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The Physical and Biological Influence of Spawning Fish on Fine Sediment Transport and Storage

The successful migration of Pacific salmon stocks back to their natal streams for spawning has long been known to be of economic importance, but only more recently has it been documented as being ecologically significant due to the contributions of organic nutrients to the aquatic ecosystem (Wipfli et al., 1998; Gende et al., 2002; Johnston et al., 2004). The return of adult Pacific salmon to their spawning streams results in a transfer of oceanic biomass and nutrients to these freshwater systems. This semelparous species invests all their reproductive energy in this one season to dig a nest (redd) to deposit and fertilize their eggs after which they die, in the vicinity of their redd. While there is a nutrient contribution of excretion products from live fish and the decaying of dead eggs and sperm, the most significant contribution is from the decomposition of the fish carcasses (Johnston et al., 2004). In building a redd the fish rework the gravel bed streams to a depth of approximately 30 cm. In this process the finer sediments stored in the gravel matrices (e.g. Soulsby et al., 2001) are resuspended into the flowing water and moved downstream (Chapman, 1988). The gravel stored sediment which is available to be released to the water column during redd building has been observed to be a combination of both sand and sand-sized aggregates comprised of silts, clays and organic matter (Petticrew, 1996). The settling rate of some of these larger aggregates is similar to that of fine sands (Petticrew and Droppo, 2000), indicating that the fine sediment (silts and clays) and the organic matter constituting the aggregates are not directly advected out of the system as hydrodynamic models would predict for these constituent grain sizes. Terrestrial nutrients delivered from the watershed spiral down through the aquatic system (Vannote et al., 1980; Webster and Meyers, 1997), but it is not clear what portion of the nutrients delivered from salmon carcass decay are retained in the system and for what length of time. Algae, periphyton and benthic insects as well as sediment-associated nutrients are potential retention pools within the stream system. While natural stream and mesocosm investigations of the effect of salmon nutrients on primary productivity and insects have been undertaken (Ritchie et al., 1975; Schuldt and Hershey, 1995; Wipfli et al., 1998), the retention by sediment has not been assessed directly. Salmon die-off in productive spawning streams has been documented as contributing in excess of 250 g C/m2 (Johnston et al., 2004). These salmon carcass nutrients are readily