| Aquatic food webs use energy derived from two types of primary producers: autochthonous (aquatically-derived) and allochthonous (terrestrially-derived) carbon sources. The objective of this study was to use stable isotope analysis to determine which carbon sources contribute to the autochthonous pathway, and subsequently quantify the relative contribution of autochthonous versus allochthonous carbon sources, both spatially and temporally, to aquatic food webs in the Upper Salmon and Big Salmon Rivers, NB (Canada). In July and November, samples were collected from headwaters to river mouth (above head of tide) reflecting the sub-basins and stream orders present, and included primary producers, aquatic invertebrate consumers, and fishes.;Macrophytes, cyanobacteria, filamentous algae, and biofilm were more 13C enriched than scraper-type invertebrates; additionally, poor correlations and scarce availability indicated these carbon sources were not contributing greatly to autochthonous pathways. Unexpectedly, results suggest that in these low productivity, nutrient-limited rivers, a reduced availability of more preferred food sources (e.g. epilithic algae) might cause primary consumers to switch to marginal food sources, such as bryophytes.;In headwater sites, allochthonous and autochthonous food sources had distinct energy pathways that became integrated by higher trophic levels. With an increase in stream order, the two food chains overlapped due to a progressive enrichment of 13C in the autochthonous carbon source. Both linear and Bayesian mixing models demonstrated a progressive switch from an allochthonously driven headwater (1st -- 3rd stream orders) system to an autochthonously driven food web in the lower reaches, in agreement to the theory proposed by the River Continuum Concept. Seasonal changes were minimal. |
