia

Abstract. 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, R) 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 R 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 R graphs also indicate that the mesoplankton specimens tended to be oriented vertically in the lower part of the oxycline and hypoxic zone.

Alexander G. Ostrovskii et al.

 
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Alexander G. Ostrovskii et al.

Data sets

Metadata of Aqualog profiler sreveys in 2013-2020 Ostrovskii, A. G. https://doi.org/10.13140/RG.2.2.30667.41766

Directional acoustic backscatter versus dissoved oxygen for individual months of 2014-2020 Ostrovskii, A. G. https://doi.org/10.13140/RG.2.2.28470.73285

Alexander G. Ostrovskii et al.

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Short summary
Paper concerns with developing methods for 24/7/365 monitoring of plankton while controlling key environmental conditions at precise depths. The method is based on using high-frequency acoustical device in a manner that gets it sufficiently close to the animals of interest. Such data are used to visualize the breathtaking dynamics of the sound scattering plankton and month-to-month variations of plankton diel vertical migrations and diapause displacements in the hypoxic zone of the Black Sea.