Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 8501
Title: Multiple pathways of carbon and nitrogen incorporation by consumers across an experimental gradient of salmon carcass inputs
Author: Peter M. Kiffney, Naman Sean, Cram Jeremy, Martin Liermann, D. G. Burrows
Publication Year: In press
Journal: Ecosphere
Keywords: organic matter,nitrogen,Carbon,salmon carcasses,streams,food webs

Abstract × Resource subsidies delivered by migratory species, such as Pacific salmon, have variable effects on recipient food webs. To identify sources of this variation, we used C and N stable isotopes to investigate how biotic characteristics of native lotic invertebrates and fish influenced their assimilation of marine organic matter across a gradient of salmon carcass inputs in flow-through stream mesocosms. We quantified the relationship (shape, magnitude) between carcass biomass density (0 to 4 kg/m2 wet mass) and assimilation of salmon organic matter (SOM) by multiple feeding guilds (scrapers, collectors, omnivores, carnivores) and trophic levels (primary consumers, primary and secondary carnivores). Although all feeding guilds and trophic levels incorporated SOM, these relationship varied by isotope, consumer, and trophic group. Invertebrate d15N and d13C values increased with carcass loading and indicated that scrapers and collectors (Ephemeroptera: Baetidae and Heptageniidae) and an omnivore (Diptera: Chironomidae) represented key pathways of SOM incorporation. However, these functional relationships were linear for d13C and generally saturating for d15N indicating different assimilative dynamics for the two elements. The linear relationship between d13C of mayflies, chironomids, and sculpin (Cotttus rhotheus, fish carnivore) across the range of carcass inputs indicated isotopic enrichment by consuming salmon flesh (direct pathway). A positive, but saturating relationship between invertebrate d15N values and salmon biomass indicate assimilation of marine organic matter via consumption (indirect pathway) but in a concentration-dependent manner. While changes in the isotopic composition of primary carnivores (Plecoptera and sculpin) with carcass biomass indicated assimilation of SOM, the strength of this relationship was weaker than for primary consumers and omnivores. Based on an aggregate primary consumer response (baetids, heptageniids, and chironomids), we also observed that the relative importance of direct vs. indirect assimilation of SOM varied as a function of carcass biomass, with indirect likely the principal pathway at low to moderate inputs (~0.1 ¿ 0.5 kg/m2) and direct plus indirect operating at high inputs (>0.5 kg/m2). Regardless of the pathway, these loading values are about an order of magnitude lower than current adult salmon inputs based on natural spawning populations in our study system, the Cedar River, WA, USA. These results give a mechanistic insight into factors that contribute to the context-dependency of SOM incorporation into lotic food webs and potentially provide insight into ecosystem-based management of salmon populations.

Theme: Ecosystem approach to improve management of marine resources
Foci: Provide scientific support for the implementation of ecosystem-based management
Characterize the interaction between marine, freshwater and terrestrial ecosystem components.