Search

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

The importance of allochthonous organic matter contributions to aquatic food webs has long been debated. We studied the aging process of terrestrial particulate organic matter (tPOM) in aquatic ecosystems by incubating senescent plant material in the presence of natural microbial communities in either lake water or artificial growth medium. We hypothesised that the aging would change the biomarker composition of tPOM, and would improve its nutritional quality for Daphnia due to microbial conditioning. We also hypothesised that when algal availability is low, Daphnia can partially compensate for the low quality of tPOM when more of this resource is available to them.The fatty acid biomarkers of tPOM changed very little during short‐ and long‐term incubations. In particular, the proportion of long‐chain saturated fatty acids was quite stable even during long‐term incubations. A decrease in carbohydrate and ω‐3 polyunsaturated fatty acid content, and an increase in the bacterial and fungal lipid biomarkers indicated that microbial decomposition of tPOM took place during the incubation. The δ15N value of aged tPOM increased, which indicated microbial colonisation, while the δ13C value did not change during aging.Daphnia magna survival and reproductive output were low when fed diets containing fresh or aged tPOM. The reproductive output of Daphnia was slightly enhanced in the presence of aged tPOM as compared to fresh tPOM, but only when tPOM comprised ≥ 95% of the diet.When Daphnia were offered low (0.5 mg C/L) concentrations of algae, even high additions of tPOM (up to 15 mg C/L) did not increase Daphnia growth rates. Daphnia reproduction increased with tPOM additions, presumably due to the sterols and ω‐3 polyunsaturated fatty acid in the tPOM that Daphnia retained from their diet.Our results indicate that tPOM is a poor resource for pelagic cladocerans, even when aged in the presence of microbes or offered in large quantities. [ABSTRACT FROM AUTHOR]