How Relevant Is Allochthony For Juvenile Chinook Salmon From The Merced River?

Allochthony modeling scenarios for juvenile Chinook salmon were assessed with a two end-member Bayesian mixing model and different combinations of terrestrial and aquatic primary carbon sources. In 2011, juvenile Chinook salmon from upstream sites of the Merced River (e.g., Merced River Ranch and Robinson’s restoration reach) were more dependent on allochthonous carbon sources than downstream sites (e.g., Gallo’s property). In 2012, the reverse allochthony longitudinal pattern was observed; juvenile Chinook salmon from downstream sites (e.g., Gallo’s and Livingston’s properties) were more dependent on allochthonous carbon sources than upstream sites (e.g., Merced River Ranch). In 2011, our findings are fully compatible with River Continuum Concept (i.e., allochthony decreases downstream), and in 2012 with the Flood Pulse Concept, indicating the importance of terrestrial organic carbon contributions from local sources downstream (e.g., floodplains). Regardless of the simulated carbon isotope fractionation value and trophic position of juvenile Chinook salmon, allochthony increased: 8.9%, 7.7%, 6.8%, and 6.0% at the Merced River Ranch, and 26.1%, 22.5%, 19.8%, and 17.7% at Gallo’s property for aquatic carbon sources at -21‰, -20‰, -19‰, and -18‰, respectively. Regardless of the simulated aquatic carbon sources, salmonid allochthony was higher 2.93 times at Gallo’s property than the Merced River Ranch. Increased salmonid allochthony at Gallo’s property may have resulted from terrestrial carbon contributions from local floodplains or nutrient enrichment.