| Summary: | Whether carbon and nitrogen stable isotope analysis (δ13C and δ15N) can resolve herbivore diets across fine spatial scales is particularly unclear in the rapidly changing Arctic. Here, we used stable isotope analysis to assess fine-scale summer dietary differences of Rangifer tarandus platyrhynchus (non-migratory reindeer) in Nordenskioldland, Svalbard. We sampled reindeer hair from Colesdalen and Sassendalen systems between 1994 and 2006 and again in 2012. In linear regression models, current and last years July temperature significantly and positively correlated with δ13C and δ15N of Svalbard reindeer, with July precipitation also being positively correlated to reindeer δ15N. δ13C and δ15N analyses could record annual changes in herbivore diet among valleys separated by less than 50 km. Reindeer from inner valleys within a system were enriched in 13C and depleted in 15N compared to outer valleys. Bayesian mixing model results indicated 10% greater consumption of graminoids in Colesdalen than in Sassendalen. Such spatial variation in diet suggest high site fidelity and localized differences in forage availability. These results probably reflect the earlier growing season onset in Colesdalen and outer valleys than in Sassendalen and inner valleys. Isotopic and MODIS remote sensing data suggested an early-season switch from moss to graminoid dependence in these valleys. Our results illustrated the importance of collecting food sources concurrently with herbivore tissue growth and emphasized the value of using local, time-overlapping source data when modeling diets in the Arctic. Additionally, our analyses quantified an increase in 13C discrimination with rising atmospheric pCO2 and falling atmospheric δ13CO2 and quantified how hair C:N and reindeer age class (e.g. calf versus adult) influence δ13C and δ15N patterns. These data illustrate the ability of δ13C and δ15N to detect subtle temporal and spatial dietary differences and provide evidence that isotopic studies can monitor dietary shifts in the changing Arctic.
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