MEPS 293:131-142 (2005)  -  doi:10.3354/meps293131

Egestion of non-pellet-bound fecal material from the copepod Acartia tonsa: implication for vertical flux and degradation

Michael Olesen1,*, Solvita Strake2, Andris Andrushaitis2

1Marine Biological Laboratory, University of Copenhagen, 3000 Helsingør, Denmark
2Institute of Aquatic Ecology, University of Latvia, 2169 Riga, Latvia

ABSTRACT: Only a minor fraction of copepod defecation appears to leave the upper water column as fast-sinking fecal pellets in coastal waters. This study suggests that most egested matter from copepods is retained in the water column because (1) >50% of fecal matter is released as small, slowsinking particles that are not surrounded by a peritrophic membrane and (2) small fecal pellets sink slowly and are degraded rapidly. The production, appearance and fate of fecal material from the calanoid copepod Acartia tonsa (fed on 2 different phytoplankton species, the cryptophyte Rhodomonas salina and the diatom Skeletonema costatum) was followed in association with the grazing activity of the copepod in a laboratory experiment. For both diets, >50% of the defecation was released as dispersed small (<10 mm) non-pellet-bound particles. The diatom was less suitable as a food item than the flagellate and led to a 3 times higher rate of grazing and egestion. Nevertheless, specific assimilation and egg production per female were 2 times higher for the Rhodomonas diet versus the Skeletonema diet. As a result, the total egestion comprised 18% of the ingestion of Rhodomonas and 27% of the Skeletonema ingestion. In terms of vertical loss, sinking rates for both types of fecal pellets were ca. 5 m d–1 and, in terms of degradation, ca. 0.5 d–1 (18°C) in the absence of copepods. Transferring these findings to similar neritic conditions suggests that 60% of the fecal pellets from copepods will be recycled within a 15 m deep mixed layer and that >80% of the total fecal matter can be expected to be retained when the unbound fecal material is also included.

KEY WORDS: Zooplankton · Grazing · Chlorophyll · Fecal pellets · Egestion · Assimilation · Degradation · Vertical flux

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