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Assessment of Food Web Recovery from Fish Introductions and Trophic Structure in Lakes of the Sierra Nevada, California Using Stable Isotopes


Stocking of salmonid fish in naturally fishless Sierra Nevada lakes over the last century has had a profound impact on aquatic foodwebs. Current research has focused on removal of fish from several lakes to observe recovery of zooplankton, benthic invertebrates and, in particular, the endangered southern mountain yellow-legged frog (Rana muscosa). Biological samples from two currently fish containing lakes and two lakes where fish have been removed were collected over the last 15 years to determine recovery rates of species found in the lakes. Due to the remote location of the lakes, the samples were preserved with ethanol (EtOH) for later analysis. However, long-term preservation methods can have significant impacts on the δ13C values of stable isotopes by resulting in enrichment in isotope values. I developed a rinse method to remove EtOH from the archived samples and to reduce the impact preservatives on stable isotope

analysis. After removing the preservative, samples were rinsed for. The rinsing method resulted in enrichment of 13C suggesting a much shorter rinsing time (< 1 day) as well as species-specific corrections for δ13C content were required to improve the accuracy of stable isotope analysis of preserved samples.

I examined the trophic structure of lakes containing and lacking fish using carbon and nitrogen stable isotopes. δ15N values revealed distinct trophic fractionation values for each type of lake. Trophic fractionation for fish-containing and fish-removed lakes was 2.03 / ± 0.517 (n=116) and1.76 / ± 0.384 (n=186) respectively. Nitrogen values in fish-containing lakes species tended to be enriched relative to fish-removed lakes, suggesting increased P availability. Time-series of δ15N for multiple species indicated small variations in nitrogen suggesting changes in diet from year to year. However significant depletion of 4.42 / was noted for the copepod Leptodiaptomus signicauda and correlated with the reemergence of Daphnia melanica post fish removal. Computations of trophic position indicated that frogs and fish occupy the same trophic level and likely rely on the same food sources. This finding suggests that fish cause extirpation of frogs both through direct predation and competition for food resources.

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