Stable isotope evidence indicates the incorporation into Japanese catchments of marine‐derived nutrients transported by spawning Pacific Salmon

Summary P acific salmon ( O ncorhynchus spp.) transport marine‐derived nutrients ( MDN ) and organic matter to freshwater ecosystems, which enhances the productivity of N orth P acific ecosystems. Relatively few studies, however, have evaluated the MDN subsidy to both the aquatic system and the terrestrial catchment simultaneously. Using stable isotope analysis, we tested how the dynamics of MDN differed between the river and adjacent riparian forest in rivers of the S hiretoko W orld N atural H eritage S ite in eastern H okkaido ( J apan). In addition, we accounted for temporal and spatial variations in the stable isotope signatures of freshwater organisms due to the presence or absence of spawning salmon. We analysed carbon and nitrogen stable isotopes (δ 13 C and δ 15 N ) of biofilm, invertebrates, fish, riparian plants and brown bear ( U rsus arctos ) in the R usha R iver during the pre‐spawning and spawning periods and in the A kai R iver (where there are no salmon). W illow leaves were collected along the 50‐m transects to evaluate how far MDN are incorporated within the riparian area. We counted the number of pink salmon ( O . gorbuscha ) carcasses in riparian areas and categorised their mode of transport. In addition, we examined the stomach contents of D olly V arden ( S alvelinus malma ). The δ 13 C and δ 15 N of aquatic organisms increased by 1–4‰ and 1–6‰, respectively, with the arrival of salmon spawners. Aquatic organisms incorporated 23% of their nitrogen from salmon (range: 7–46%). The diet of D olly V arden switched from aquatic invertebrates to salmon eggs during the salmon spawning run. More salmon carcasses were transported from the stream to riparian areas by flooding than by brown bears. The δ 13 C and δ 15 N of blowflies ( C alliphora spp.) and brown bears increased significantly during the spawning run. Riparian vegetation, with the exception of M anchurian alder ( A lnus hirsuta ), incorporated 25% of its nitrogen from salmon. The δ 15 N values of riparian willow ( S alix spp.) were correlated negatively with distance from the stream. The proportion of MDN incorporated in the freshwater biota was lower than that reported for N orth A merican rivers, potentially due to the influence of dams and modification of the river environment in this J apanese example. The riparian forest incorporated a relatively high fraction of MDN , however, mainly due to the transport of salmon carcasses from the channel by brown bears and, particularly, flooding. The dynamics of salmon‐derived nutrients thus differed between river and adjacent riparian zones. These results suggested the importance of linkages between freshwater and riparian ecosystems for the extent of the marine nutrient subsidy.