Zooplankton community structure in the Western Mediterranean sea related to mesoscale hydrodynamics
Résumé
In the Liguro-provencal basin (Northwestern Mediterranean sea), an heterogeneous hydrodynamic structure was observed in May 1986 (Pinca & Dallot, 1995). The hydrological system was characterized, as usual for this region, by the cyclonic Ligurian current with its associated thermohaline coastal fronts separating the coastal area from a central divergence zone, but was complicated in 1986 by the presence of an anticyclonic eddy in the middle of the central area. The sampling sites were consequently separated into four zones according to this physical structure. Mesozooplankton community structure related to the physical organization of the water masses is presented here. In order to obtain a description of the community structure, we studied both structure indices (species richness, evenness, dominance) and Rank-Frequency Diagrams. These descriptors showed a certain patchiness: species diversity and evenness were both minimal in the central area and increased towards the zones that surrounded the eddy and the coastal regions. However, almost everywhere we observed a stage 1 in the ecological succession as depicted by Frontier (1976), indicating that the development of the community structure (Margalef, 1967) was still at a juvenile state, else, that it was disturbed by the rapidly growing population of a single species. The most striking characteristic of the data set was actually the strong predominance of one or two copepod species. Centropages typicus prevailed: its population increased during the spring bloom, particularly in the area of the anticyclonic eddy. In order to better identify the organization of the region in terms of different ecological maturity stages and to search for a relation between mesoscale physical structure and community patterns, we grouped the sites on the basis of relative abundance of ranked species. We used an agglomerative cluster analysis with a new dissimilarity index, corrected to limit the weight of the most abundant species. We then tested for differences among the seven obtained groups in terms of structure indices, biological and environmental parameters. The physical environment could be related to the organization of the plankton community. In the central area, where the divergence and the eddy were located, a higher concentration of phytoplankton, total mesozooplankton abundance and biomass were recorded. The eddy was characterized by a higher concentration of relatively bigger and omnivorous copepods (Euchirella rostrata, Pleuromamma gracilis, Centropages typicus), probably favored by their stronger swimming ability in this highly energetic environment. However, the more specific organization of the community was regulated by more complicated biological interrelations that gave rise to a higher level of patchiness in the ecological structure.