Responses of stream biofilm to Pacific salmon (Oncorhynchus spp.) spawners: The role of environmental context and scale

Pacific salmon (Oncorhynchus spp.) provide resource subsidies to stream and riparian ecosystems throughout much of the northern Pacific Rim. The dynamics of salmon runs and environmental conditions of streams in which they spawn can vary in space and time, potentially creating or reinforcing heterogeneity in these recipient ecosystems. Enrichment of stream biota by salmon-derived nutrients and energy may be offset by spawner-mediated physical disturbance of sediments during migration and nest construction. These two contrasting roles of salmon are normally studied at reach scales but may vary at larger scales for enrichment and at smaller scales for salmon-induced physical disturbance. I studied the response of benthic biofilm (i.e., the complex of autotrophic and heterotrophic organisms growing on submerged surfaces) to the presence of salmon spawners in seven southeast Alaska streams. Autotrophs and heterotrophs were nutrient-limited by N or P (or both) in all study streams before the salmon run. During the salmon run, biofilm nutrient limitation was partially alleviated, but biofilm responses to salmon varied from net biomass accrual to net loss among streams and years. When data from multiple streams and years were considered, the 95% confidence interval of relative salmon enrichment was positive only for chlorophyll a (16--411% increase). The wide range of biofilm responses reflected, in part, variability among streams and years in environmental conditions, such as in temperature and nutrient concentrations. However, those environmental variables explained only a small amount of the overall biofilm variability, even if partitioned among spatial scales (e.g., stream, reach, riffle). The only consistent predictor across scales was sediment size, especially during the run, demonstrating the importance of salmon disturbance. High-frequency sampling revealed that effects attributed to salmon spawners may reflect other environmental variables that change during the salmon run, such as increased discharge later in the run. Salmon tissue decomposition and macroinvertebrate assemblages differed between terrestrial and aquatic habitats likely due to necrophagous guilds in terrestrial ecosystems. Integration of different spatial and temporal scales indicates that the ecological effects of salmon on recipient ecosystems are complex, and constrained by multiple environmental factors potentially operating at scales different from those of biotic responses.