A synthesis of tissue-preservation effects on carbon and nitrogen stable isotope signatures

Stable-isotope analysis is a powerful method for characterizing flows of mass and energy through ecosystems. Long-term food-web studies using stable isotopes are valuable but rare because the required samples are not readily available. We examine the feasibility of using preserved specimens from natural-history collections as a source of long-term data for food-web studies and test whether chemical preservation affects the stable-isotope signature of tissues. We experimentally determined the effects of tissue preservation and fixation with 75% ethanol and 10% formalin, respectively, on delta(13)C and delta(15)N of three aquatic consumers: Sacramento sucker, Catostomus occidentalis, Asian clam, Corbicula fluminea, and a caddisfly, Hydropsyche sp. Using both our results and previously published literature results, we characterize preservation effects across many different consumer taxa including invertebrates, fish, and birds. Overall, only formalin fixation systematically affected isotope signature, causing an average depletion of 1.65parts per thousand in delta(13)C, a bias that can easily be corrected for prior to interpreting data. Preservation affected mean delta(15)N values with far lower frequency and magnitude, although variability increased with preservation for some taxa but not others. These findings suggest that preserved specimens may be used for stable-isotope analysis and open up the possibility of using archived collections to reconstruct food webs and biogeochemical changes at scales of tens to hundreds of years.