Response of dissolved nutrients and periphyton to spawning Pacific salmon in three northern Michigan streams

The ecological effects of spawning runs of native Pacific salmon (Oncorhynchus spp.) on stream ecosystems of the Pacific Northwest and Northern Pacific Rim have been studied extensively. However, little is known about how nonnative Pacific salmon affect stream ecosystems in the Great Lakes Basin, especially given the difference in environmental context between the regions. Mechanisms by which salmon spawners alter stream ecosystems include nutrient enrichment from excretion by live adults, carcass decomposition, and physical disturbance of the substrate during redd construction. The objective of our study was to quantify changes in water chemistry and benthic periphyton in 3 streams in northern Michigan that have spawning populations of Pacific salmon. In each stream, dissolved nutrients (soluble reactive P [SRP], NH ₄ ⁺, dissolved organic C [DOC], NO ₃ ^-), and periphyton on gravel were sampled before, during, and after the spawning run in reaches upstream and downstream of a salmon barrier. Nutrients increased in reaches downstream of the barrier when salmon were present, but the magnitude of increase was low relative to increases observed in Pacific Rim streams. During and after the spawning run, periphyton biomass declined significantly in reaches where high densities of salmon spawners were present. Our results suggest that disturbance by spawning salmon may override their enrichment effects in northern Michigan streams, but this pattern may in part be driven by environmental context, especially the presence of finer substrates.