Assessing allochthonous inputs to stream food webs using dual carbon and nitrogen stable isotope tracers

Quantifying the relative contributions of allochthonous and autochthonous energy sources to stream food webs is key to understanding community and ecosystem level processes, but presents methodological challenges. A great deal of previous research has examined nutrient cycling and food webs in streams using ¹⁵N labeled tracers. Stable carbon isotope tracers are much less common, but can be used in a similar manner to follow detrital food web pathways. We conducted a dual isotope tracer experiment (using both ¹³C-acetate and ¹⁵N-ammonium) in a small, headwater stream in the Adirondack Mountains of upstate New York. By examining flows of both ¹³C, which is assimilated selectively by heterotrophic microorganisms, and ¹⁵N, which is assimilated by both heterotrophs and autotrophs, we were able to determine the relative importance of detrital pathways in the food web. In this system, bacteria and benthic macroinvertebrates were highly enriched with ¹³C, indicating that there are very strong connections between heterotrophs and consumers. This result is consistent with the prediction that first-order streams rely on allochthonous energy inputs because of low light availability and low primary production. Dual isotope tracers present a novel and promising method to assess pathways of terrestrial vs. aquatic energy use in streams.