Landuse change alters the quantity and supply of stream energy resources from allochthonous (catchment-derived) to autochthonous (stream-derived) sources. This alteration has prompted the need for better understanding of C and N pathways through stream food webs. We added stable 15N-NH4 and 13C-acetate isotopes as tracers to experimental stream channels containing 1 of 4 key basal food resources: autotrophic epilithon, heterotrophic epilithon, fine particulate organic matter (FPOM), and coarse particulate organic matter (CPOM). Our goal was to evaluate the importance of these foods to 2 scrapers (Deleatidium sp. [mayfly] and Potamopyrgus antipodarum [snail]) and a facultative shredder (Olinga feredayi [caddisfly]). A 5th potential food source, new heterotrophic biofilm, grew on all surfaces during the course of the experiment. Isotope levels in the food resources and the invertebrates were measured weekly during the 3-wk isotope addition and for 2 wk after the addition. Changes in nutrient concentrations between the channel inflows and outflows provided measures of instream net uptake. Net uptake of P by autotrophic epilithon (mean = 0.09–0.24 mg dissolved reactive P [DRP] m−2 h−1) was 3× higher than uptake by other basal resources. Autotrophic epilithon removed substantial quantities of dissolved N, predominantly as NO3-N (2.1–9.3 mg NO3-N m−2 h−1), whereas the other basal resource types removed only NH4-N at a rate that was 2 orders of magnitude slower (mean = 0.04–0.08 mg NH4-N m−2 h−1) than uptake by autotrophic epilithon. NH4-N and dissolved organic C isotope tracers were taken up predominantly by heterotrophic epilithon and new biofilms. We developed a dynamic mixing model to separate the contribution to the invertebrates of each treatment food source relative to the new heterotrophic biolfilm. Mayflies obtained almost 100% of their body C and N from new biofilm except when offered autotrophic epilithon, which contributed >50% to their diets. Olinga and Potamopyrgus obtained most of their C and N from the treatment food sources, but in the particulate organic matter treatments (FPOM and CPOM) the contribution of new biofilm was much larger (∼10–60%). Our results suggest that heterotrophic biofilms are a significant component of stream food webs.
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