The nutritional importance of polyunsaturated fatty acids and their use as trophic markers for herbivorous zooplankton:: Does it contradict?

Fatty acids of field caught Daphnia spp. (Phyllopoda) and Eudiaptomus spp. (Copepoda) representing a non-selective and a selective grazer were analyzed and compared to the respective fatty acids of their potential food, i. e. seston < 30 mu m. In addition, values in daphniids from the field were compared to D. galeata fed cultured algae (Scenedesmus obliquus, Cryptomonas erosa, Nitzschia palea). In Daphnia spp. from the field their EPA content varied least among all omega 3-polyunsaturated fatty acids (omega 3-PUFA) and less than in their food. Hence, it can be considered to be more homeostatic than in the food. This becomes especially evident when comparing fatty acid profiles of D. galeata feeding on the cultured algae which had very different fatty acid compositions. However, the variation in daphniids' EPA and omega 3-PUFA content is much greater than what is observed for phosphorus (P) content in daphniids, owing the fact that PUFA and P are under different physiological constraints. Although the fatty acid pattern of D. galeata was greatly influenced when feeding on the algal cultures in the laboratory, no significant correlations were found between seston and Daphnia spp. and Eudiaptomus spp. for most PUFA and fatty acid ratios used as trophic markers. Thus, different to the laboratory situation when food was superfluous, a more homeostatic omega 3-PUFA profile occurred under limiting conditions in the field.