Fatty acid stable isotopes add clarity, but also complexity, to tracing energy pathways in aquatic food webs

Abstract

Tracing the flow of dietary energy sources, especially in systems with a high degree of omnivory, is an ongoing challenge in ecology. In aquatic systems, one of the persistent challenges is in differentiating between autochthonous and allochthonous energy sources to top consumers. Bulk carbon stable isotope values of aquatic and terrestrial prey often overlap, making it difficult to delineate dietary energy pathways in food webs with high allochthonous prey subsidies, such as in many northern temperate waterbodies. We conducted a feeding experiment to explore how fatty acid stable isotopes may overcome the challenge of partitioning autochthonous and allochthonous energy pathways in aquatic consumers. We fed hatchery‐reared Arctic Char (Salvelinus alpinus) diets of either benthic invertebrates, terrestrial earthworms, or a mixture of both. We then compared how the stable carbon isotopes of fatty acids (δ13CFA) distinguished between diet items and respective treatments in S. alpinus liver and muscle tissues, relative to bulk stable isotopes and fatty acid profiles. Although a high degree of variability of fatty acid stable carbon isotope values was present in all three measures, our results suggest that the ability of this method to overcome the challenges of bulk stable isotopes may be overstated. Finally, our study highlights the importance of further experimental investigation, and consideration of physiological and biochemical processes when employing this emerging method.

Ice-algal carbon supports harp and ringed seal diets in the European Arctic: evidence from fatty acid and stable isotope markers

“…Including isotopic fractionation terms into the model would certainly be more accurate, but to our knowledge only few data are available for higher marine organisms (see above). Furthermore, experimental work has shown that δ 13 C FA values of PUFAs can also be modified via fractionation when being incorporated into lipids in various tissues (Gladyshev et al 2016), or by metabolic processes (Chiapella et al 2021). Although it is difficult to map the individual steps of isotopic fractionation into total metabolism, we recognize a need for increased attention towards the kinetics of metabolic processes.…”

Assessing the potential of amino acid δ13C and δ15N analysis in terrestrial and freshwater ecosystems

“…Compound‐specific stable isotope analysis (CSIA) of individual AAs or fatty acids (FAs) continues to grow as a robust proxy for tracing energy sources and estimating consumer TL. For example, FA δ 2 H and δ 13 C values are promising tracers for food web studies (Anparasan & Hobson, 2021; Budge et al, 2011; Burian et al, 2020; Chiapella et al, 2021; Wang et al, 2015), especially those in which consumers rely on both terrestrial and aquatic resources. Aquatic algae and terrestrial plants typically have distinct FA profiles, with algae containing higher concentrations of polyunsaturated fatty acids (PUFAs; Gladyshev et al, 2013; Hixson et al, 2015).…”

Chytrids enhance Daphnia fitness by selectively retained chytrid‐synthesised stearidonic acid and conversion of short‐chain to long‐chain polyunsaturated fatty acids

“…We quantified each δ 13 C PUFA value to follow dietary PUFA sources for Daphnia by calculating the difference in δ 13 C of each FA between the consumer ( δ 13 C C ) and its respective diet sources ( δ 13 C D ), i.e. Δ 13 C FA , following Chiapella et al ( 2021 ): Δ 13 C FA = δ 13 C C − δ 13 C D . In a similar way, we calculated Δ 13 C FA differences within Daphnia between LIN ➔ ARA, ALA ➔ SDA, ALA ➔ EPA, and SDA ➔ EPA conversion pathways.…”