Vertical Migrations and Feeding Rhythms of Mesozooplanktonic Organisms in the Rhône River Plume Area (North-west Mediterranean Sea)

The vertical distribution of mesozooplankton was studied for a 24-h period at a fixed station. The most abundant taxa (Clausocalanus spp., Paracalanus parvus, Temora stylifera, Centropages typicus, Acartia clausi, Microsetella spp., appendicularians, doliolids and chaetognaths), exhibited nocturnal upward migration into the chlorophyll-rich and low-salinity plume waters (surface to 1 m depth) while the cladocerans Podon polyphemoides and Penilia avirostris exhibited reverse migrations. Feeding activity (gut fluorescence) was investigated for T. stylifera, C. typicus and A. clausi . The chlorophyll gut contents of these copepods were highest at night in the plume waters (0 and 1 m). Nocturnal feeding and migration appeared synchronized for T. stylifera but not for C. typicus, which migrated later (middle of the night) than the other copepods in the upper layer. Respiration rates were measured and compared, on a carbon basis, to algal ingestion rates. The budgets suggested the energetic advantage of the nocturnal migration in the plume for C. typicus and T. stylifera which appeared able to produce significantly more organic matter in the plume waters than in the deeper waters. Acartia clausi could not balance its metabolic budget in both plume and marine waters suggesting a poor adaptation of this species to the algal trophic conditions. Metabolic O:N ratios measured for total zooplankton were lower in marine water (about 10) than in plume water (about 20) suggesting other trophic behaviour than herbivorous feeding (carnivorous, detritivorous …) for animals living under the plume.