Autumnal deep scattering layer from moored acoustic sensing in the subtropical Canary Basin

TL;DR

This study reports a seasonal deep scattering layer in the NE-Atlantic's bathypelagic zone, likely composed of zooplankton or similar organisms, showing slow vertical movement and reduced diel migration during autumn.

Contribution

It provides new insights into deep scattering layers' seasonal presence, composition, and behavior in the subtropical Atlantic, using moored acoustic sensing.

Findings

Deep scattering layer observed at 1650 m depth during autumn.

Layer exhibits slow vertical movement of 2-5 m per day.

Diel vertical migration is significantly reduced during the layer's presence.

Abstract

An enhanced acoustic scatterer reflectance layer was observed in the bathypelagic zone around 1650 m in the subtropical NE-Atlantic Ocean for about two months during autumn. It resembles a classic pattern of diapause-resting, possibly of large zooplankton, shrimp and/or Cyclothone, at great depths well below any sunlight penetration, which is more commonly found at higher latitudes. The observed slow sink and rise of about 2-5 m per day into and out of this deep layer is considerably slower than the more than 1000 m per day of diel vertical migration (DVM). During the two-month period of deep scattering, DVM is observed to be greatly reduced.