Grass-Fed Salmon? Using Stable Isotopes to Discriminate Energy Pathways in Headwater Streams, Kenai Lowlands, Alaska

We partitioned allochthonous and autochthonous energy sources for two headwater streams in the Kenai Lowlands (Alaska) using dual-isotope mixing models (δ¹³C/δ¹⁵N and δ²H/δ¹⁵N) in a Bayesian framework.  Our first objective was to estimate the trophic base of production for juvenile coho salmon (Oncorhynchus kisutch) and Dolly Varden (Salvelinus malma).  We hypothesized that consumers would be reliant on autochthonous sources (filamentous algae and periphyton) due to the open canopy and lower quality litter inputs provided by the herbaceous vegetation, primarily bluejoint grass (Calamagrostis canadensis), associated with the adjacent upland and wetland habitat complexes.  Our second objective was to evaluate the potential for using stable hydrogen isotopes to trace energy pathways in a northern-latitude ecosystem.  We hypothesized that δ ²H-based models would provide more precision, in terms of source partitioning estimates, due to greater source separation relative to the δ¹³C-based models.  Results consistently indicated that allochthonous source contributions exceeded autochthonous sources for all species and size classes at both study sites.  However, diet shifted during ontogeny and larger Dolly Varden demonstrated an increased reliance on autochthonous sources compared to smaller individuals of both species.  Finally, we demonstrated good correspondence and similar levels of precision between the δ¹³C- and δ ²H-based models despite greater source separation by δ ²H.  Results indicated that stable hydrogen isotopes can be an effective food web tracer in northern-latitude streams.  In addition, we highlighted the importance of allochthonous sources in headwater streams suggesting that litter inputs from grasses may be an under-appreciated subsidy to salmon production.