Assessing levels of marine-derived nutrients in the Elwha River basin prior to dam removal

The removal of two long-standing dams from the Elwha River represents an unparalleled opportunity to study large-scale river restoration. In the Pacific Northwest, the nutrients provided by migrating salmon through spawning and carcass deposition can be an important contribution to freshwater and terrestrial food webs.  For over 90 years, salmon migration has been limited to the lower 8 km of the Elwha River and thus the reaches between and above the dams have lacked this nutrient subsidy. Measurements of water chemistry collected during this study support previous evidence that the Elwha River is oligotrophic. We also collected data on marine-derived nutrient levels above, between, and below the Elwha River dams by assessing stable isotope ratios for carbon and nitrogen.  At summer low flows, we sampled benthic macroinvertebrates, algae, and fish and then analyzed dried tissues for their isotope ratios. Preliminary comparisons of reaches with and without salmon showed elevated levels of heavy nitrogen in resident fish and some invertebrate taxa where salmon still have access. This trend was not evident in periphyton. Documenting baseline values of stable isotopes, in concert with data on benthic and fish community structure, in the years prior to dam removal will be important for understanding the ecological effects of salmon restoration in the Elwha River ecosystem.