Ecosystem response to a salmon disturbance regime: Implications for downstream nutrient fluxes in aquatic systems

Recent work in salmon spawning streams has shown that sediment resuspended during nest construction flocculates with salmon organic matter to form suspended composite particles characterized by increased size and settling velocities. In a river system, these flocs have the potential to interact with benthic biofilms, suggesting a mechanism for the incorporation of organic matter into aquatic food webs. Using the Horsefly River spawning channel in central British Columbia, the spatial scale of biofilm floc trapping was evaluated for a salmon disturbance regime, which consists of the active digging of redds, spawning, and carcass decay. We stocked two sequential enclosures in the spawning channel with sockeye salmon (Oncorhynchus nerka) and established one upstream control enclosure. Biofilms were sampled for chlorophyll a, trapped sediment, and marine isotope tracers (delta N-15 and delta C-13). In the active-spawn period, biofilm abundance was reduced due to spawning disturbance, with isotope values indicating low utilization of marine-derived nutrients (MDNs). During the post-spawn period, downstream biofilm abundance exceeded pre-spawn values, indicating a near-field nutrient pulse with isotope values reflecting biofilm utilization of MDNs. At the same time, an increase in biofilm trapping efficiency occurred in concert with a significant increase in the in situ particle size of suspended sediment, suggesting that flocs were a temporary storage site of MDNs. The retention of MDNs over short spatial scales acts to retard the flushing of MDNs to downstream rearing lakes. The magnitude of these processes has ecological implications on the downstream lake's productivity, thereby influencing the success of future salmon stocks.