MESOZOOPLANKTON AND MACROZOOPLANKTON COMPOSITION PATTERNS RELATED TO HYDRODYNAMIC STRUCTURES IN THE LIGURIAN SEA (TROPHOS-2 EXPERIMENT, APRIL-JUNE-1986)
Résumé
The distributional patterns of zooplanktonic fauna based on samples taken during the Trophos-2 cruise (spring 1986) in the western Liguro-Provencal Basin, Mediterranean Sea, are described. This region is permanently exposed to a cyclonic circulation with an associated thermohaline front which encloses a central divergence zone. In the middle of this central area, we encountered a mesoscale (about 70 km diameter) anticyclonic eddy. Taking into account the hydrological structure of the 0 to 200 m upper level, the area could be divided into 4 zones: (1) peripheral and frontal, (2) central Riviera, (3) central Corsica and (4) the eddy. These zones differed in the abundance of several zooplankton species. In particular, some of them showed a higher concentration in the eddy, such as the radiolarian Aulacantha scolymantha, the euphausiid calyptopis larvae, and the copepods Centropages typicus, Pleuromamma gracilis, Mesocalanus tenuicornis, Heterorhabdus papilliger and Euchirella rostrata. We suggest the central eddy is a favorable environment for these large species. Thus, increased primary and secondary production due to the influence of hydrodynamic phenomena appears not to be Limited to the frontal zone. Within the eddy, zooplankton growth could be enhanced, thus favoring export production. To detect zooplankton species assemblages, the stations have been clustered into 5 groups based on their taxonomic composition and abundance. These groups had a rather good geographic coherence although they had a more complex distribution than the physical structure. This higher variability of spatial organization of the 5 plankton assemblages can be explained by behavioral and trophic phenomena.
Domaines
Océanographie
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