Responses in stream ecosystem function to salmon subsidies

Pacific salmon serve dual ecological roles in their natal freshwater ecosystems, providing both an enrichment via the contribution of salmon-derived nutrient (SDN) subsidies and a benthic disturbance as ecosystem engineers. The balance between their roles as subsidies and engineers is mediated by the environmental characteristics of their natal streams. My dissertation research examined the effects of annual salmon migrations on stream ecosystem function across gradients in environmental characteristics at the scale of the biome (e.g., climate, underlying geology, vegetation) and the watershed (e.g., large wood volume, benthic sediment size, timber harvest intensity).

I found that streams with more structure, measured as large wood volume, enhanced channel transient storage, and transient storage metrics were robust to changes in discharge (e.g., baseflow vs. stormflow) within a stream. Before the salmon run, export of dissolved nutrients was controlled by transient storage, but not during the run when nutrient concentrations and export were elevated due to the presence of salmon. The density of live salmon best predicted changes in nutrient concentration, whereas the density of carcasses had only a negligible effect on streamwater nutrients.

In addition to the nutrient enrichment, salmon increased stream ecosystem function from before to during the run. Salmon created a 'hot moment' in nitrification as rates increased 3-fold during the salmon run. Variation in nitrification rates was best explained by streamwater and exchangeable ammonium concentrations, which increased during salmon, demonstrating that SDN are actively transformed in stream ecosystems. For whole-stream metabolism, the presence of salmon increased ecosystem respiration across all streams and biomes. In Southeast Alaskan streams, gross primary production doubled during the run, but responded inconsistently to salmon in sand-bottomed Great Lakes tributaries. Thus, dissimilatory and assimilatory processes were generally enriched by SDN despite differences in environmental characteristics among watersheds and biomes.

Overall, my dissertation has demonstrated the ecological significance of salmon on stream ecosystem structure and function in multiple novel ways, both in their native and introduced ranges. The results also confirm the intimate link between Pacific salmon and their natal freshwaters, which may inform management to ensure the conservation of salmon and their vital ecological roles.