Date of Award

Spring 2018

Document Type


Degree Name

Master of Science (MS) in Biology




Nitrogen is often the most limiting nutrient to productivity in terrestrial ecosystems, and can have large effects on ecosystem processes. Two sources of nitrogen to Pacific Northwest riparian areas are marine derived nitrogen (MDN) via anadromous pacific salmon and symbiotic nitrogen fixation via Alnus rubra. The recent removal of two large dams on the Elwha River, WA, opened up ~60 km of previously inaccessible river habitat for pacific salmon. I used naturally abundant stable nitrogen isotopes (denoted as ‰ δ15N) to establish baseline data to monitor the influx of MDN to riparian zones of Elwha River tributaries, post dam removal. I sampled riparian soil and vegetation along three tributaries, representing either the lower (undammed reference), middle (accessible since 2012), or upper Elwha (no anadromous salmon control). I was not able to detect MDN in soil or vegetation at any of the tributaries, including the reference tributary. However, the understory vegetation at the middle tributary had a higher δ15N than the other tributaries (1 ‰, p < 0.05), which may be due to MDN inputs, or upstream anthropogenic nitrogen sources. Periodical monitoring of these sites, and establishing sites further upstream on the main stem of the Elwha River will allow us to trace the return of MDN to the watershed. IV I also compared soil food webs of A. rubra and a non-nitrogen fixing riparian tree species, Acer macrophyllum, by using nematodes as a focal organism. Alnus rubra soil food webs had more predaceous nematodes than A. macrophyllum stands, but this difference decreased with increased sand in the soil (p = 0.034). This could be due to resource quality, as the C:N ratio of A. rubra leaf litter was lower than that of A. macrophyllum (p < 0.001). I then compared riparian soil food webs to those of adjacent upland sites. Total nematode and bacterivorous nematode abundance increased with soil moisture, but only in upland soils (p = 0.004, p = 0.001, respectively). This varied response could be due to riparian and upland soils hosting different taxonomic groups not seen by classifications used here.