Stable isotope fractionation of fatty acids of Daphnia fed laboratory cultures of microalgae

•We did compound specific isotope analyses of fatty acids (CSIA-FA) in laboratory.•CSIA-FA was done for Daphnia fed batch cultures of Chlorella and Cryptomonas.•All essential polyunsaturated FA in Daphnia were lighter than those in microalgae.•There was no constant value of isotope fractionation of FA in Daphnia.•The results contradicted common way of interpretation of CSIA-FA in field studies.