Abstract
Copepod 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.
Highlights
Carbon-rich particles sinking from the euphotic zone to the deeper, less productive parts of the ocean are important elements of the biological pump
In a series of experiments conducted in Gullmarsfjorden, Sweden, we investigated degradation rates (r, d−1) of large copepod faecal pellets (FP) exposed to the following treatments: (1) a natural assemblage of dinoflagellates (96 μg C l−1), (2) the copepod Centropages hamatus (10 copepods l−1) and (3) a combination of the 2 treatments
The experiments revealed that dinoflagellates had a significant effect on FP degradation (r = 0.18 d−1), but that C. hamatus were 3-fold more efficient (r = 0.60 d−1) in terms of degradation rates
Summary
Carbon-rich particles sinking from the euphotic zone to the deeper, less productive parts of the ocean are important elements of the biological pump The efficiency of this pump is related to primary productivity and degradation of organic material. Copepod faecal pellets (FP) are possibly important contributors to the downward flux of carbon, and mechanisms for FP degradation have received increased attention during the last decade (Poulsen & Kiørboe 2005, Reigstad et al 2005, Iversen & Poulsen 2007, Ploug et al 2008, Svensen et al 2012). Samples for chl a (total and the >11 μm fraction) were filtered onto GF/F filters and extracted in ethanol for 24 h in 96% ethanol (Parsons et al 1984)
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