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

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’.

💡 Research Summary

This paper presents a unique long‑term, high‑resolution dataset obtained from the ANTARES deep‑sea neutrino telescope site off the coast of Toulon, France, where an Acoustic Doppler Current Profiler (ADCP) was permanently moored at a depth of 2 475 m. The ADCP measured three‑dimensional currents (east‑west, north‑south, and vertical) as well as acoustic backscatter (echo intensity) in 50 vertical bins of 2.5 m, sampled every ten minutes. In parallel, the optical modules (photomultiplier tubes, PMTs) of the ANTARES detector recorded ambient light levels, which are interpreted as bioluminescent activity of deep‑sea organisms, mainly zooplankton.

During the late winter and early spring of 2006 the ADCP recorded several episodes of strong downward vertical velocity, reaching up to –0.03 m s⁻¹. These events coincided with a ten‑fold increase in acoustic backscatter, indicating a substantial rise in suspended particles, and with a marked rise in PMT light counts, indicating intensified bioluminescence. Horizontal currents were also strong, attaining speeds of about 0.35 m s⁻¹. The authors link these observations to the formation of dense water in the Ligurian sub‑basin during winter 2006, a process that can generate deep convection plumes and transport surface‑derived material to the abyss.

However, the observed 10–20 day quasi‑periodic episodes of high backscatter, elevated light, and strong downward currents persisted well into the summer, a pattern that cannot be explained solely by winter dense‑water formation. The authors therefore propose that the dominant mechanism for later‑year vertical transport is local advection driven by instabilities along the rim of the Northern Current (NC), a mesoscale boundary current that flows counter‑clockwise around the Ligurian and Provençal sub‑basins. Seasonal changes in the NC’s width and depth produce stronger shear and meander activity in winter, generating narrow, intense downwelling zones (order 10⁻² m s⁻¹) that are spatially limited compared with broader, weaker upwelling regions. These rim‑associated instabilities can intermittently lift suspended material and zooplankton from the upper water column and convey them to depths of >2 000 m.

Methodologically, the paper details the ADCP’s four‑beam configuration, the use of the fourth beam to compute an “error velocity” that serves as an internal consistency check for vertical velocity estimates, and the correction of tilt and heading biases. The authors argue that the instrument’s tilt accuracy (±0.5°) limits possible vertical bias to ±3 × 10⁻³ m s⁻¹, far smaller than the observed vertical motions, confirming the reliability of the measurements. The temporal correlation between ADCP‑derived vertical currents, acoustic backscatter, and PMT light intensity demonstrates a tight coupling between physical forcing and biological response at abyssal depths.

The study’s significance lies in providing the first simultaneous acoustic‑optical record of deep‑sea particle fluxes and bioluminescent activity linked to mesoscale dynamics. It extends the concept of surface‑to‑deep nutrient and organism transport—previously documented only in the upper few hundred meters—to the abyssal realm. The findings suggest that boundary‑current instabilities, rather than solely deep convection, can episodically inject organic material and planktonic fauna into the deep sea, potentially influencing deep‑sea food webs and carbon cycling.

Future work recommended by the authors includes coupling these observations with high‑resolution numerical models of the Northern Current, expanding the sensor suite to include direct particle sampling and molecular biodiversity assays, and investigating the seasonal evolution of the observed processes over multiple years. Such integrated approaches would clarify the relative contributions of dense‑water convection and rim‑instability‑driven advection to deep‑sea ecosystem dynamics.