Learning more about the role of salmon-derived nutrients in Southeast Alaska watersheds
After salmon return from the ocean and spawn upstream, they die and their spent carcasses wash up along the streambanks. As they decompose, their nutrients are cycled back into the terrestrial system. It has long been assumed that these salmon-derived nutrients play an important role in the watershed, and there was some concern about the impact declining salmon runs might have on terrestrial ecosystems. However, the underlying assumption about the role of salmon-derived nutrients in the watershed had never been explicitly tested.
Forest Service scientists and collaborators tested this assumption by delineating riparian zones along salmon spawning channels on Prince of Wales Island in southeast Alaska. They found two distinct soil types in those zones and tested the soil's response to the presence of decaying salmon by measuring the amount of the nitrogen isotope 15N present. The 15N isotope, which is abundant in the ocean but not as common in terrestrial systems, has historically been used to trace presumed salmon-derived nutrients in riparian systems. Contrary to conventional wisdom, the scientist found the soil closest to the stream (which happened to be the younger soil) had significantly lower concentrations of the nitrogen isotope 15N compared to the older soil found farther away from the stream.
From this research, the scientists developed a model that provides a way to constrain the natural variability encountered in studies of riparian nutrient cycles associated with the feedbacks between salmon-derived nutrients and terrestrial ecosystems. This will help improve estimates of the fate of salmon-derived nutrients in soils and vegetation.
Forest Service Partners