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An experiment and a mark‐recapture field study of juvenile coho salmon (Oncorhynchus kisutch) were conducted to identify controls of key energy flow chains in river food webs. In the small‐scale experiment, we investigated the individual and interactive effects of physical habitat structure (PHS) as small wood and resource availability (tissue of adult Chinook salmon, O. tshawytscha) on nutrients, algae, invertebrates, and fish predators including juvenile coho. In the field, we quantified the effects of natural variation in prey availability (invertebrate drift biomass), PHS (wood), and local fish density on summer growth of juvenile coho across multiple stream reaches. Adding salmon tissue to experimental channels resulted in strong bottom‐up effects on select invertebrates including increased population biomass of chironomids and baetids, the numerically dominant invertebrates, and faster growth of juvenile coho. We link the enhanced growth of coho to chironomid productivity: for instance, adult chironomid flux was 4.3× higher and coho consumption of these animals 3× higher in salmon‐subsidized channels. PHS in experimental channels was associated with reduced algal biomass, potentially in response to increased invertebrate consumption, and invertebrate flux or export. The field study revealed coho growth was negatively related to PHS and total fish density and positively related to Diptera drift biomass; however, the effects of fish density and drift biomass on coho growth were relatively weak. The field study also indicated that prey resource availability and coho growth were associated with differences in canopy cover, with prey biomass and coho growth 2–4× higher in reaches receiving more sunlight. As in the experiment, coho in natural stream reaches predominantly fed on adult chironomids and other Diptera, indicating that these taxa and life‐stages are a key link between the benthic food web and mobile vertebrate predators. Our study showed that bottom‐up processes initiated by salmon subsidies and possibly light flux determined key trophic interactions in the Cedar River food web. Moreover, we speculate that PHS may modify some of these interactions indirectly through its effects on the movement of organisms through the environment.

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Research showing that salmon carcasses support the productivity and biodiversity of 3 aquatic and riparian ecosystems has been conducted over a variety of spatial and temporal scales. 4In some studies, carcasses were manipulated in a single pulse or loading rate or manipulations 5 occurred during summer and early fall, rather than simulating the natural dynamic of an extended 6 spawning period, a gradient of loading rates, or testing carcass effects in late fall-early winter 7 when some salmon stocks in the US Pacific Northwest spawn. To address these discrepancies, period. We found little evidence that carcasses influenced primary producer biomass or fish 13 growth; however, nutrients and some primary consumer populations increased with loading rate. 14 These effects varied through time, however. We hypothesize that the variable effects of carcasses 15 were a result of ambient abiotic condition, such as light, temperature and disturbance that 16 constrained trophic response. There was some evidence to suggest peak responses for primary 17 producers and consumers occurred at a loading rate of ~1.0 -2.0 kg/m 2 , which was similar to 18 other experimental studies conducted during summer. 19 20