Use of carbon-13 and carbon-14 natural abundances for stream food web studies

We review the use of stable carbon isotope ratios (delta C-13) and radiocarbon natural abundances (Delta C-14) for stream food web studies. The delta C-13 value of primary producers (e.g., periphytic algae, hereafter periphyton) in streams is controlled by isotopic fractionation during photosynthesis and variable delta C-13 of dissolved CO2. When periphyton delta C-13 differs from that of terrestrial primary producers, the relative contribution of autochthony and allochthony to stream food webs can be calculated. Moreover, the variation in periphyton delta C-13 can reveal how much stream consumers rely on local resources because each stream habitat (e.g., riffle vs. pool, open vs. shaded) usually has a distinctive delta C-13. However, periphyton delta C-13 often overlaps with that of terrestrial organic matter. On the other hand, periphyton Delta C-14 is less variable than delta C-13 among habitats, and reflects the Delta C-14 of dissolved CO2, which could be a mixture of aged (Delta C-14 < 0 aEuro degrees) and modern (Delta C-14 > 0 aEuro degrees) carbon. This is because the Delta C-14 is corrected by its delta C-13 value for the isotopic fractionation during photosynthesis. Recent studies and our data indicate that many stream food webs are supported by aged carbon derived from the watershed via autochthonous production. The combined use of delta C-13 and Delta C-14 allows robust estimation of the carbon transfer pathway in a stream food web at multiple spatial scales ranging from the stream habitat level (e.g., riffle and pool) to watershed level (autochthony and allochthony). Furthermore, the Delta C-14 of stream food webs will expand our understanding about the time frame of carbon cycles in the watersheds.