Do transparent exopolymer particles (TEP) inhibit grazing by the euphausiid Euphausia pacifica?

The hypothesis that ubiquitous, sticky transparent exopolymer particles (TE P) formed from phytoplankton exudates will adhere to and coat the feeding s tructures of marine zooplankton grazers, and thus depress feeding on phytop lankton, was tested using the euphausiid, Euphausia pacifica, as a model or ganism. During two feeding experiments, E. pacifica were offered cells of t he diatom Thalassiosira weissflogii, TEP, or both TEP and T. weissflogii ce lls. Ingestion rates on cells were lower in the presence of TEP. However, c ontrary to the hypothesis, grazing on cells was not inhibited by TEP. Rathe r, TEP-clusters, aggregates which formed from TEP and nano-sized particles normally too small for the filtering apparatus of E. pacifica to retain, se rved as an alternative food source for E. pacifica, reducing their ingestio n of cells. These clusters were very similar in form to the TEP actually av ailable to marine grazers in nature. TEP-clusters were similar to cells in size and food quality, and were grazed at similar rates. When feeding on TE P-clusters, euphausiids short circuit the food web by feeding on nano- and picoplankton directly, bypassing the microbial loop. Thus, the presence of TEP appears to enhance, rather than depress, macrozooplankton grazing.