Time variability of acoustic backscatter from zooplankton in the Ibiza Channel (western Mediterranean)

One year of mean volume backscattering strength (MVBS) data obtained from an acoustic Doppler current profiler (ADCP) moored at 240 m in the Ibiza Channel is analysed. It shows the prominent effect of light on the zooplankton distribution in the surface layer. Two communities of zooplankton can be differentiated, both of which perform diel vertical migrations (DVM). They initiate descent at dawn, reach a maximum depth at noon, and finish ascent at dusk. Fast “primary” migrators, which quite homogeneously filled the surface layer at night, generally descended below the ADCP during day-time, whereas slower “secondary” migrators always remained above the instrument. The vertical motion of primary zooplankton at dawn and dusk was strong enough to be directly measured by the ADCP (±3–4 cm s−1). The vertical velocity of secondary zooplankton inferred from the slope of MVBS contours did not exceed a few tenths of cm s−1. Variations in DVM show a great dependence on light intensity: (1) The depth attained by zooplankton at noon was modulated by the annual cycle of sunlight and was maximum/minimum around the summer/winter solstice. (2) Some primary zooplankton remained above the ADCP during day-time on days of heavy cloud cover in winter. (3) The preferential depth of zooplankton at night followed the cycle of moonlight (29.5 days). Primary zooplankton were present all year at the mooring, whereas seasonal variations are seen in the abundance of secondary zooplankton. Maximum abundance is found in summer with enhanced concentration in the thermocline (upper 60 m), and minimum abundance in early winter after the erosion of the thermocline. A bloom is also observed in late winter. These observations are discussed in the context of past zooplankton surveys in the area. Fast primary migrators could consist of euphausiids (alternatively, micronekton, e.g. myctophids). A fraction of slow secondary migrators very likely are copepods.