We examined the role of marine derived nutrients (MDN) on the food web of an array of rivers around the Pacific Rim that have variable salmon runs.  The rivers also differ in geomorphology (complex, simple) and water chemistry (brown, clear). Rivers included the Kol and Utkholok in Russia, the Kwethluk in Alaska and the Skeena and Kitlope Rivers in British Columbia.  The primary hypothesis was that MDN in foodweb members would be greater in rivers receiving more salmon and that rivers of higher geomorphic complexity would exhibit higher levels of foodweb MDN.  Natural abundances of stable carbon (C) isotopes were used to trace the flow of organic matter from primary producers to consumers while stable nitrogen (N) isotopes served as an indicator of salmon-derived N and provided information on trophic structure.  A comparison of percent N and C:N ratios for foodweb members were also used to assess potential nutrient subsidy from salmon carcasses.

Results/Conclusions

Marine N and C subsidy was stronger in streams receiving greater runs of salmon and was significantly higher following high returns of pink salmon in some rivers. One of the major differences observed between food webs of rivers with robust salmon runs and those with few salmon was the lack of a marine signal in the invertebrates in rivers with fewer salmon.  Within the Kol River watershed, several aquatic insect species and juvenile fishes showed higher delta ¹⁵N in tributaries receiving millions of salmon in comparison to tributaries receiving a few thousand salmon.  Enrichment of ¹³C in foodweb members was also common in rivers receiving more salmon but the reasons for this remain unclear.  In general, non N-fixing plants exhibited higher delta ¹⁵N in high salmon rivers.  Preliminary observations suggest that MDN influence was higher in complex systems and that there are major differences in the processing of carcasses in rivers of different hydrogeomorphology.  We concluded that food webs are strongly influenced by MDN but the outcome is moderated by the ecological setting.