Picoplankton and nanoplankton aggregation by appendicularians: Fecal pellet contents of Megalocercus huxleyi in the equatorial Pacific

The content of fecal pellets of the freshly collected warm water appendicul arian Megalocercus huxleyi was studied by light and electron microscopy and by flow cytometry in the superficial 100 m of the water column at 2 degree s N, 165 degrees E, in September 1994, during the Flux dans l'Ouest du Paci fique Equatorial (Joint Global Ocean Flux Study-France) oceanographic cruis e. Microscopic observations showed that the fecal pellet contents of M. hux leyi reflected the natural composition of the nanophytoplankton and small m icrophytoplankton (<50 mu m). Larger cells were excluded from entering the filtering system by the inlet filters. Coccolithophorids appeared as the ma in component found in the feces. Evidence for ingestion of "naked" cells by this appendicularian is given. Analysis of picoplankton in fecal pellets b y flow cytometer confirmed that appendicularians efficiently collect small particles. Cyanobacteria, similar to 1 mu m in diameter, were found in larg e quantities and showed high fluorescence in the fecal pellets. Most of the se cyanobacteria in the pellets appeared to be intact, and thus may be good indicators of the appendicularian ingestion rate. The situation was differ ent for the prochlorophyte Prochlorococcus abundant in the seawater and for picoeucaryotes (<2 mu m). These were found at very low quantities in the l arvacean fecal pellets. The calculations showed that with an average concen tration of 5 M. huxleyi m(-3), >7% of the small particulate matter will be daily removed from the water. Some of this matter will be assimilated, some trapped in the houses, and the rest aggregated into rapidly sinking fecal pellets. Ingestion of large quantities of coccolithophorids indicates that appendicularians are important not only in the cycle of organic carbon but also of inorganic carbon. Moreover, if appendicularians successfully aggreg ate and assimilate Prochlorococcus and picoeucaryotes, then their grazing a ctivity can represent a major pathway of carbon transformation in the tropi cal ecosystem.