Contributions of salmon-derived nitrogen to riparian vegetation in the northwest Pacific region

Abstract

We examined the relationship between the annual escapement of salmon and the δ15N of willow (Salix spp.) leaves to evaluate the contribution of marine-derived nutrients (MDN) to riparian vegetation around the Pacific Northwest and Northeast regions. Foliar δ15N values ranged from −3.42° to 4.65°. The value increased with increasing density of carcasses up to 500 fish/km and 1500 fish/km. δ15N values were variable at carcass densities below 500 fish/km. Possible factors affecting the fluctuation of δ15N at reference sites are: (1) denitrification; (2) the presence of N2-fixing trees, such as alder; and (3) agricultural runoff. δ15N values at the sites with carcass densities over 500 fish/km were consistently high, while a value of δ15N below zero was observed at only one site (Rusha River; δ15N = −1.87°). At this site, most adult pink salmon returned to limited locations near the estuary because steeper channel gradients acted as a migration barrier, resulting in the negative δ15N value. Nevertheless, we concluded that our results showed evidence of the feedback of MDN to terrestrial vegetation, although the use of the δ15N value as a terrestrial end member at spawning sites is limited. If the relationship between the enrichment index, which is expressed as the values using a mixing model, and salmon abundance was estimated, the availability of MDN in riparian ecosystems could possibly be evaluated and will lead to the establishment of escapement goals.