Examination of the Carbon Base in Southern Ontario Streams Using Stable Isotopes

Dependence of the invertebrate and fish community on terrestrial versus aquatic carbon sources in southern Ontario streams was studied using stable carbon isotope analysis. Algal carbon was significantly more ¹³C depleted (-35.5‰) than terrestrial carbon (-27‰), but high variability in algal δ ¹³C precluded use of a quantitative mixing model. δ ¹³C values at the unforested sites show a dependence of invertebrates on primarily autochthonous carbon. δ ¹³C values of many taxa at the forested sites also show a substantial dependence of invertebrates on algal carbon during the spring. Fish appear to derive more carbon from terrestrial sources than do aquatic invertebrates at both the forested and unforested sites. Carbon isotope analysis has the potential to elucidate carbon pathways in streams only when local conditions cause algae to be ¹³C depleted relative to terrestrial carbon.