Polyunsaturated fatty acids in zooplankton: variation due to taxonomy and trophic position

1. Food quality has major effects on the transfer of energy and matter in food webs, and essential long-chained polyunsaturated fatty acids (PUFAs) can affect the quality of phytoplankton as food. In a study of oligotrophic lakes in north-western Sweden, we investigated the fatty acid composition of four planktonic cladocerans and two calanoid copepods, representing herbivorous and carnivorous species. We also collected seston samples. 2. The proportions of long-chain PUFAs in the organisms increased with their increasing trophic position. Thus, both their quality as food for other organisms, as well as their requirement for fatty acids (FAs), differed among taxa and depended on their trophic position. 3. We found taxon-specific differences in the FA composition of zooplankton that were not related to sestonic FA composition. This implies that the variation in zooplankton FA composition is constrained by phylogenetic origin, life history characteristics, or both. 4. The cladoceran taxa contained 12-23% eicosapentaenoic acid (EPA) but only 0.9-2.1% docosahexaenoic acid (DHA) of the total FA content. In contrast, the calanoid copepods contained 7-11% EPA and 14-21% DHA. Thus, our results show that differences in the PUFA content among zooplankton species could have repercussions for both food web structure and function.