Quantifying allochthonous subsidies of terrestrial organic matter to aquatic systems is difficult, yet essential for understanding aquatic food webs. Recent work suggests that natural abundance deuterium-to-hydrogen stable isotope ratios (δD) may distinguish aquatic and terrestrial primary production, facilitating quantification of subsidies. In this study, we used δD to quantify allochthonous support of zoobenthos, zooplankton, and fishes in four north-temperate lakes. We had previously estimated allochthony for these consumers in these lakes using whole-lake ¹³C additions.

Results/Conclusions

δD-derived allochthony estimates were generally similar to ¹³C-derived estimates. Differences reflected the importance of detrital pathways in lake food webs, which may supply autochthonous organic matter to consumers yet go undetected by the ¹³C approach for estimating allochthony. For all of the consumers considered, allochthony varied across the four study lakes in response to variation in the availability of terrestrially-derived organic matter. This effect was weaker for zoobenthos and fishes than for zooplankton, perhaps because much of the production of zoobenthos and fishes occurs in the littoral zone ecotone, where the availability of terrestrial organic matter is high regardless of its overall importance in the lake’s carbon budget. While methodological challenges remain, deuterium-to-hydrogen stable isotope ratios may provide a powerful tool for estimating terrestrial subsidies to aquatic systems.