Increased degradation of copepod faecal pellets by co-acting dinoflagellates and Centropages hamatus

International audienceCopepod faecal pellets (FP) are carbon-rich particles possibly of great importance for the biological pump. However, most FP are degraded within the euphotic zone, by processes not yet fully understood. In a series of experiments conducted in Gullmarsfjorden, Sweden, we investigated degradation rates (r, d(-1)) of large copepod FP (average length and width: 466 x 62 mu m) exposed to the following treatments: (1) a natural assemblage of dinoflagellates (96 mu g C l-1), (2) the copepod Centropages hamatus (10 copepods l(-1)) and (3) a combination of the 2 treatments. FP incubated in filtered seawater served as a control and a measure of degradation by pellet-associated bacteria. Bacterial degradation of FP was low, only 0.04 d(-1), while the natural community of dinoflagellates degraded FP at a rate of 0.18 d(-1). FP incubated with C. hamatus were degraded at a rate of 0.6 d(-1), but degradation was faster when the dinoflagellates and C. hamatus acted together. The resulting degradation rate (1.12 d(-1)) was higher than the sum of the degradation rates measured under each individual condition (bacteria + dinoflagellates + C. hamatus). We suggest an interactive effect of dinoflagellates and copepods acting together-that the copepods were mechanically breaking up large FP, making the FP more available for further degradation by the dinoflagellates. This finding may have implications for understanding FP fluxes and carbon export in the ocean and points to a more complex 'coprophagous filter' than originally suggested