Carbon and nitrogen stable isotope assessment of the Lake Roosevelt aquatic food web
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The purpose of this study was to investigate the carbon and nitrogen stable isotope signatures of the common aquatic organisms in Lake Roosevelt, a large volume storage reservoir in the upper Columbia River, Washington, USA where annual water level fluctuations limit benthic and littoral productivity and diversity. Carbon isotope signatures were used to indicate the relative importance of benthically vs. pelagically fixed carbon in each species diet. Nitrogen isotope signatures were used to identify the trophic level occupied by each species. We expected a limited benthic component in the aquatic community and a need for benthic fishes to use a high proportion of pelagically fixed carbon. Distinct pelagic vs. benthic carbon signatures were detected for primary producers and primary consumers (benthic organisms possessed greater 13C), which meant that the relative importance of the two carbon sources in the diet could be determined. Our collections revealed few benthic organisms, and of those, only one trophic level of benthic consumers was detected from isotope data. Most of the animal diversity in Lake Roosevelt utilizes pelagically fixed carbon. Even those fishes that are usually considered obligate benthivores (cyprinids, catostomids, and cottids) collected more than 65% of their carbon from pelagic sources.