1017 Fatty acid retention under temporally heterogeneous dietary intake in a cladoceran Apostolos-Manuel Koussoroplis, Martin J. Kainz and Maren Striebel A.-M. Koussoroplis (apostolos.koussoroplis@wcl.ac.at), M. J. Kainz and M. Striebel, WasserCluster – Biologische Station Lunz, Dr. Carl Kupelwieser Promenade 5, AT-3293 Lunz-am-See, Austria. Omega-3 ( ω3) and -6 ( ω6) polyunsaturated fatty acids (PUFA) are essential for all aquatic animals, but their dietary availability can be highly heterogeneous in space and time. Te way consumers retain PUFA across such heterogeneous feeding conditions remains poorly understood. In a series of feeding experiments, we investigated how retention efficien- cies (i.e. amount in consumer biomass/amount ingested) of PUFA and bulk carbon responded to heterogeneous PUFA intake in Daphnia magna. Heterogeneous PUFA intake was achieved by exposing D. magna to algal diets of different PUFA content and composition for specific time periods. Te retention efficiency of carbon did not change among dietary treatments. At shorter exposure to PUFA-rich diet, retention efficiencies of most PUFA were 2–3 times higher than that of bulk carbon, clearly indicating PUFA bioaccumulation in D. magna. Increasing exposure to PUFA-rich diet caused exponential decrease of retention efficiencies for most PUFA. However, D. magna receiving more PUFA were richer in these compounds despite lower retention efficiency. Eicosapentaenoic (20:5 ω3) and arachidonic acid (20:4 ω6) and their precursors were always supplied in the same proportions (3.6:1), but the 20:5 ω3/20:4 ω6 ratio in D. magna (an important measure of nutritional quality for consumers) increased with exposure time to these PUFA from 2.2:1 to 3.8:1, thus eventually matching the diet. Our results suggest that D. magna is an efficient gatherer, accumulator, and repository of PUFA under low/fragmented dietary availability. However, at higher availabilities, PUFA are not always bioaccumulated in D. magna. Hence, the efficiency of PUFA transfer by daphnids in food webs may depend on temporal PUFA availability and its range of variation. Finally, we show that heterogeneity in PUFA intake may also affect higher trophic levels by influencing nutritionally critical PUFA ratios of zooplankton. Consumers experience spatial and temporal variability in their food availability and quality. Tis variability is linked to temporal successions (Sommer 1986) of the consumed resources or to movements of organisms within a hetero- geneous environment and thus between different food patches. In such dynamic environments, ecological success of consumers may depend on diet quantity, but also on their ability to efficiently locate and acquire physiologically essential dietary nutrients whenever available. Consumers with the ability to store essential dietary nutrients when available in excess (compared to their immediate require- ments) and supplement growth and/or reproduction when exposed to low quality diets may be less sensitive to natural variability of food quality (Hood and Sterner 2010). Recently, efforts have been made to understand how tempo- ral heterogeneity of resource quality is related to fitness in aquatic consumers (Sterner and Schwalbach 2001, Becker and Boersma 2005, Hood and Sterner 2010). To link temporal resource heterogeneity to fitness of consumers it is necessary to understand how consumers retain essential nutrients when their dietary intake is heterogeneous in time. In aquatic ecosystems, omega-3 ( ω3) and -6 ( ω6) poly- unsaturated fatty acids (PUFA) are synthesized by primary producers and certain heterotrophic protists, but the synthe- sis of these PUFA is highly variable among species and environmental conditions (Jain et al. 2007, Desvilettes and Bec 2009, Guschina and Harwood 2009). Terefore, the availability of PUFA for animals is highly heterogeneous in space and time (Olsen 1998, Müller-Navarra et al. 2004, Bec et al. 2010, Ravet et al. 2010). Te various PUFA are not mutually substitutable resources and often have differ- ent physiological functions (Wacker and von Elert 2001). For example, the most physiologically active PUFA are the 20-22 carbon chain (C 20-22 ) PUFA: 20:4 ω6 (arachidonic acid; ARA), 20:5 ω3 (eicosapentaenoic acid; EPA), and 22:6 ω3 (docosahexaenoic acid; DHA) (Olsen 1998). In many animals these molecules are important constituents of cellular membranes where they help regulate physical properties such as fluidity. ARA and EPA are also precursors for eicosanoids, which are signalling molecules affecting immunity, inflammation, mineral balance and reproductive processes (Sardesai 1992, Stanley-Samuelson and Pedibhotla 1996, Calder 2009). ARA-derived eicosanoids are more biologically active than those derived from EPA, which competitively inhibits the formation of eicosanoids derived from ARA (Sargent et al. 1999, Schmitz and Ecker 2008). Oikos 122: 1017–1026, 2013 doi: 10.1111/j.1600-0706.2012.20759.x © 2012 Te Authors. Oikos © 2012 Nordic Society Oikos Subject Editor: Jotaro Urabe. Accepted 2 October 2012