Stable isotope fractionation of fatty acids of Daphnia fed laboratorycultures of microalgae

tWe tested a comparatively new method of tracing of natural food webs, compound-specific isotopeanalysis (CSIA) of fatty acids (FA), using laboratory culture of Daphnia galeata fed Chlorella vulgaris andCryptomonas sp. In general, Daphnia had significantly lighter carbon stable isotope composition of mostfatty acids, including essential, than those of their food, microalgae. Thus, our results did not support thepivotal premise of the FA-CSIA application for food web analysis, i.e., transmitting the isotope ‘signal’ ofessential FAs to consumers from their food without any modification. Moreover, the values of isotopefractionation of particular FAs in the consumer relative to its food were not constant, but varied from1.35‰ to 7.04‰. The different isotope fractionation (depletion) values of diverse FAs in consumer wereprobably caused by different processes of their synthesis, catabolism and assimilation. More work isevidently to be done for correct interpretation of results of FA-CSIA during field studies for tracing ofnatural food webs