Opposing forces: Evaluating multiple ecological roles of Pacific salmon in coastal stream ecosystems

Abstract

Resource flows and disturbance from species migrations can alter the productivity, structure and function of an ecosystem. Annual mass migrations of Pacific salmon (Oncorhynchus spp.) to coastal watersheds import vast quantities of potentially limiting nutrients that have been shown to increase primary and secondary productivity in streams and lakes. Substrate disturbance during spawning can also export nutrients and reduce primary and secondary production. Here we study the impacts of these dual roles of salmon on stream invertebrates.We collected benthic macroinvertebrates in 15 streams prior to and following peak salmon spawning on British Columbia's central coast. Along with other habitat measurements including stream water chemistry, temperature, and watershed size, we investigated the effects of salmon on invertebrate δ15N, δ13C and biomass density (mg/m2) among 15 streams and within 5 streams, upstream and downstream of barriers to spawning salmon. We found that stream invertebrates assimilate salmon-derived nutrients in proportion to availability but invertebrate biomass density declines in both seasons with increasing salmon density. Benthic disturbance appears to be the cause of this decline in the fall, but the decline in the spring may be due to the slow recovery of invertebrates from substrate disturbance the previous fall or salmon nutrients may be indirectly driving declines in spring invertebrate biomass by subsidizing other trophic levels and eliciting a trophic cascade.