Acoustical Scattering Layers of Two Mesozooplanktons as a Tool for Hydrographic Features of the Black Sea

Abstract

Two dominant acoustical scatterers of fodder zooplankton (Calanus euxinus and Sagitta setosa) performed diel emigrational
speeds depending highly on the profile of dissolved oxygen (DO) in the Black Sea: C. euxinus started accelerating upon entering
the oxycline while S. setosa accelerated only after entering well-oxygenated subsurface water. The speed and their daytime depths
suggest profile of the DO, sub-region classification and thickness of the oxic layer. Oxygen profile: layer where Sagitta migrate
very fast is subsurface maximum oxygen and chl-a (σ θ=14.0-14.7), layer where Sagitta speed down whereas Calanus still swim fast is oxycline (σ θ =15.3-15.9), layer where Sagitta spend their daytime is a zone just above oxygen minimum zone (OMZ) (σ θ =15.9-16.0), and layer where Calanus spend their daytime is OMZ (σ θ =16.15 to 16.20). Sub-regions: If Calanus start the upward migration very fast, the region is downwelling. If they perform slow ascendance and then speed up (this means that they settle down in a layer with σ θ =16.15 to 16.20 during day), the region is upwelling. Thickness of oxic layer is depth between surfaces to a depth where Calanus are found during the daytime; Calanus and Sagitta coexist in OMZ during the cold season. Depth preference of Calanus in sub-regions must be taken into account of the thickness estimates.