Camera trap image of black bear (Ursus americanus) consuming a salmon carcass.
Using Stable Isotopes to Track Salmon-Derived Nutrients in Riparian Food Webs
Melanie Truan (1,2), Andrew Engilis Jr (1,2), Avery Cook (1,2), Ona Alminas (1,2), Lisa Thompson(1), Ken Davis(3), Marisa Escobar(4), Chris Mosser(1), David Purkey(4), and Peter Moyle(1)
(1) Department of Wildlife, Fish & Conservation, UC Davis, firstname.lastname@example.org; (2) Museum of Wildlife & Fish Biology, UC Davis, (3) Wildlife Survey & Photo Service, (4) Stockholm Environment Institute
As part of a larger, EPA-funded project investigating climate change effects on California’s spring-run Chinook salmon, the MWFB has been looking at salmon-delivered nutrient subsidies and their role in the riparian food web. Since they spend the majority of their life in the ocean, anadromous salmon deliver large quantities of marine-derived nutrients to freshwater systems when they return to spawn. These nutrients help increase the productivity of freshwater ecosystems, contributing to an integrated network of nutrient pathways and cycling processes. Loss of these nutrient subsidies can have profound cascading effects on riparian ecosystems (Naiman et al. 2002).
Nutrient flux through food webs can be tracked through the use of stable isotopes. Stable isotopes move in predictable ways through food chains and can be used to identify dietary sources at the base of food webs (Chaloner et al. 2002). Marine sources are generally enriched in the heavy isotopes of nitrogen and carbon and can thus be used to trace marine inputs to freshwater systems.
Butte Creek hosts the largest remaining population of wild, naturally-spawned spring-run Chinook salmon (Oncorhynchus tshawytscha) in California. But these populations are much reduced from historic levels. Linked watershed and salmon population dynamics models created by this study predict that long term survival of this federally-endangered species is unlikely under current climate change scenarios (Thompson et al. 2011). Loss of salmon-derived nutrient subsidies to the Butte Creek ecosystem could have grave implications for the region’s biodiversity and nutrient cycling processes.
To test the importance of salmon-delivered marine-subsidies to the Butte Creek food web we collected tissue samples (whole body, hair, feathers, muscle tissue) prior to and during the spawning period at three spawning sites and one control site upstream of a barrier to salmon migration. Samples were dried, homogenized, and encapsulated, then analyzed for δ13C and δ15N (enriched relative to nonmarine sources) at the UC Davis Stable Isotope Facility.
Salmon Food Webs in the News
- Climate Change Threatens California Salmon (as featured on NPR)
- Researchers Find Salmon Vital to Biodiversity in California (as featured on NPR)
We also documented salmon consumers using motion sensor cameras (Moultrie Game Spy 4.0, StealthCam Sniper Pro, Trailmaster 1500) placed at salmon-baited camera trap stations at spawning sites. A variety of organisms were photographed, shown above, including (left to right): turkey vultures, ringtailed cat, mule deer, and black bear (top of page right).
We found that tissue samples from spawning sites were significantly enriched in marine-derived nitrogen compared with samples from sites above the barrier to salmon migration. Black bear and terrestrial invertebrate tissues carried the most marine-derived nitrogen (43% and 37% respectively), while bear and aquatic invertebrate tissues carried the most marine-derived carbon (36% and 42% respectively). These patterns reflected a heavy reliance on marine-derived nutrients in these organisms. Epilithic biofilm (benthic microbial aggregates of autotrophic and heterotrophic prokaryote and eukaryote micro-organisms) was also highly marine-derived nitrogen and carbon enriched, reflecting a close association with salmon and a substantial assimilation of dissolved marine-derived nutrients. Figure 1 plots these values against each other for the organisms sampled.
Fig. 1. Biplot of δ13C and δ15N values in tissues of sampled organisms taken from sites above and below a barrier to salmon migration. Closed circles represent mean values from tissues at spawning sites. Open circles are mean values from sites located above a waterfall barrier to salmon migration. In most cases, open circles plot lower than closed circles, indicating that organisms from nonsalmon sites are less enriched in MDN than similar taxa at spawning sites. (Truan 2011, unpublished data.)
NPR’s Capital Public Radio recently aired a two-part story highlighting this important research. Visit the following pages for these features.
Part 1: Climate Change Threatens California Salmon
Part 2: Researchers Find Salmon Vital To Biodiversity in California
Chaloner, D.T., K.M. Martin, M.S. Wipfli, P.H. Ostrom, and G.A. Lamberti. 2002. Marine carbon and nitrogen in southeastern Alaska stream food webs: evidence from artificial and natural streams. Can. J. Fish. Aquat. Sci. 59: 1257-1265.
Naiman, R.J., R.E. Bilby, D.E. Schindler, and J.M. Helfield. 2002. Pacific salmon, nutrients, and the dynamics of freshwater and riparian ecosystems. Ecosystems 5:399-417.
Thompson, L.C., Escobar, M.I., Mosser, C.M., Purkey, D.R., Yates, D., Moyle, P.B. (2011). "Water management adaptations to prevent loss of spring-run Chinook salmon in California under climate change." J. Water Resour. Plann. Manage., 10.1061/(ASCE)WR.1943-5452.0000194 (Aug. 31, 2011).
Funded by United States-Environmental Protection Agency US-EPA STAR Grant Program, RD–83301701–0.