Macroinvertebrate community dynamics: strong negative response to salmon redd construction and weak response to salmon-derived nutrient uptake

Abstract

Densely spawning salmon (Oncorhynchus spp.) affect macroinvertebrates negatively through redd construction and positively through nutrients released during spawning and carcass decomposition. We investigated the long-term characteristics of this interaction by measuring density, biomass, and C and N sources in benthic macroinvertebrates over 1 y. Total macroinvertebrate community biomass decreased during spawning and redd construction. However, the percentage of macroinvertebrate biomass derived from salmon increased. The highest percentage (56%) and biomass (2.71 g/m2) of salmon-derived tissue in the macroinvertebrate community occurred 3 mo after spawning. Shredders accumulated the most salmon-derived biomass per individual; however, collector-filterers, scrapers, and predators accumulated the most salmon-derived biomass at the population level, a result that suggests multiple pathways of salmon nutrient acquisition. Salmon-derived macroinvertebrate biomass was never <22% of macroinvertebrate community biomass in the spawning reach, but 6 mo after spawning, no clear density or biomass differences were found between reaches with and without salmon. This pattern held true at all scales examined, from whole community to functional feeding group to individual taxa in prespawning samples from both years. These results suggest that increased macroinvertebrate production from the nutrient and energy subsidy from salmon was restricted to the recovery period following the disturbance of redd construction and might have been obscured by emergence and transfer to higher trophic levels.