Fertilization of Terrestrial Vegetation by Spawning Pacific Salmon: The Role of Flooding and Predator Activity

Spawning Pacific salmon (Onchorhynchus) transport marine-derived nutrients into streams and rivers. Subsequently, these marine-derived nutrients are incorporated into freshwater and terrestrial food webs through decomposition and predation. In this study, we investigated the influence of spawning Pacific salmon on terrestrial vegetation using stable isotope analysis. We hypothesized that terrestrial vegetation near streams or in areas with activity of piscivorous predators will show higher δ 15 N values compared with the same species growing elsewhere. The influence of spawning Pacific salmon as observed in elevated δ 15 N in terrestrial consumers was also investigated. Data collected from five species of plants in 18 transects from the stream to the upland forest (0 to 1000 m) indicated that a significant decrease in δ 15 N values occurred with increase in distance and relative elevation from the stream in three of the five plant species sampled. Values of δ 15 N in plants at sites actively used by piscivorous predators were higher than those of the same plants growing elsewhere, and similar to values measured near the stream. A decrease in values of δ 15 N and increase in values of δ 13 C in muscles of small mammals, with increase in distance from the stream, indicated that this signature was not a result of direct consumption of salmon carcasses but rather an indirect assimilation of marine-derived nitrogen through terrestrial vegetation. These results indicate that salmon carcasses contribute to the nitrogen pool available to riparian vegetation. The spatial distribution of the marine-derived nitrogen is apparently determined by flooding and the activity patterns of piscivorous predators. The importance of these nitrogen additions to the riparian zone, however, will depend on whether nitrogen is a limiting factor to plant growth in this system, and requires further investigation.