FERTILIZATION OF TERRESTRIAL VEGETATION BY SPAWNING PACIFIC SALMON - THE ROLE OF FLOODING AND PREDATOR ACTIVITY

Spawning Pacific salmon (Onchorhynchus) transport marine-derived nutri ents into streams and rivers. Subsequently, these marine-derived nutri ents are incorporated into freshwater and terrestrial food webs throug h decomposition and predation. In this study, we investigated the infl uence of spawning Pacific salmon on terrestrial vegetation using stabl e isotope analysis. We hypothesized that terrestrial vegetation near s treams or in areas with activity of piscivorous predators will show hi gher delta(15)N values compared with the same species growing elsewher e. The influence of spawning Pacific salmon as observed in elevated de lta(15)N in terrestrial consumers was also investigated. Data collecte d from five species of plants in 18 transects from the stream to the u pland forest (0 to 1000 m) indicated that a significant decrease in de lta(15)N values occurred with increase in distance and relative elevat ion from the stream in three of the five plant species sampled. Values of delta(15)N in plants at sites actively used by piscivorous predato rs were higher than those of the same plants growing elsewhere, and si milar to values measured near the stream. A decrease in values of delt a(15)N and increase in values of delta(13)C in muscles of small mammal s, with increase in distance from the stream, indicated that this sign ature was not a result of direct consumption of salmon carcasses but r ather an indirect assimilation of marine-derived nitrogen through terr estrial vegetation. These results indicate that salmon carcasses contr ibute to the nitrogen pool available to riparian vegetation. The spati al distribution of the marine-derived nitrogen is apparently determine d 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 i n this system, and requires further investigation.