Copepod grazing on diatom aggregates: Influences on particles size, sinking velocities and carbon export
Résumé
Within the marine field, zooplankton grazing may contribute to carbon export through faecal pellets emission or delay vertical sinking flux sinking phytoplankton aggregates. To assess this process, artificial monospecific aggregates of two diatom species (Chaetoceros neogracile and Skeletonema marinoi) and natural aggregates of Melosira sp. were incubated with different copepod species (Acartia clausi, Temora longicornis, Calanus helgolandicus, Euterpina acutifrons and Calanus hyperboreus). We measured a direct grazing on aggregates with ingestion rates ranging from 0.39 to 6.71 µg pigment ind-1 d-1. The relation between aggregates equivalent spherical diameters and sinking velocities have not changed, showing that copepods grazing do not affect their densities. Regarding the particle flux, we observed three main trends: 1) Fragmentation: copepods activities increase the number of particles and decrease sinking velocities, the case of incubation with T. longicornis, the number of aggregates per litter increased with significant decrease of aggregates sinking velocities; 2) Re-aggregation: copepods activities increase aggregates size spectra and consequently the sinking velocities, the average size spectra of C. neogracile aggregates significantly increased during incubation with A. clausi but not with E. acutifrons; 3) No impact, when S. marinoi aggregates were incubated with E. acutifrons or C. helgolandicus. Our results showed that copepod grazing could modulate biological pump of carbon through fragmenting or re-aggregating but not through change of aggregate density.