Vertical distribution and nutritional behaviour of Cyclothone braueri, Nematoscelis megalops, Meganyctiphanes norvegica and Salpa fusiformis in the NW Mediterranean mesopelagic zone

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Vertical distribution and migration, stomach content and population structure of four characteristic macrozooplanktonic species were determined during 4 cruises in the Ligurian Sea, NW Mediterranean. Two instruments were used: the BIONESS multiple net system for the sampling, and the Underwater Video Profiler for the “in situ” estimations. The non migrating gonosthomatiid Cyclothone braueri forms the main macrozooplanktonic biomass from 300 to 600 m deep in the studied stations. Its stomach content shows that it is a carnivore, feeding efficiently on migrating mesozooplanktonic populations. Although the euphausiid Nematoscelis megalops can be observed from the surface to the depth of 800 m the major part of its population lives in the 300 - 600m layer. Only one part of the population is migrating. In the aphotic layer its nutritional behaviour is mainly carnivorous, while in the subuperficial layer N. megalops is omnivorous. Meganyctiphanes norvegica displays a strong migrating behaviour and feeds intensely during the night in the superficial layer both on animal prey and on phytoplanktonic/detritic material. During the daytime M. norvegica seems to be dispersed in the mesopelagic layer. Salpa fusiformis is a filter feeding tunicate. The oozooids and the large blatozooids seem to migrate, while the small blastozoids remain in the superficial layer during the day. The role of these organisms in the mesopelagic food web and in the carbon cycling will be discussed.