Macroinvertebrate response to salmon carcass analogue treatments: exploring the relative influence of nutrient enrichment, stream foodweb, and environmental variables

In Pacific Northwest (USA) streams, historical levels of marine-derived subsidies of C, N, and P from spawning salmon and steelhead are either greatly diminished or no longer present. A novel nutrient enrichment form, pasteurized salmon carcass analogue (SCA), has been developed as a management tool to increase freshwater productivity. We analyzed macroinvertebrate assemblages in 4 central Idaho (USA) streams to explore the relative influence of reach-scale SCA treatments on macroinvertebrate assemblages. We stratified study streams into upstream (control) and downstream (treatment) reaches and applied a single SCA treatment in mid-September to the downstream reaches of 2 treatment streams. We measured chemical, physical, and biological variables in all study streams and used nonmetric multidimensional scaling (NMDS) ordination of macroinvertebrate communities to illustrate relationships to foodweb and environmental variables. Macroinvertebrate assemblages differed significantly among streams (p < 0.001) and between treated and control reaches in treatment streams (p < 0.030). No reach-level differences were found in control streams (p > 0.458) or in treatment streams before SCA additions (p > 0.130). Variables that were significantly and positively correlated with NMDS ordination scores and suggested an SCA response included presence of SCA, elevated periphyton and macroinvertebrate delta N-15, increased periphyton ash-free dry mass, and increased relative abundances of dipterans, collectors, and Chironomidae. A weaker autotrophic response in 1 treatment stream relative to the other appeared to be explained partially by differences in canopy shading, as indicated by periphyton autotrophic index values. Increased autotrophic productivity and the absence of major shifts in treatment stream macroinvertebrate community composition and structure suggest that SCA is a viable nutrient-enrichment strategy.