Multiple stable isotopes identify sediment organic matter as the primary nutritional source to American brook lamprey larvae along a stream gradient

Multiple stable isotope ratios (δ2H, δ13C, δ15N, and δ34S) of American brook lamprey (Lethenteron appendix) ammocoetes and their potential sources were measured along a stream gradient. Isotopic ratios were successful at differentiating three potential sources: terrestrial organic matter (TOM; upland plants), sediment organic matter (SOM; benthic organic material), and algae. Bayesian mixing models (MixSIAR) utilizing δ13C, δ15N, and δ34S estimated ammocoetes were highly reliant on SOM at all sites (>95%), but inclusion of δ2H increased the importance of both algae and model uncertainty. Ammocoete δ2H was more depleted than anticipated even after lipid extraction and after accounting for trophic water contributions, suggesting δ2H in consumer tissues (especially for long-lived animals) is still incompletely understood. Model sensitivity testing with trophic fractionation values did not substantially change the results. Regardless of the mixing model investigated, none indicated more algae in ammocoete diets in downstream rather than upstream sites, in contrast with expectations about river productivity. Stable isotopes suggest SOM, and not algae, is nutritionally most important to ammocoetes, even in downstream reaches of rivers.