Three years of experimental carbon andnutrient additions in upper Salmon River Basin streams using salmon carcass analogs: what have we learned so far?

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Anadromous fishes represent an important linkage between marine and inland aquatic and terrestrial ecosystems. These fishes carry organic matter and marine-derived nutrient (MDN) subsidies across a vast landscape, often with profound influences on recipient ecosystem food web structure and function. In the Columbia River Basin, century-long declines in the abundance of anadromous fish populations have lead to the implementation of mitigation efforts designed to address MDN deficits. As part of the Salmon River Basin Nutrient Enhancement project, we are conducting a large-scale experiment designed to evaluate the efficacy of artificial carbon and nutrient treatments (e.g., salmon carcass analog or SCA) intended to increase the freshwater productivity of streams in the upper Salmon River Basin. Specific project objectives include the quantification and assessment of both structural and functional components of the stream food web in treatment streams receiving two levels (high and low) of SCA additions and control streams that do not receive SCA additions. We used upstream-downstream and before-after comparisons in control and treatment streams(Multiple Before-After Control-Impact or MBACI design) across a broad spatial and temporal scale. Our preliminary results indicate that stream food web response to SCA is influenced by loading rates (i.e., treatment level) and is at times equivocal. For example, dissolved nutrient concentrations were higher following SCA treatments, while biofilm standing crop did not respond to any treatment level. Meanwhile, whole-stream and benthic gross primary productivity and community respiration increased markedly, but only in a stream receiving high analog treatments. Concurrently, responses were evident for stream food web consumer groups. Benthic and drift macroinvertebrate abundance and biomass increased following SCA additions. However, bioenergetics modeling demonstrated that although drift abundance increased in the short-term, the proportion of suitable habitat available for salmonids, as measured by net energy intake rates, did not respond significantly. Finally, trophic (or direct) transfer of SCA toautotrophic and heterotrophic biofilm communities and macroinvertebrate and fish consumer groups was evident in nitrogen stable isotope measures, but the pathways for assimilation of MDN from SCA and into these producer and consumer groups remains unclear.

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Western Division American Fisheries Society 2013 Annual Meeting