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Abstract

Herbivorous zooplankton species are considered to occupy a central position in pelagic food webs since they play an important role in structuring algal communities and, at the same time, provide food for higher trophic levels. Consequently, the commonly observed diel vertical migrations of the zooplankton must affect the food webs by creating diii rhythms of algal mortality and reducing the zooplankton production. Hypotheses on the adaptive value of diel vertical migrations are discussed. Little evidence is available for the validity of hypotheses proposing a metabolic or demographic advantage of migrating
over non-migrating animals. However, there is evidence from two Daphnia species in Lake Constance and from Cyclops in Polish lakes that diel vertical migration is a mechanism for avoiding consumption by planktivorous fish. Diel shifts of zooplankton biomass cause considerable quantitative and qualitative fluctuations of grazing in the euphotic zone of lakes. The mortality of іdiblе algae is low during day
and high at night. The algae can grow unimpeded during the day, but are 'hагvеѕtеd at night. Mathematical models show that this rhythm enhances primary production. Thus, plаnКtіvoгоuѕ fish create effects cascading through the whole ecosystem structure from the highest to the lowest trophic level.