Limnetica 38

In two Saharan freshwater lakes (gueltas), 3-6 m deep, one with and one without fish, zooplankton migrated vertically. The guelta with fish had a very impoverished zooplankton, mainly composed of copepodids of cyclopoid copepods. They showed a strong diurnal migration. The guelta without fish had Daphnia, Asplanchna, and the fairy shrimp Streptocephalus. Daphnia migrated normally at first, but was later disturbed by Streptocephalus that moved around in swarms and physically interfered with the smaller zooplankton. Interference competition is a factor to be considered whenever several species of zooplankton migrate side by side. This may result in a vertical stratification of species, and in extreme cases, in reverse migration.