Short term changes in zooplankton community during the summer-autumn transition in the open NW Mediterranean Sea: species composition, abundance and diversity

Short term changes in zooplankton community were investigated at a fixed station in offshore waters of the Ligurian Sea (DYNAPROC 2 cruise, September-October 2004). Mesozooplankton were sampled with vertical WP- II hauls (200µm mesh-size) and large mesozooplankton, macrozooplankton and micronekton with a BIONESS multi- net sampler (500µm mesh-size). Temporal variations of to- tal biomass, species composition and abundance of major taxa were studied. Intrusions of low salinity water masses were observed two times during the cruise. The first one, which was the most intense, was associated with changes in zooplankton community composition. Among copepods, the abundance of Calocalanus, Euchaeta, Heterorhabdus, Meso- calanus, Nannocalanus, Neocalanus, Pleuromammaand also calanoid copepodites increased markedly. Among non- copepod taxa, only small ostracods abundance increased. Af- ter this low salinity event, abundance of all taxa nearly re- turned to their initial values. The influence of salinity on each zooplankton taxon was confirmed by a statistical anal- ysis (Perry's method). The Shannon diversity index, Pielou evenness and species richness were used to describe temporal variations of large copepod (>500µm) diversity. The Shan- non index and Pielou evenness decreased at the beginning of the low salinity water intrusions, but not species richness. We suggest that low salinity water masses contained its own zoo- plankton community and passed through the sampling area, thus causing a replacement of the zooplankton population.