Monitoring the Return of Marine-Derived Nitrogen to Riparian Areas in Response to Dam Removal on the Elwha River, Washington

Abstract

Two sources of nitrogen to Pacific Northwest riparian areas are marine-derived nitrogen (MDN) via anadromous Pacific salmon (Oncorhynchus spp.) and atmospheric nitrogen fixation via red alder (Alnus rubra). The recent removal of two large dams on the Elwha River, WA, opened up about 60 km of previously inaccessible river habitat for anadromous salmon. We used naturally abundant stable nitrogen isotopes to establish baseline data to monitor the influx of MDN to riparian zones of Elwha River tributaries post dam removal. We sampled riparian soil, overstory, and understory vegetation in sites with nitrogen-fixing A. rubra and sites with bigleaf maple (Acer macrophyllum) at an undammed reference site, and along three tributaries, one between the former dams (accessible to anadromous salmon since 2012) and the others upstream of the former dams (no anadromous salmon). Based on δ15N measurements of soil and vegetation, we did not detect MDN at any of the tributaries, including the reference tributary. However, the understory riparian vegetation between the former dams had a higher δ15N than the other tributaries, which may be due to upstream anthropogenic nitrogen sources. Although A. rubra foliage was isotopically distinct from A. macrophyllum, and A. rubra litter had higher total nitrogen, soil and understory vegetation in A. rubra and A. macrophyllum sites did not differ isotopically. Monitoring of these areas and those further upstream on the Elwha River will allow us to trace the return of MDN to the watershed, and help to clarify the role that anadromous fish play in riparian ecosystems.