Competition vs. predation: are small heterotrophic dinoflagellates competitors for food or prey for bigger ciliates?

We investigated the grazing impact and competition patterns of two different groups of microzooplankters, dinoflagellates and ciliates, preying upon the flagellate Scrippsiella sp.. Two model predators isolated from Helgoland Roads (North Sea) were chosen for the experiments, the big tintinnid Favella sp. and the small dinoflagellate Gyrodinium sp.. The grazing impact of both species as single predators and both predators in combination on the prey Scrippsiella sp. was investigated. Since Gyrodinium sp. ranges in the same size as the prey organism Scrippsiella sp., its role as a potential prey item for the ciliate Favella sp. was additionally investigated. The aim of the present study was to elucidate potential competition patterns between both predators and, furthermore, to answer the question if the tintinnid can be considered as a consumer of predominantly autotrophic or heterotrophic prey items. Our data indicate that Favella sp. prefers autotrophic prey, but the results of the present feeding experiments will be discussed in the light of food quality aspects, too