Quantitative links between Pacific salmon and freshwater ecosystem structure

Spawning Pacific salmon affect freshwater ecosystems through substrate disturbance and the marine-derived nutrient pulse they deliver. I examined relations between a) salmon abundance and stream periphyton after spawning, and b) salmon abundance and invertebrate communities in the spring. I used 24 sockeye salmon (Oncorhynchus nerka) spawning streams in central British Columbia, Canada. After spawning, periphyton was enriched in salmon nitrogen but abundance was negatively related to salmon abundance, likely from substrate disturbance during spawning. Thus nutrient enrichment does not always translate into increased abundance. In the spring, the abundance of grazing mayflies and predatory stoneflies was positively related to salmon abundance, probably from increased algal growth caused by salmon nutrients delivered in previous years. Thus the salmon nutrient pulse can have ecological effects that extend long after spawning. The influence of spawning salmon on freshwater ecosystems differs through the year, across ecosystem components, and in relation to salmon abundance.