Diel variations in the vertical distribution of crustacean zooplankton and food selection by planktivorous fish in a shallow turbid lake

The variations in the vertical migrations by zooplankton and food composition of fish in the shallow, clay-turbid Mustionselka basin of Lake Hiidenvesi were studied (water depth 3 m). Daphnids showed signs of reverse migration, their density in the surface layer being highest during the day. The reverse migration was probably due to predation threat by the predatory cladoceran Leptodora kindtii. Chydorus sphaericus, which was consumed especially by the day active bleak (Alburnus alburnus), showed normal vertical migration. Copepods showed no clear vertical movements. Leptodora kindtii, as well as the large herbivore Limnosida frontosa, showed no vertical migrations, but their overall abundance increased considerably during the night, indicating horizontal movement towards the open water at night. Due to the shallowness of the water, vertical migrations may not be a good predator-avoidance strategy for the largest cladocerans, which may be able to perform horizontal migrations.