Vertical distributions of zooplankton across the Almeria–Oran frontal zone (Mediterranean Sea)

The vertical distribution of zooplankton in the upper 700 m of the water column was investigated across the Almeria–Oran front (Alboran Sea) during winter 1997/98 (Almofront 2 cruise). Eight sites corresponding to three hydrodynamic structures (the Atlantic jet, an anticyclonic gyre and the surrounding Mediterranean waters) were explored, by day and at night, using multiple-net sampling. Results are presented for the major species of copepods (>500 m), macroplankton and microplankton. Species inhabiting the surface layer or occupying it at night extended deeper in the gyre. On the other hand, mesopelagic species (e.g. Pleuromamma borealis, Pleuromamma abdominalis, Euphausia krohni and Cyclothone braueri) were found to live deeper, at least by day for the migrants, in the Mediterranean waters compared with the two other structures. These changes in preferential depths across the front are discussed in relation to environmental features, such as light level, abundance of food and mixed layer depth. This study also documented the vertical distribution of some overwintering copepod species (Calanus helgolandicus, Eucalanus hyalinus and Eucalanus monachus).