THE FEEDING ECOLOGY OF THE CYCLOPOID COPEPOD DIACYCLOPS THOMASI IN LAKE ONTARIO

Diacyclops thomasi is the dominant crustacean zooplankton in Lake Onta rio, making up much of the mesozooplankton biomass. However, the role of Diacyclops in the food-web dynamics of Lake Ontario is relatively u nexplored. The feeding ecology of Diacyclops in Lake Ontario was exami ned by predator manipulation experiments, performed monthly from April 1991 until September 1991. Diacyclops consumed a variety of food item s including: a mixotrophic dinoflagellate (Gymnodinium helveticum); ci liates (choreotrichines); and other-protozoans (heliozoans). Prey item s were generally microplankton-sized (15-100 mu m), motile, and withou t any hard external covering. Diacyclops had no effect on non-motile a lgae, small flagellates (<15 mu m), or on organisms with tests or lori cas. Keratella cochlearis, a loricate rotifer; was the most abundant r otifer through most of the stratification period but was not consumed by Diacyclops. However, we present evidence that Diacyclops preys on t he eggs of this species without consuming the females to which the egg s are attached. Our results indicate that adult Diacyclops are selecti ve omnivores in the pelagic food chain of Lake Ontario.