Temporal variations of zooplankton biomass in the Ligurian Sea inferred from long time series of ADCP data

Abstract

Abstract. Three years of 300 kHz acoustic doppler current profiler data collected in the central Ligurian Sea are analysed to investigate the variability of the zooplankton biomass and the diel vertical migration in the upper thermocline. After a pre-processing phase aimed at avoiding the slant range attenuation, hourly volume backscattering strength time series are obtained. Despite the lack of concurrent net samples collection, different migration patterns are identified and their temporal variability examined by means of time-frequency analysis. The effect of changes in the environmental condition is also investigated. The highest zooplankton biomasses are observed in April-May just after the peak of surface primary production in March-April. The main migration pattern found here points to a "nocturnal" migration, with zooplankton organisms occurring deeper in the water column during the day and shallower at night. Also, twilight migration is highlighted during this study. The largest migrations are recorded in November-December, corresponding to lowest backscattering strength values and they are likely attributable to larger and more active organisms (i.e. euphausiids and mesopelagic fish). The results suggest further applications of the available historical acoustic doppler current profiler time series.

ADCP observations of migration patterns of zooplankton in the Cretan Sea

“…Seasonal and monthly variability, evident in the backscatter coefficient and vertical velocity, was dictated by the duration of day-time and moon phase respectively. Twilight effects on migration patterns using data from a downward looking 300 kHz ADCP measuring from the surface down to 80 m were also reported by Bozzano et al (2014) in the Ligurian Sea, while smaller amplitude changes of the extent of vertical migration due to changes in cloudiness can also be found in the results of Pinot and 15 Jansá (2001). According to the results presented here, during full moon the zooplankton preys almost 50 m deeper than during the new moon, a possible behavioral response to increased light conditions.…”

“…The intensity of the backscattered sound can also be used in 15 conjunction with net samples for estimating the biomass of zooplankton (Ashjian et al, 2002). Although the estimated ADCP backscatter is a by-product (Bozzano et al, 2014) and thus more suitable for qualitative than quantitative analysis (Brierley et al, 1998), field studies complemented with ADCP-derived sound scattering have been used to describe biological patterns in the interior of the ocean, such as zooplankton aggregations (Zhou and Dorland, 2004) and vertical migration (Postel et al, 2007) with remarkable detail. 20…”

Wintertime water dynamics and moonlight disruption of the acoustic backscatter diurnal signal in an ice‐covered Northeast Greenland fjord

“…However, sound scattering by zooplankton is more complex compared to that by sediment particles [Stanton et al, 1994]. Nevertheless, the acoustic backscatter has been found to be correlated to zooplankton biomass [Flagg and Smith, 1989;Brierley et al, 1998;Fielding et al, 2004;Berge et al, 2009;Hamilton et al, 2013;Bozzano et al, 2014]. It was noted, however, that ADCPs, unlike echo-sounders, are limited in deriving accurate quantitative estimates of biomass due to calibration difficulties because their acoustic beams are narrow and are inclined to the vertical [Sato et al, 2013;Vestheim et al, 2014;Brierley et al, 1998].…”

Impact of tidal dynamics on diel vertical migration of zooplankton in Hudson Bay

“…The diel vertical migration (DVM) of zooplankton is a synchronized movement of individuals through the water column and is considered to be the largest daily synchronized migration of biomass in the ocean (Brierley, 2014). This migration is majorly controlled by two biological factors:…”