Persistent ecological effects of a salmon-derived nutrient pulse on stream invertebrate communities

Abstract

Pulsed resource subsidies can have ecological effects that persist over time. These subsidies can be particularly important in aquatic ecosystems, which are often resource-limited. Anadromous salmon (Oncorhynchus spp.) deliver annual nutrient pulses to many freshwater ecosystems around the North Pacific. The persistent ecological consequences of this nutrient subsidy are poorly understood across the range of Pacific salmon and likely depend on stream habitat, background nutrient dynamics, and the abundance of spawning salmon. Using a model selection approach, we examined relationships among spawning salmon density, stream habitats, and the abundance and diversity of stream invertebrates ten months after salmon spawning, across 21 streams in central British Columbia, Canada. Total invertebrate abundance increased with salmon density and with higher stream temperatures. Invertebrate diversity was more closely related to stream habitat characteristics than to salmon density. These results suggest that salmon nutrients have a greater impact on stream invertebrate population sizes than on the variety of taxa that inhabit these streams. The three most common invertebrate families—grazing mayflies (Heptageniidae), predatory stoneflies (Chloroperlidae), and chironomid midges (Chironomidae)—all increased in abundance with salmon density. Stream habitat variables (temperature, pH, and substrate size) also explained significant variation in the abundances of the three groups. These results suggest that salmon nutrients retained in the watershed from previous years help support greater abundances of some invertebrate taxa. Thus the pulsed nutrient subsidy provided by spawning salmon may have ecological effects that persist many months, or even years, after it is delivered.