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Abstract

1. The ultimate explanation for diel vertical migration (DVM) of zooplankton is the avoidance of visual predation in surface waters. Studies on migrating zooplankton have shown that remaining in the cold and food-poor hypolimnion during the day, however, has demographic costs. Higher temperatures and greater food concentrations in the surface waters are thought to be the main reasons why Daphnia species move upwards at night. 2. In this study, we investigated the growth condition of daphniids raised on seston taken from different depths from a lake with and without a deep-water chlorophyll maximum. 3. Juvenile growth rates of Daphnia galeata x hyalina from the lake without a deep-water chlorophyll maximum were similar for all treatments. After temperature correction, however, growth rates were significantly higher on seston taken from the surface layers. 4. In contrast, in the lake with the deep-water chlorophyll maximum, D. galeata growth rates were higher in deeper strata, even after temperature correction. Although this lake had a weak temperature gradient, D. galeata left the food- rich strata at night and migrated into the surface food-poor environment. Invertebrate predation and oxygen depletion are probably not the reasons for the nocturnal upward migration into the surface strata. Therefore, we assume that D. galeata migrates upwards to take advantage of higher temperatures. Using several temperature-egg-development models, we could not, however, fully explain this behaviour.