Department of Biology, University of Utah, Salt Lake City, UT 84112, USA

now at: Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106, USA

National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Boulder, CO 80305, USA

Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO 80309, USA

Institute for Arctic and Alpine Research, University of Colorado, Boulder, CO 80302, USA

Institute for Arctic and Alpine Research, University of Colorado, Boulder, CO 80302, USA

Department of Biology, University of Utah, Salt Lake City, UT 84112, USA

Stable Isotope Ratio Facility of Environmental Research, University of Utah, Salt Lake City, UT 84112, USA