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

One year of mean volume backscattering strength (MVBS) data obtained from a n acoustic Doppler current profiler (ADCP) moored at 240 m in the Ibiza Cha nnel is analysed. It shows the prominent effect of light on the zooplankton distribution in the surface layer. Two communities of zooplankton can be d ifferentiated, both of which perform diel vertical migrations (DVM). They i nitiate descent at dawn, reach a maximum depth at noon, and finish ascent a t dusk. Fast "primary" migrators, which quite homogeneously filled the surf ace layer at night, generally descended below the ADCP during day-time, whe reas 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 veloc ity of secondary zooplankton inferred from the slope of MVBS contours slid not exceed a few tenths of cm s(-1). Variations in DVM show a great depende nce on light intensity: (1) The depth attained by zooplankton at noon was m odulated by the annual cycle of sunlight and was maximum/minimum around the summer/winter solstice. (2) Some primary zooplankton remained above the AD CP during day-time on days of heavy cloud cover in winter, (3) The preferen tial depth of zooplankton at night followed the cycle of moonlight (29.5 da ys). Primary zooplankton were present all year at the mooring, whereas seas onal variations are seen in the abundance of secondary zooplankton. Maximum abundance is found in summer with enhanced concentration in the thermoclin e (upper 60 m), and minimum abundance in early winter after the erosion of the thermocline. A bloom is also observed in late winter. These observation s are discussed in the context of past zooplankton surveys in the area, Fas t primary migrators could consist of euphausiids (alternatively, micronekto n, e.g, myctophids). A fraction of slow secondary migrators very likely are copepods. (C) 2001 Elsevier Science Ltd, All rights reserved.