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

Martin J. Kainz,Angela L. Strecker,Ariana M. Chiapella

doi:10.1002/ecs2.3360

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.

Highlights

A central goal in ecology is to understand the flow of energy within and across ecosystems (Teal 1962, Odum 1968), and theories related to energy flow, trophic dynamics, and cross-ecosystem interactions continue to evolve
Differences in fatty acid profiles between diets were largely driven by differences in alpha-linolenic acid (ALA) and arachidonic acid (ARA) content, with some differences in LIN, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) (Table 1, Fig. 2)
We determined if polyunsaturated fatty acids (PUFA) content was significantly different between treatments with a mixed-effects ANOVA, used a pairwise post hoc test of Tukey-adjusted least-squares means (LSM) differences to determine which treatment combinations were significantly different from each other (Table 1)

Summary

Introduction

A central goal in ecology is to understand the flow of energy within and across ecosystems (Teal 1962, Odum 1968), and theories related to energy flow, trophic dynamics, and cross-ecosystem interactions continue to evolve. Lake food webs are often simplified into two major diet trajectories (benthic and pelagic), a wealth of evidence supports strong habitat coupling, and significant allochthonous subsidies, especially in small lakes and ponds and northern temperate waterbodies (Schindler and Scheuerell 2002, Vander Zanden and Vadeboncoeur 2002, Pace 2004, Carpenter et al 2005, Milardi et al 2015). These phenomena are observed both at the base of the food web (via carbon and nutrient cycling; e.g., Pace 2004, Solomon et al 2011) and the apex of the food web (via cross-habitat foraging and terrestrial insect subsidies; e.g., Vander Zanden and Vadeboncoeur 2002, Milardi et al 2015). As a new approach to this persistent challenge, compound-specific stable isotopes may serve as more informative tracers, but their use in freshwater systems is far limited (Bec et al 2011, Boecklen et al 2011, Larsen et al 2013)

Methods

Results

Conclusion