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

Resource subsidies can alter the productivity, structure and function of ecosystems. These effects can be particularly strong when the resource is in limited supply within recipient habitats. Migratory species can act as subsidy vectors, transporting vast quantities of nutrients to recipient habitats. They can also significantly disturb the physical, chemical and biological landscape of recipient habitats through their behaviour. This thesis explores how adult spawning salmon affect 1) algal and biofilm biomass, 2) benthic macroinvertebrate biomass, 3) benthic macroinvertebrate diversity and 4) food web structure in streams of the Pacific Northwest. I draw spatial comparisons across multiple streams, compare pre-spawn with post-peak spawning periods, and test for habitat characteristics that mediate these effects. In Chapters 2 and 3, I show that algal and invertebrate biomass in streams decline after salmon spawn, a likely result of substrate disturbance. Through the use of stable isotopes, I also show that algae and invertebrates readily incorporate salmon-derived material. However, only algal biomass in the spring increases with salmon density. Spring invertebrate biomass is low in streams with high salmon densities, despite being enriched in both salmon-derived nitrogen and carbon. This may be due to a slow recovery from the previous fall or in response to salmon nutrients subsidizing higher trophic levels and eliciting a trophic cascade. Chapter 4 reveals that this decline in invertebrate biomass may also be linked to life history traits. Invertebrate family composition shifted significantly across a gradient in salmon density and stream temperature, both prior to and post spawning. However, invertebrate family richness was not related to salmon density; it declined with drainage size, a composite measure of stream and terrestrial habitat measurements. Finally, Chapter 5 reveals that aquatic invertebrates and resident fish species in these coastal streams consume a variety of resources throughout the year and that salmon are an important dietary source for many of these organisms, regardless of season. This thesis demonstrates that salmon are a major structural component of coastal stream food webs, as a resource to benthic production, a major contributor to the diets of freshwater consumers, and as a source of disturbance that contributes to losses in lower trophic levels. Declines in wild Pacific salmon populations would significantly impact how coastal stream food webs function.