Multiple pathways of C and N incorporation by consumers across an experimental gradient of salmon carcasses

Abstract Numerous studies link resource subsidies to consumers, yet manipulations of subsidy gradients are rare, limiting our ability to predict their effects. We measured natural abundance levels of the stable isotopes of carbon (δ13C) and nitrogen (δ15N) in consumers representing two taxonomic (invertebrates, fish) and multiple trophic groups (primary, secondary, and tertiary consumers) exposed to a gradient of Pacific salmon carcasses (0–4 kg/m2, eight treatment levels) in 16 experimental streams from September to December. We hypothesized (1) that consumer δ13C and δ15N values would increase with carcass loading and (2) that these relationships would vary among consumers and trophic levels. Within taxonomic groups, both δ13C and δ15N values increased with carcass loading indicating direct consumption of salmon organic matter (δ13C increase) and indirectly by consumption of salmon‐enriched prey (δ15N increase). The relative importance of these two pathways for invertebrates primary consumers varied with carcass loading, with an abrupt increase in direct consumption above ~0.5 kg/m2 salmon carcass wet mass. Both the rates of δ13C and δ15N increase and the shape of the response to the carcass gradient varied among invertebrates. The only fish that responded positively to carcasses was a small benthic, secondary consumer, and only for δ13C. Overall, benthic invertebrate primary consumers were the dominant pathway by which salmon organic matter entered the trophic system, followed by benthic secondary consumers, and then tertiary consumers (juvenile coho salmon and cutthroat trout) that occupy the water column. The attenuating response to subsidies at higher trophic levels was possibly due to a combination of low decomposition and metabolic rates during fall and winter, composition of the subsidy, and habitat use. Our results demonstrate that decomposing salmon carcasses provide variable but potentially important sources of C (range: ~0–60%) and N (range: ~0–35%) supporting benthic consumers during fall and winter, when environmental conditions constrain primary productivity, with some evidence of saturation at carcass loadings >0.1–0.6 kg/m2. Furthermore, stable isotope analysis showed that consumers exploited salmon organic matter in different ways, potentially alleviating competition and allowing for a more efficient processing of this resource subsidy.

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