Effects of formalin preservation on invertebrate stable isotope values over decadal time scales

ISSN： 1480-3283

Source： Canadian Journal of Zoology, Vol.90, Iss.11, 2012-11, pp. : 1320-1327

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Abstract

Stable isotope values derived from chemically preserved organisms are a valuable resource for documenting long-term ecosystem changes. However, isotopic correction factors of preservation effects applied to samples stored for decades are frequently based on studies lasting only months, assuming that the effects of preservation stabilize within a short time frame. Very few studies test this critical assumption. We validated this assumption for formalin-preserved invertebrate tissues, finding no significant difference between mean isotopic δ¹³C and δ¹⁵N values of material stored 1–15 years across taxa. Preservation effects were evaluated for Amphipoda, Chironomidae, Dreissenidae, Ephemeroptera, Gastropoda, Isopoda, Sphaeridae, Oligochaeta, and Trichoptera. On average, freshwater benthos δ¹³C was lower by approximately 2‰ after formalin fixation, whereas δ¹⁵N values were not different from control samples. Fixation effects were similar among taxa, but were more pronounced in Gastropoda and Sphaeridae for δ¹³C and in Trichoptera for δ¹⁵N. We reviewed the literature to show that preserved freshwater zooplankton δ¹³C were slightly but significantly lower relative to control samples (–0.2‰) and higher in δ¹⁵N (+0.25‰). The mean decline among marine invertebrate δ¹³C was greater than for freshwater invertebrates after 1+ years of formalin preservation, but effects on δ¹⁵N were not different between marine and freshwater invertebrates.