Daphnia fatty acid composition reflects that of their diet

We conducted a series of experiments feeding Daphnia pulex nine different phytoplankton monocultures with widely varying fatty acid composition and nutritional values to test the extent to which Daphnia fatty acid composition was affected by diet. In general, Daphnia fatty acid composition matched that of their diet much more closely than it did the fatty acid composition of Daphnia consuming other diets. However, Daphnia had consistently less saturated fatty acids and more arachidonic acid than did their diet, and Daphnia consuming cyanobacteria had substantially less saturated fatty acids and more monounsaturated fatty acids than their diets. Daphnia that consumed cryptophytes, which are rich in v3 polyunsaturated fatty acids (PUFAs), had on average 47% 6 8% (61 SD) v3 PUFAs within their fatty acid pool, whereas Daphnia that consumed v3 PUFA–poor cyanophytes only had 6% 6 3% v3 PUFAs. The ratio of v3 to v6 fatty acids in Daphnia was also strongly dependent on diet, and averaged < 10 : 1, 2 : 1, and 1 : 1 for Daphnia that consumed cryptophytes, chlorophytes, and cyanophytes, respectively. Furthermore, the sum of C20 and C22 v3 and v6 fatty acids in Daphnia was highly correlated with that of their diet (r2 5 0.94). These results suggest analyses of Daphnia fatty acid composition may be a powerful means of inferring diet in the field. These results also suggest the nutritional benefits of consuming v3-rich phytoplankton will transfer up the food web, making zooplankton both more efficient at converting phytoplankton biomass to their own biomass as well as much more nutritious for the zooplanktivorous fish that consume them. There is tremendous interest in developing approaches for inferring diet in organisms based on their elemental and biochemical composition. The classic approach is to use analyses of stable isotopes to infer dietary source (carbon and sulfur) and trophic position (nitrogen), thereby providing three variables with which to characterize an organism’s trophic niche (Peterson and Fry 1987). However, this approach can only be used to infer dietary sources if the sources of interest have distinct stable isotope ratios, which is not always the case, especially for generalist herbivores. A number of authors have explored the utility of using fatty acids as trophic markers of dietary composition, i.e., the FATM concept (Dalsgaard et al. 2003). This concept has been most commonly applied to marine systems (Dalsgaard et al. 2003), but has also been applied to studies of freshwater streams (Desvilettes et al. 1997; Napolitano 1999; Heintz et al. 2004). Fatty acids (FAs) are critical constituents of all biota, where they comprise the main component of neutral and polar lipids. Polar lipids provide the basic cellular membrane matrix into which other membrane constituents such as cholesterol and proteins are embedded (Vance 1996). The FATM concept is based on the fact that the major primary producer taxa have distinctive fatty acid profiles that may be, to varying degrees, transferred conservatively to consumers. FAs can be incorporated into the neutral lipids of primary consumers virtually unaltered, especially when catabolic activity is low, such as when accumulating lipid reserves. However, because consumers selectively metabolize FAs and can convert some forms to others, FAs can only be used as semiquantitative food web tracers (Dalsgaard et al. 2003). Because the fatty acid composition within phytoplankton is largely dependent on systematic affiliation, primary producers may ‘‘lay-down’’ the basic fatty acid pattern in aquatic food webs (Jefferies 1970; Ahlgren et al. 1996). The main phytoplankton groups, such 1 Corresponding author ([email protected]). 2 Present address: Center for Marine and Climate Research, Institute of Hydrobiology and Fisheries Research, University of Hamburg, D-22767 Hamburg, Germany. 3 Present address: Nicholas School of the Environment, Division of Earth and Ocean Sciences, Duke University, Box 90229, Durham, North Carolina 27708-0229. Acknowledgments We thank Anne Liston for overseeing the fatty acid analyses and Carolyn Burns, Norm Yan, and two anonymous reviewers for their very helpful comments to an earlier version of this paper. The study was supported by a National Science Foundation grant (DEB-0075591) to M.T.B., C.R.G., and D.C.M.N. Limnol. Oceanogr., 51(5), 2006, 2428–2437 E 2006, by the American Society of Limnology and Oceanography, Inc.