Search

International audience; So far, appendicularians role in the biogeochemical cycling of organic matter has been overlooked. Appendicularians represent only a fraction of total mesozooplankton biomass, however these ubiquitous zooplankters have very high filtration and growth rates compared to copepod, and produce numerous fecal pellets and filtering houses contributing to the export production by aggregating small marine particles. To study their quantitative impact on biogeochemical fluxes, we have included this group in the Biogeochemical Flux Model (BFM), using a recently developed ecophysiological model. One dimensional annual simulations of the pelagic ecosystem including appendicularians were conducted with realistic surface forcing for year 2000, using data from the DyFAMed open ocean station. The appendicularian grazing impact was generally low, but appendicularians increased detritus production by 8% and export production by 55% compared to a simulation without appendicularians. Therefore present biogeochemical models lacking appendicularians probably under, or misestimate the detritus and export production by omitting the pathway from small sized plankton to fast sinking detritus. Detritus production and export rates are 60% lower than estimates from mesotrophic sites, showing that appendicularians' role is lower but still significant in oligotrophic environments. The simulated annual export at 200 m exceeds sediment trap values by 44%, suggesting an intense degradation during the sinking of appendicularian detritus, supported by observations made in other sites. Thus degradation and grazing of appendicularians detritus need a better quantification if we are to accurately assess the appendicularian role in the export flux.

Abundance and composition of microplankton were studied over a period of 2 years at two depths in Villefranche Bay (Ligurian Sea, NW Mediterranean Sea). Diatoms dominated the microplankton in late spring and autumn, whereas dinoflagellates composed the major part of the microplankton in summer. The silicoflagellate Dictyocha fibula and the diatom Thalassionema frauenfeldii dominated in winter. Ciliates showed low variability throughout the year with the lowest abundance in February and an increase which coincided with the diatom maxima during autumn in both years. In 1998, the spring bloom (in May) was mainly composed of dinoflagellates near the surface and of diatoms in deeper layers. Subsurface diatom maxima were observed in August-September and November. In 1999, diatoms peaked in May both at the surface and at the depth of 50 m. They showed a strong maximum in October. Dinoflagellates and tintinnids showed maxima in early November. Comparisons with previous studies reveal that (i) changes in species composition have not been significant, (ii) the silicoflagellate's abundance is lower during the present study, (iii) the sequential spring bloom is composed of a pico-nanoplankton bloom in March and microphytoplankton in May, whereas in other western Mediterranean areas the spring microphytoplankton bloom is reported in February and March, (iv) high water transport through the Corsica channel coinciding with low or negative winter values of North Atlantic Oscillation (NAO) index are associated with the anomalous strong development of the spring diatom blooms in the Bay of Villefranche, whereas the usual trend is the lack of or weak development of the spring diatom bloom. This feature may determine the nature and the fate of primary production and the interannual variability in the relative importance of the microbial food web versus the microbial loop.