Structure et importance des peuplements zooplanctoniques dans la baie d'Arcachon : milieux ouverts et lagunes aménagées de Certes

Specific composition and distribution of the zooplankton in Arcachon Bay varie along a salinity gradient from oceanic to neritic and lagoonal condi­tions. A peak of density and biomass is recorded in spring due to an increase of holoplanktonic species as well as larvae of benthic Invertebrates. A second maxi­mum of abundance is observed in autumn. The community structure of copepods and polychaets larvae is considered in detail. The diversity (Shannon index, Sanders's rarefaction) decreases from the mouth of the Bay to the inner neritic water mass. Demographic parameters indi­cate that the populations structure is affected by the salinity gradient. Biomass increases significantly from the oceanic water to the inner neritic water mass while the species richness is decreasing.In the lagoonal impoundments at Certes, there are no truly autochthonous planktonic forms. Allochthonous populations enter the ponds when the latter are flooded with sea water. The planktonic immigrants cannot colonize the fish ponds for a long time. Ubiquitous communities dominate the zooplankton. These popula­tions are not strictly dependent on a distributional pattern and have affinity with benthos. They show meiobenthic characters, the importance of which is empha­sized in lagoon ecosystems.