Photosynthetic protists (i.e. microscopic algae; the phytoplankton) constitute the base of the food chains in most regions of the ocean. However, in some regions such as the Ross Sea, Antarctica, the algae form large colonies and/or noxious materials that make them less palatable to the minute animals in the plankton. Thus, it is not clear how the zooplankton in these regions obtain their nutrition. In this project we test the hypothesis that heterotrophic protists (rather than the algae themselves) constitute an important food source for zooplankton in the Ross Sea.