The relative influence of landscape vs local processes on variation in stream C and energy sources to lotic consumers remains unresolved, but is a key component in understanding pathways through stream food webs. I investigated potential linkages between the distribution of δ13C signatures of primary producers (periphyton), terrestrial organic matter (detritus), and secondary consumers (aquatic invertebrates and fish) and: 1) drainage area and 2) stream reach geomorphology in 3 mountain watersheds of northern Idaho, USA. When considered alone, drainage area explained variation in the δ13C signatures of periphyton (R2 = 0.30) and sculpin (R2 = 0.15) only. Subsequent General Linear Models (GLMs) showed that stream morphology explained significant variation in δ13C of periphyton (79%), Diptera (93%), sculpin (51%), and trout (63%). Drainage area was not significant in these models, a result indicating that once δ13C signatures are explained as a function of geomorphic type, the contribution of drainage area becomes less important. On the whole, consumer δ13C signatures related more strongly to periphyton δ13C than detrital δ13C, though this relationship varied among geomorphic types. Collectively, results indicate that the morphology of a stream reach (including factors such as substrate size and storage elements, floodplain development, etc.) may be an important factor in the assimilation and distribution of C in food webs in forested mountain streams.
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