Recent field studies indicate that dinoflagellates are key degraders of copepod fecal
pellets. This study is the first to publish direct evidence of pellet grazing by dinoflagellates. Feeding
and growth on copepod fecal pellets were studied for both heterotrophic (4 species) and mixotrophic
dinoflagellates (3 species) using a combination of classic incubation experiments and video
recordings of feeding behavior. Fecal pellets were produced by adult Acartia tonsa feeding on
Rhodomonas salina. Two mixotrophic species (Karlodinium armiger, a gymnodinoid dinoflagellate,
Gy1) and all heterotrophic dinoflagellates (Gyrodinium dominans, Gyrodinium spirale, Diplopsalis
lenticula, Protoperidinium depressum) studied fed on fecal pellets. Using natural concentrations of
dinoflagellates and copepod fecal pellets, average ingestion rates of 0.2 and 0.1 pellets cell-1 d-1 and
clearance rates of between 0.2 and 0.3 ml cell-1 d-1 were obtained for G. spirale and P. depressum, respectively.
Pellet feeding resulted in average growth rates of 0.69 and 0.08 d-1 with growth yields of
0.58 and 0.50 for G. spirale and P. depressum. Important factors for the grazing impact of the dinoflagellates
on fecal pellets in this study were: dinoflagellate concentration, the dinoflagellate-to-pellet
size ratio, the feeding mechanism, pellet food source, and pellet age. This study reveals a new trophic
role for dinoflagellates as detritivores, and shows that large (>20 µm) heterotrophic dinoflagellates
alone can account for reported pellet degradation rates in field studies. Thus, dinoflagellates can
function as an effective ‘protozoan filter’ for fecal pellets in the water column.