1. Trophic fractionation was studied in short-term laboratory feeding experiments with larvae of the deposit-feeding midge Chironomus riparius. Larvae were fed food of terrestrial (oats, peat) and aquatic origin (Spirulina, Tetraphyll®).
2. By analysing both whole larvae and isolated gut contents we were able to distinguish between the isotopic signature of recently ingested food and that of assimilated carbon and nitrogen in body tissue. Additionally we studied the effects of microbial conditioning, i.e. the colonisation and growth on food particles of microbes, on the isotopic signal of food resources.
3. Nitrogen fractionation for the different food types ranged from 0.67‰ to 2.68‰ between consumer and diet and showed that isotopic fractionation can be much lower than the value of 3.4‰ that is commonly assumed.
4. Microbial degradation of food particles resulted in an approximate doubling of the δ15N in 8 days, from 6.24 ± 0.05‰ to 11.36 ± 0.56‰. Values for δ13C increased only marginally, from −20.66 ± 0.11‰ to −20.34 ± 0.12‰. These results show that microbial conditioning of food may affect dietary isotope signatures (in particular N) and, unless accounted for, could introduce an error in measures of trophic fractionation. Microbial conditioning could well account for some of the variation in fractionation reported in the literature.