Quantitative Use of Stable Carbon Isotope Analysis to Determine the Trophic Base of Invertebrate Communities in a Boreal Forest Lotic System

Stable carbon isotope mixing models were combined with inventories of macroinvertebrate biomass to quantitatively assess the trophic base of three contrasting lotic ecosystems in a boreal forest drainage system in spring/early summer. In a second-order partially shaded stream and in a fourth-order unshaded river, 13C/12C ratios of autochthonous and allochthonous food sources were distinct, allowing the use of isotopic mixing equations to calculate the relative contribution of both sources to consumer biomasses. In a small shaded lake outlet, isotopic compositions of autochthonous and allochthonous carbon were similar but distinct from that of lake seston which was relatively 13C depleted. The mixing model for the lake outlet thus discriminated between feeding on lake-derived food and on combined autochthonous and allochthonous food sources. Percent food utilization calculated for each invertebrate taxon by a site-specific mixing equation was weighted by biomass data to define the food bases of primary consumer communities. The food base was clearly dominated by lacustrine inputs (which represented >80% of the total food base) in the lake outlet, by allochthonous inputs (≈75%) in the second-order site, and by autochthonous production (≈60%) in the fourth-order site.