The potential of fatty acid isotopes to trace trophic transfer in aquatic food-webs

Abstract

Compound-specific isotope analyses (CSIA) of fatty acids (FA) constitute a promising tool for tracing energy flows in food-webs. However, past applications of FA-specific carbon isotope analyses have been restricted to a relatively coarse food-source separation and mainly quantified dietary contributions from different habitats. Our aim was to evaluate the potential of FA-CSIA to provide high-resolution data on within-system energy flows using algae and zooplankton as model organisms. First, we investigated the power of FA-CSIA to distinguish among four different algae groups, namely cyanobacteria, chlorophytes, haptophytes and diatoms. We found substantial within-group variation but also demonstrated that δ 13 C of several FA (e.g. 18:3 ω 3 or 18:4 ω 3) differed among taxa, resulting in group-specific isotopic fingerprints. Second, we assessed changes in FA isotope ratios with trophic transfer. Isotope fractionation was highly variable in daphnids and rotifers exposed to different food sources. Only δ 13 C of nutritionally valuable poly-unsaturated FA remained relatively constant, highlighting their potential as dietary tracers. The variability in fractionation was partly driven by the identity of food sources. Such systematic effects likely reflect the impact of dietary quality on consumers' metabolism and suggest that FA isotopes could be useful nutritional indicators in the field. Overall, our results reveal that the variability of FA isotope ratios provides a substantial challenge, but that FA-CSIA nevertheless have several promising applications in food-web ecology. This article is part of the theme issue ‘The next horizons for lipids as ‘trophic biomarkers’: evidence and significance of consumer modification of dietary fatty acids’.

Spatial and Temporal Variability of Ice Algal Trophic Markers—With Recommendations about Their Application

“…Furthermore, the stable isotope signal of single fatty acids also showed temporal and spatial variability similar to the bulk signal, in response to environmental and growth conditions as shown in this study. In the context of food web studies, it should also be noted that isotopic ratios of essential PUFAs that are not metabolically modified by consumers have the greatest potential to provide reliable information on trophic structures [64], as the isotopic signal of a fatty acid molecule is determined by the number of metabolic steps during biosynthesis.…”

The critical importance of experimentation in biomarker-based trophic ecology

“…In small, short-lived species, particularly arthropods, a similar approach could be used, allowing the animals to progress through several growth stages or moults to ensure that a substantial mass gain has occurred. As with finfish, it can also be feasible to monitor a change in total mass [33]. A doubling or tripling in size is a rough indicator used for initial experiments with previously unstudied macroinvertebrates [6]; however, we note that in the six-eight week experiments with juvenile crabs by Thomas et al [31, this issue], crabs that did not moult or grow appreciably still incorporated and reflected the fatty acid profiles of their different diets.…”

“…With primary consumers, it is relatively easy to provide fresh, single species of phytoplankton or macroalgae as foods that are likely representative of nature since many short-lived primary consumers feed on temporally limited algal blooms. In this special issue, numerous authors employed this approach and focused on feeding pure algal diets [20,21,29,30,33,34]; with their springtail experiments Kühn et al [35, this issue] were also able to offer mixed dried diets of bacteria, microalgae, fungi and plants. At intermediate trophic levels, it can be difficult to meet the nutritional requirements of consumers by feeding single species, particularly for longer-duration experiments, since such predators are rarely so specialized [28,31].…”

Compound‐specific stable hydrogen isotope (δ2H) analyses of fatty acids: A new method and perspectives for trophic and movement ecology

“…When trying to identify potential diets, the isotopic values of δ 13 C of essential fatty acids are reflected in consumers and it is likely that the same holds true for 2 H. 41,42 Especially, both terrestrial plants and algae can synthesize LIN and ALA de novo; however, they are essential fatty acids for most consumers, which can only further convert them to long-chain PUFA such as ARA, EPA, and DHA at a species-dependent and usually limited rate. 43,44 Terrestrial plant-or algal-derived n-6 PUFA and n-3 PUFA isotopic δ 2 H values are thus unlikely to change significantly during trophic transfers, and consumers grazing entirely on algae should reflect similar FAME δ 2 H values.…”

“…In contrast, the correlation of fish and biofilm and especially leaves δ 2 H values deviated significantly, indicating a negligible role for both as direct food source. Future studies might further explore the value of δ 2 HFA for food web analysis as site-independent marker.When trying to identify potential diets, the isotopic values of δ 13 C of essential fatty acids are reflected in consumers and it is likely, that the same holds true for 2 H41,42 . Especially, since both terrestrial plants and algae can synthesize LIN (C18:2n-6) and ALA (C18:3n-3) de novo, however, they are essential fatty acids for most consumers, which can only further converte them to LC-PUFA such as ARA, EPA and DHA at a species dependant and usually limited rate43,44 .…”