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

Radiocarbon natural abundance (Delta C-14) 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 Delta C-14 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 Delta C-14 values (-43.1 +/- 21.8 parts per thousand, 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 Delta C-14 values of leaf-shredding insect larvae (21.7 +/- 31.9 parts per thousand,N = 5) were close to those of contemporary atmospheric CO(2)except at the SF Eel River where algal production was highest. The Delta C-14 values of predators (-6.1 +/- 35.7 parts per thousand,N = 14) were intermediate between those of grazers and shredders. In a Bayesian mixing model, Delta C-14 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 (delta C-13). These results demonstrate that Delta C-14 can be used, in combination with delta C-13, to more precisely estimate organic matter sources to stream animals.