- Pacific salmon (Oncorhynchus spp.) transport marine-derived nutrients (MDN) and organic matter to freshwater ecosystems, which enhances the productivity of North Pacific 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 Shiretoko World Natural Heritage Site in eastern Hokkaido (Japan). 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 (δ13C and δ15N) of biofilm, invertebrates, fish, riparian plants and brown bear (Ursus arctos) in the Rusha River during the pre-spawning and spawning periods and in the Akai River (where there are no salmon). Willow 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 Dolly Varden (Salvelinus malma).
- The δ13C and δ15N 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 Dolly Varden 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 δ13C and δ15N of blowflies (Calliphora spp.) and brown bears increased significantly during the spawning run. Riparian vegetation, with the exception of Manchurian alder (Alnus hirsuta), incorporated 25% of its nitrogen from salmon. The δ15N values of riparian willow (Salix spp.) were correlated negatively with distance from the stream.
- The proportion of MDN incorporated in the freshwater biota was lower than that reported for North American rivers, potentially due to the influence of dams and modification of the river environment in this Japanese 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.