Biogeomorphic impacts of migration and disturbance: Implications of salmon spawning and decay

Geomorphologic processes often involve a biotic element that acts to regulate landform development. This biotic element can be plant or animal-based with a feedback that ultimately benefits the ecology of the organism. Pacific salmon (Oncorhynchus sp.) are an example of an animal biogeomorphic agent exhibiting such feedbacks and, because of long migrations from the sea to freshwater spawning grounds, are a species of interest that act on both local and regional scales. Upon returning to their natal streams, salmon generate a dual disturbance, resuspending large amounts of sediment as they construct nests while at the same time generating a substantial nutrient pulse through post-spawn die-off and decay. The retention and export of these nutrients are of importance to any hypothesized productivity boost driven by the marine derived nutrients (MDNs). Using experimental enclosures in the Horsefly River spawning channel in north-central British Columbia, our objectives for this study were to i) quantify the magnitude of organic and inorganic sediment export and retention from an active-spawning area and ii) determine the contribution of fine sediment MDN storage. Using a suspended sediment mass balance model, marine isotope enrichment and a time series of gravel bed sediment infiltration, we found strongly linear relationships between sediment infiltration and marine-derived nutrient enrichment. Elevated suspended sediment produced by salmon redd (nest) construction acted as an effective vector for MDN infiltration into the gravel bed. This study demonstrated that localized patterns of sediment deposition are regulated by salmon activity which in turn act to control MDN storage within, and release from, the gravel bed. Furthermore, this study demonstrates the ability of a biogeomorphic agent like salmon to establish a feedback mechanism that creates favorable conditions which ultimately benefit the organism.