Fate of copepod faecal pellets and the role of Oithona spp.

Abstract

Because the downward flux of zooplankton faecal pellets (FP) measured in sediment traps is typically lower than FP production in overlaying waters, it is inferred that FP are recycled. One suggested explanation for low export fluxes of FP is retention through processes such as FP ingestion (coprophagy) and FP fragmentation (coprorhexy). It has become accepted that the globally important cyclopoid copepod Oithona spp. are one of the main organisms causing high retention of copepod FP, however, the experimental evidence supporting this hypothesis is limited. We conducted 3 experiments to test whether the raptorial feeding Oithona spp. removed sinking FP material produced by Calanus finmarchicus either by grazing or fragmentation. No such evidence was found. We therefore suggest that alternative explanations for pellet recycling should be sought. We believe that Oithona spp. may be indicator species for a 'retention ecosystem', but they are probably not the single factor explaining FP retention in the world oceans.

Particulate organic carbon export in the upper twilight zone during the decline of the spring bloom

“…This calculation gives potential ingestion rates of 147, 87 and 177 mg C m -2 d -1 by the migrating zooplankton population in the upper water column on Days 2, 4 and 6, respectively. These ingestion rates could have been underestimated due to potential coprophagy/coprorhexy by the ubiquitous small calanoid copepod Oithona similis (Reigstad et al 2005) which was the most abundant zooplankton species in the upper 100 m of the water column throughout the study period (E. Head, Bedford Institute of Oceanography, pers. comm.).…”

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

“…Vials were placed in the dark in a submerged plankton wheel at 17°C rotating at 1 rpm to keep the material in suspension (see e.g. Reigstad et al 2005). Oxygen was monitored every 6 to 8 h for 48 h with an oxygen sensor microsensor (Unisense AS) and never dropped below 15 to 20% (Renaud et al 2007).…”

“…While cyclopoid copepods of the genus Oithona have for some time been considered the main grazers on FP (González & Smetacek 1994, Svensen & Nejstgaard 2003, more recent studies suggest that this is not always the case and that other flux-feeding organisms must also be taken into consideration (Reigstad et al 2005, Iversen & Poulsen 2007. For instance, protozooplankton are able to use FP as a direct food source (Jacobsen & Azam 1984, Noji 1991, Turner 2002, Poulsen & Kiørboe 2006, Poulsen & Iversen 2008.…”

“…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. However, the contribution of FP to the downward flux of carbon is highly variable both seasonally (Wexels Riser et al 2010) and geographically (Wexels Riser 2007).…”

Feeding by the cyclopoid copepod Oithona similis on the microplankton assemblage in the Oyashio region during spring

“…Lampitt & Gamble 1982;Drits & Semenova 1984;Uchima & Hirano 1986). Oithonids have also been observed to feed on fecal pellets of other zooplankton (González & Smetacek 1994), though their coprophagous feeding is still controversial (Reigstad et al 2005;Iversen & Poulsen 2007). Recent experimental studies employing natural prey assemblages have revealed that oithonids feed selectively on microzooplanktonic protists, such as ciliates and heterotrophic dinoflagellates (e.g.…”