Movers and shakers: nutrient subsidies and benthic disturbance predict biofilm biomass and stable isotope signatures in coastal streams

SUMMARY 1. Nutrient subsidies and physical disturbance from migrating species can have strong impacts on primary producers. In the north Pacific, adult salmon (Oncorhynchus spp.) transport marine-derived nutrients back to freshwater streams and can also significantly disrupt the substratum during spawning events. We tested for effects of spawning pink (O. gorbuscha) and chum (O. keta) salmon on stream biofilm. Biofilm is a mix of algae, fungi and bacteria that provides food and habitat and forms the base of these aquatic food webs. 2. We collected rock biofilm samples to compare stable isotopes and biomass prior to and following peak salmon spawning in 16 catchments on the central coast of British Columbia, Canada. 3. We conducted two separate analyses. The first was a within-stream comparison, which focused on 5 catchments that had a barrier to pink and chum salmon migration. The second was an amongstream analysis that included all 16 catchments and explicitly considered biotic and abiotic factors, in addition to salmon density, known to influence biofilm growth and isotope ratios. 4. Salmon density proved to be the best predictor of biofilm d 15 N. Biofilm d 13 C was best predicted by salmon density and catchment size. While spring chlorophyll a increased with mean salmon density, it was on average lower during spawning in the autumn, probably due to physical disturbance from spawning salmon. 5. These results show that of the several variables considered to affect biofilm isotopes and biomass, salmon density and catchment size are among the most influential in coastal streams where salmon spawn.