We investigated the effect of food quality on somatic growth and reproduction of zooplankton at different temperatures (12uC, 15uC, 20uC, and 25uC). Standardized growth experiments of two cladocerans, Daphnia magna and Simocephalus vetulus, were performed on (1) high-quality food (Cryptomonas sp.), (2) relatively low-quality food (Scenedesmus obliquus), and (3) intermediate-quality food (Cryptomonas : Scenedesmus mixture). Food quality constraints on somatic growth and reproduction of the two cladocerans decreased with increasing temperature. For D. magna and for S. vetulus, differences between clutch size and growth rate of individuals fed on the three food sources were highly pronounced when they were reared at 12uC and 15uC; however, such differences decreased at 20uC and were negligible at 25uC. Variations in food quality constraints with temperature can be explained by the variability of dietary polyunsaturated fatty acids, such as eicosapentaenoic acid and stearidonic acid requirements of these cladocerans. We conclude that dietary constraints exerted by food quality for zooplankton development vary as a function of different temperature conditions.Since the conceptual work of trophodynamics by Lindeman (1942), determining factors that affect matter transfer efficiency is a key issue in ecology. In aquatic food webs, the efficiency of energy transfer at the plant-animal interface is highly variable (Brett and Mü ller-Navarra 1997), consequently resulting in variable secondary production. Thus far, several studies have been motivated by this variability of dietary energy transfer at the phytoplankton-zooplankton interface, and it has been clear for many years that these variations can be attributed to the variation in food quality of algae for herbivorous zooplankton (Ahlgren et al. 1990; Mü ller-Navarra and

Summary1. Fatty acid-specific 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 d 13C 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 13C 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 particular FA to consumers in the field.