Melanie Hartwich, Dominik Martin-Creuzburg, Alexander Wacker

• In aquatic food webs, consumers, such as daphnids and copepods, differ regarding their accumulation of polyunsaturated fatty acids (PUFAs). We tested if the accumulation of PUFAs in a seston size fraction containing different consumers and in Daphnia as a separate consumer is subject to seasonal changes in a large deep lake due to changes in the dietary PUFA supply and specific demands of different consumers. We found that the accumulation of arachidonic acid (ARA) in Daphnia increased from early summer to late summer and autumn. However, ARA requirements of Daphnia appeared to be constant throughout the year, because the accumulation of ARA increased when the dietary ARA supply decreased. In the size fraction 140 m, we found an increased accumulation of docosahexaenoic acid (DHA) during late summer and autumn. These seasonal changes in DHA accumulation were linked to changes in the proportion of copepods in this size fraction, which may have increasingly accumulated DHA for active overwintering. We show that consumer-specific PUFAIn aquatic food webs, consumers, such as daphnids and copepods, differ regarding their accumulation of polyunsaturated fatty acids (PUFAs). We tested if the accumulation of PUFAs in a seston size fraction containing different consumers and in Daphnia as a separate consumer is subject to seasonal changes in a large deep lake due to changes in the dietary PUFA supply and specific demands of different consumers. We found that the accumulation of arachidonic acid (ARA) in Daphnia increased from early summer to late summer and autumn. However, ARA requirements of Daphnia appeared to be constant throughout the year, because the accumulation of ARA increased when the dietary ARA supply decreased. In the size fraction 140 m, we found an increased accumulation of docosahexaenoic acid (DHA) during late summer and autumn. These seasonal changes in DHA accumulation were linked to changes in the proportion of copepods in this size fraction, which may have increasingly accumulated DHA for active overwintering. We show that consumer-specific PUFA demands can result in seasonal changes in PUFA accumulation, which may influence the trophic transfer of PUFAs within the food web.…