Zooplankton distribution related to the hydrodynamic features in the Algerian Basin (western Mediterranean Sea) in summer 1997

This work represents a first step in understanding the impact of hydrodynamic features on the zooplankton dynamics in the Algerian Basin (southwestern Mediterranean Sea). The mesoscale distribution of mesozooplankton abundance, biomass, specific composition and size structure was investigated during ELISA-1 campaign (1997) in the framework of the program ELISA (Eddies and Leddies Interdisciplinary Study off Algeria, 1997–1998), partly dedicated to study the mesoscale features during 1997. Physical, biogeochemical and biological measurements were made on transects through two hydrodynamic features, one anticyclonic eddy and a small secondary shear cyclonic eddy. The use of combined zooplankton descriptors, i.e. biomass, abundance, size structure (e.g. NB-SS slope) and taxonomic structure, and of cumulative function allowed us to extract spatial trends in the anticyclonic eddy (AE 96-1). It is hypothesized here that the eastern edge of AE 96-1, characterized by thick layer of chlorophyll (between 100 and 150 m) due to the downward entrainment of chlorophyll down to 200 m, was favorable for small organisms (Paracalanus/Clausocalanus, Calocalanus, and Calanus) while higher abundance of large active swimmer such as chaetognaths was observed in the center. In the cyclonic eddy, the highest abundance of filter-feeders (ostracods, cladocerans, doliolids and salps) was related to enhance trophic conditions, i.e. highest chlorophyll concentration (4–8 mg m−3). These results show that cyclonic and anticyclonic eddies strongly influence the mesoscale characteristics of zooplankton in the Algerian basin during summer. Higher chlorophyll concentrations observed in spring suggest that such eddies can have an even more pronounced impact on the structuring of the ecosystem during high productive season.