Impacts of In-Situ Salmon Decay: Interactions Between Biofilms, Sediment and Nutrient Delivery

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 settling velocities. In a river system, these fast settling flocs have the potential to interact with benthic biofilms suggesting a mechanism for the incorporation of salmon organic matter into the aquatic food web over short downstream distances. We evaluated the ability of biofilms to trap flocculated salmon organic matter and fine sediment in the Horsefly River spawning channel during the active-spawn and post-spawn periods. We stocked two sequential enclosures in the spawning channel with sockeye salmon (Oncorhynchus nerka) and established one upstream exclusion zone as a spatial control for salmon spawning. Biofilms were sampled for chlorophyll a, trapped sediment and a marine isotope tracer (δ¹⁵N). In the active spawn period biofilm was reduced in abundance due to salmon spawning activity, with isotope values signifying low utilization of marine-derived nutrients. During the post-spawn period, downstream biofilm abundance exceeded pre-spawn values indicating a near-field nutrient pulse while δ¹⁵N values reflected an upstream decaying salmon nutrient source. With the re-established post-spawn biofilm layer, the mass of fine sediment trapped at the streambed surface increased. This increase in biofilm trapping efficiency occurred in concert with a significant increase in the in-situ particle size of suspended material. These larger, fast settling composite particles have a higher probability of becoming trapped by surface biofilms, thereby aiding in streambed delivery of marine derived nutrients. Increases in each of fine sediment mass, biofilm abundance and δ¹⁵N values indicate that these larger composite particles comprised of inorganic sediment and salmon organic matter were trapped and rapidly processed by downstream biofilms.