Compound‐specific stable isotopes resolve sources and fate of polyunsaturated fatty acids in biota of headwater streams

Organisms at the base of stream food webs are typically poor in long-chain polyunsaturated fatty acids (LC-PUFA), especially docosahexaenoic acid (DHA), whereas consumers higher trophic levels often rich LC-PUFA. For example, fish tissues, brain, DHA-rich. This obvious mismatch between consumer LC-PUFA and their basal dietary supply may result from selective retention and/or endogenous conversion precursors to To determine which is more likely, we investigated compound-specific carbon stable isotopes PUFA (δ13CPUFA) potential resources (stream epilithon, leaf litter) (invertebrates, European bullhead, two salmonid species eye, liver, muscle tissues). We predicted that consumer-PUFA, depleted 13C values relative sources, would indicate internal de novo synthesis. Alternatively, consumer-δ13CPUFA imply aquatic (epilithon conditioned leaves) rather than terrestrial or production, irrespective levels. Invertebrate grazers predators resembled δ13C essential (δ13CEFA) benthic algae, while shredder-δ13CEFA reflected those fresh leaves. Lower eye-δ13CEFA salmonids livers indicated high sources (invertebrates epilithon). Stable isotope eicosapentaenoic suggest all retained algal EPA, insectivorous produced DHA liver. There no further evidence for local within neural tissues. Our study demonstrates δ13CPUFA can be used track these highly functional molecules highlights importance derived-PUFA oligotrophic headwater streams.