MEPS 279:13-22 (2004)  -  doi:10.3354/meps279013

ABSTRACT: We studied the ingestion of picoplankton-size particles by the oligotrich Strombidium sulcatum as a function of transparent exopolymeric particle (TEP) concentration. Fluorescent microspheres of 1 µm (FMS) were used as prey-item analogs. TEP-FMS aggregates were formed by bubbling seawater solutions containing different TEP concentrations in the presence of FMS. After bubbling, concentrations of TEP-FMS aggregates ranged from 0.5 to 1.7 ppm and from 11.9 to 17.9 ppm in the low and high TEP treatments, respectively. When the ciliates were exposed to similar FMS concentrations, but different TEP concentrations, ingestion rates were higher for the low TEP treatments (14.1 FMS ciliate^-1 h^-1) than for the high TEP treatments (5.2 FMS ciliate^-1 h^-1). The number of FMS ingested in the low TEP treatments was 2.7 times higher than in the high TEP treatments and ingestion rates were inversely correlated to TEP volume concentration. Our results suggest that the formation of TEP-FMS reduces FMS availability for micro-grazers by modifying the food size spectra and redistributing FMS to larger size classes, and that at a critical TEP concentration picoplankton-size prey items may become unavailable for the ciliate S. sulcatum, while concomitantly they may become available for large-particle grazers. Therefore, one role of TEP in the microbial food web would be to act as a trophic elevator, thus creating a direct lift for picoplankton-size prey items to higher trophic levels.

KEY WORDS: Transparent exopolymeric particles · Fluorescent microspheres · Aggregation · Grazing · Food selection · Food size spectra · Strombidium sulcatum