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.
Highlights
The Acoustic Doppler Current Profiler (ADCP) is a widely used instrument to monitor the marine currents
The general circulation of the Ligurian Basin is characterized by a permanent basin-wide cyclonic circulation involving both the surface Modified Atlantic Water (MAW) and the lower Levantine Intermediate Water (LIW) (Crepon et al, 1982; Millot, 1999); it shows important seasonal variability (Astraldi and Gasparini, 1992)
In 2004 there is a unique peak of net primary production (NPP) from March to May, while in 2005 an extraordinarily high NPP peak is recorded from April (1221 mg C/m2 day−1) to May (1070 mg C/m2 day−1), but high NPP values persist until August
Summary
The Acoustic Doppler Current Profiler (ADCP) is a widely used instrument to monitor the marine currents. Time series of these measurements span from a few days up to several years and are available for many coastal and open ocean sites. ADCP data are more qualitative than quantitative because the conversion from backscatter intensity to equivalent zooplankton biomass, usually obtained by direct comparison against coincident data from net samples, is somewhat problematic, and the resulting relationship provides only rough estimates (Pinot and Jansá, 2001; Ashjian et al, 2002; Fielding et al, 2004). Its data supply important information required in marine ecology research that cannot be satisfactorily obtained using the classical observational methodology based on discrete net sampling
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