Combined use of radiocarbon and stable carbon isotopes for the source mixing model in a stream food web

Radiocarbon natural abundance (Δ¹⁴C) has emerged as a useful dietary tracer in freshwater ecology for the past decade, yet its applicability for separating aquatic and terrestrial resources has not been examined quantitatively. Here, we report Δ¹⁴C values of stream invertebrates in different functional feeding groups collected from the upper South Fork Eel River watershed, northern California. We found that algae-grazing insect larvae show low Δ¹⁴C values (−43.1 ± 21.8‰, mean ± standard deviation, N = 6), reflecting the signal of dissolved inorganic carbon weathered from ancient inorganic carbon or respiration of old organic carbon. In contrast, the Δ¹⁴C values of leaf-shredding insect larvae (21.7 ± 31.9‰, N = 5) were close to those of contemporary atmospheric CO₂ except at the SF Eel River where algal production was highest. The Δ¹⁴C values of predators (−6.1 ± 35.7‰, N = 14) were intermediate between those of grazers and shredders. In a Bayesian mixing model, Δ¹⁴C provided a more ecologically realistic estimate for terrestrial vs. aquatic source contributions to invertebrates with lower uncertainty (i.e., narrower credible interval) than did the stable carbon isotopes (δ¹³C). These results demonstrate that Δ¹⁴C can be used, in combination with δ¹³C, to more precisely estimate organic matter sources to stream animals.