An evaluation of deuterium as a food source tracer in temperate streams of eastern Canada

Stable H isotope ratios (D/H, expressed as D) hold promise as an additional tool for elucidating food sources for consumers in aquatic ecosystems. We tested the applicability of D as a food source tracer in streams of New Brunswick, Canada. First, we analyzed D and 13C in biofilm and terrestrial leaves and compared signal-to-noise ratios (variability within sources vs variability between sources) between the 2 elements. Signal-to-noise ratios were roughly similar, and 23 of the 31 sites had isotopically distinct food sources based on D compared to 20 of 31 based on 13C. Second, we used mixing models to estimate aquatic H and aquatic C for benthic invertebrates at a subset of sites. Of 16 samples, only 1 had 13C that was outside the range of the 2 food sources (yielding aquatic C > 100), but 12 of the 16 samples had D outside the range of the food sources, a result suggesting confounding effects of water and lipids on total body H content. Last, we analyzed D in laboratory-reared consumers (brook trout and water striders) and in their diet before and after lipid extraction to estimate diettissue fractionation. Large differences between consumer and diet were apparent before lipid extraction, but no significant differences were found after lipid extraction. All of these measures indicate that D could serve as a complementary, but not alternative, isotopic method for estimating food sources for consumers in streams. Further laboratory trials are needed to explore the influence of lipids on D values.