Marine nitrogen in central Idaho riparian forests: evidence from stable isotopes

Terrestrial and marine ecosystems are linked by the annual migration of salmonids. We examined the contribution of anadromous fish to foliar nitrogen (N) in central Idaho riparian forests where anadromous fish have been in sharp decline for approximately 30 years. To determine if the amount of N from anadromous fish carcasses can be assessed from conifer foliage, we fertilized seedlings with varying proportions of N from marine sources and analyzed their foliar 15N/14N ratio (δ15N). Also, to evaluate whether N obtained from marine sources was evident in riparian forests, we analyzed the δ15N of conifer foliage, above and below waterfalls, along historically salmon-bearing and non-salmon-bearing streams and rivers in central Idaho. In the seedling study, the amount of marine N was positively correlated with foliar δ15N. Furthermore, we determined that foliar δ15N values in riparian forests were explained by the historical occurrence of salmon. Foliar δ15N declined significantly from bankside to upslope trees in transects along salmon-bearing streams and rivers, whereas there was no significant gradient along transects perpendicular to non-salmon-bearing reaches. We therefore conclude that marine nutrient sources have historically contributed to the N pool in riparian conifer forests and marine N persisted following 30 years of severe populations declines.