Protoplankton are single-celled or colonial, eukaryotic (having a true nucleus) protozoa that swim or float in water. Protoplankton differ from algae that occupy similar habitats-the phytoplankton - in lacking photosynthetic ability. However, some protoplankton have endosymbiotic, photosynthetic algal cells that supply them with food. Freshwater protoplankton live in ponds, lakes, rivers, and streams.