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Dietary polyunsaturated fatty acid supply improves
 Daphnia
 performance at fluctuating temperatures, simulating diel vertical migration

Diel vertical migration (DVM) is a common predator avoidance strategy of zooplankton. Migration to deeper water layers during the day to escape visually hunting predators is most likely to require physiological adaptations to periodically changing temperatures. Polyunsaturated fatty acids (PUFA) play crucial roles in membrane temperature acclimation. Exposure to cold temperatures typically results in an increase in the relative abundance of PUFA in cell membranes and PUFA requirements of Daphnia increase with decreasing temperatures. To assess the role of dietary PUFA in coping with temperature fluctuations experienced during DVM, we reared Daphnia magna at either constantly warm or fluctuating temperatures, simulating DVM both with and without dietary PUFA supplementation. We show that the well‐known positive effect of dietary eicosapentaenoic acid supplementation on offspring production and population growth rates of D. magna is more pronounced at alternating temperatures than at constantly warm temperatures. Exposure to alternating temperatures caused modification in body PUFA concentrations and, consequently, increased lipid peroxidation. However, detrimental effects of lipid peroxidation were not evident. Our data demonstrate that the capacity to cope with the distinct temperature fluctuations experienced during DVM increases with dietary eicosapentaenoic acid supplementation, suggesting that an adequate dietary PUFA supply is crucial especially for migrating Daphnia populations. A dietary deficiency in long‐chain PUFA may thus severely constrain predator evasion, potentially resulting in increased mortality and cascading effects on lower trophic levels.