MESOPELAGIC MICRONEKTON AND MACROZOOPLANKTON OBSERVED BY ECHOSOUNDING, MULTIPLE-NET SAMPLING AND VIDEO PROFILING ACROSS THE ALMERIA-ORAN FRONT (W MEDITERRANEAN-SEA)

The daytime vertical distribution and abundance of mesopelagic microne kton and macrozooplankton were investigated, using simultaneous echoso unding, multiple-net sampling (EZNET) and Video Profiling across the A lmeria-Oran front, during spring 1991. Three sites were studied: a Med iterranean site, an Atlantic site and a frontal site lying between the m. Observations carried out with a 38 kHz echosounder demonstrated dif ferences in the patterns and backscattering volume of deep sound scatt ering layers (SSL) between these sites. The two main SSL (SSL1 and SSL 2) were found more than 50 m deeper at the Mediterranean site than at the Atlantic site, while the frontal site wal characterized by two add itional shallow SSL (SSL3 and SSL4) situated above SSL1 and 2. These d ifferences in acoustic patterns were reflected in the depth distributi on and biovolumes of both micronekton and macrozooplankton from EZNET samples and Video Profiler recordings. Fishes appear responsible for S SL1, 2 and 3, while euphausiids are possible sources of SSL4 in the fr ontal site. Though gelatinous zooplankton (medusae, siphonophores, app endicularians, ctenophores) visualized by the Video Profiler was abund ant, it seems to be transparent to 38 kHz sound. Biological productivi ty in superficial layers probably plays a role in differences observed in patterns across the Almeria-Oran front.