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

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 delta C-13 and delta N-15 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 delta C-13 was lower by approximately 2 parts per thousand after formalin fixation, whereas delta N-15 values were not different from control samples. Fixation effects were similar among taxa, but were more pronounced in Gastropoda and Sphaeridae for delta C-13 and in Trichoptera for delta N-15. We reviewed the literature to show that preserved freshwater zooplankton delta C-13 were slightly but significantly lower relative to control samples (-0.2 parts per thousand) and higher in delta N-15 (+0.25 parts per thousand). The mean decline among marine invertebrate delta C-13 was greater than for freshwater invertebrates after 1+ years of formalin preservation, but effects on delta N-15 were not different between marine and freshwater invertebrates.