Assessing the reliability of fatty acid-specific stable isotope analysis for trophic studies

1. Fatty acidspecific stable isotope analysis (FA-SIA) is expected to encompass most of the limits encountered when using more classical trophic markers such as bulk tissue stable isotope or fatty acid analyses. However, an essential premise for using FA-SIA for trophic studies is that the individual FA d13C values of the consumer reflect those of its diet. Field studies using FA-SIA have so far made this assumption, which is not necessarily supported by the rare experimental tests. 2. A feeding experiment was conducted on Daphnia to test whether the d 13 C values of individual fatty acids in Daphnia were actually related to those of its food. 3. Only the stable isotope composition of polyunsaturated (PUFA) and branched fatty acids (BrFA) was globally transmitted from the diet to Daphnia lipids, with however a significant isotope fractionation that varied depending on the considered diet source. 4. A model was constructed to evaluate how such variability may affect the reliability of FA-SIA to track the dietary sources of consumers' PUFA and BrFA in the field. Results suggest that provided the endmembers are substantially isotopically different, FA-SIA could provide valuable insights into the pathways conveying these particularFA to consumers in the field.