Acoustic and optical variations during rapid downward motion episodes in the deep north-western Mediterranean Sea

TL;DR

This study combines acoustic and optical data from deep-sea observations in the Mediterranean to analyze episodic vertical movements and bioluminescence, revealing complex interactions possibly driven by local current instabilities.

Contribution

It provides a novel high-resolution comparison of acoustic and optical variations during deep-sea episodes in the Mediterranean, linking observed phenomena to local current dynamics.

Findings

Detected quasi-periodic episodes of high acoustic reflection and bioluminescence.

Linked deep water formation events to observed acoustic and optical variations.

Suggested local advection and boundary current instabilities as mechanisms for suspended material movement.

Abstract

An Acoustic Doppler Current Profiler (ADCP) was moored at the deep-sea site of the ANTARES neutrino telescope near Toulon, France, thus providing a unique opportunity to compare high-resolution acoustic and optical observations between 70 and 170 m above the sea bed at 2475 m. The ADCP measured downward vertical currents of magnitudes up to 0.03 m s-1 in late winter and early spring 2006. In the same period, observations were made of enhanced levels of acoustic reflection, interpreted as suspended particles including zooplankton, by a factor of about 10 and of horizontal currents reaching 0.35 m s-1. These observations coincided with high light levels detected by the telescope, interpreted as increased bioluminescence. During winter 2006 deep dense-water formation occurred in the Ligurian subbasin, thus providing a possible explanation for these observations. However, the 10-20 days quasi-periodic episodes of high levels of acoustic reflection, light and large vertical currents continuing into the summer are not direct evidence of this process. It is hypothesized that the main process allowing for suspended material to be moved vertically later in the year is local advection, linked with topographic boundary current instabilities along the rim of the 'Northern Current'.