Retesting a prediction of the River Continuum Concept: autochthonous versus allochthonous resources in the diets of invertebrates

The River Continuum Concept (RCC) predicts that food webs (and, in particular, invertebrates) of rivers in temperate, forested drainages should exhibit a longitudinal gradient from reliance on terrestrially derived organic matter (e.g., seasonally shed leaves) in the headwaters to autochthonous sources (e.g., algae) in the mid-orders, to suspended material in larger rivers. This prediction has been evaluated by longitudinal comparisons of macroinvertebrate communities in terms of functional feeding groups (FFGs), but such an approach yields only indirect evidence regarding actual food use. To retest this prediction, we investigated invertebrate diets from the Salmon River, Idaho, by examining the gut contents of archived specimens from the longitudinal set of sites sampled in the original 1976 RCC study and by collecting invertebrates from these same sites in 2009. We detected no apparent shifts in diets over the ∼30-y time span. The importance of allochthonous materials in invertebrate diets diff...