Preservation effects on C/N ratios and stable isotope signatures of freshwater fishes and benthic macroinvertebrates

Danny C. P. Lau, Kenneth M. Y. Leung, David Dudgeon

Limnol. Oceanogr. Methods 10:75-89 (2012) | DOI: 10.4319/lom.2012.10.75

ABSTRACT: This study examined the effects of fluid preservatives on carbon and nitrogen stable isotopes (δ13C and δ15N) and C/N ratios of freshwater animals. Brotia hainanensis snails, Caridina cantonensis and Macrobrachium hainanense shrimps, and Pseudogastromyzon myersi, Liniparhomaloptera disparis, and Ctenogobius duospilus fishes were collected from seven Hong Kong streams, so as to incorporate natural variations in isotopic signals among conspecifics. Samples were preserved with 10% formalin, 70% ethanol, or formalin-ethanol solution (fixation in formalin then storage in ethanol). We compared sample molar C/N, δ13C, and δ15N with frozen conspecifics after 30, 60, 90, 180, and 360 d. Increases in C/N were evident in formalin-fixed shrimps and fish only, whereas ΔC/N attributable to ethanol and formalin-ethanol preservation was insignificant in all species. Chemical preservation generally caused δ13C depletion in fishes and Δδ13C significantly declined over time in formalin-ethanol-preserved L. disparis. Formalin-induced δ13C shifts were observed in shrimps (C. cantonensis: -1.54‰; M. hainanense: -0.80‰) and snails (-0.25‰) and were relatively consistent when preservation was ≤ 60 d. The influence of formalin-ethanol on C/N and δ13C was smaller than that of formalin for all species and more consistent than ethanol preservation. δ15N of all species was unaffected (within ±1‰) by chemical preservation. Effects on isotopic signals were more predictable among fishes than shrimps or snails. Corrections of +1.11‰ and +1.24‰ should be applied to δ13C of fishes preserved with formalin and formalin-ethanol (respectively) during trophic analysis. We recommend using formalin-ethanol for macroinvertebrates to limit isotopic shifts, especially those preserved for > 60 d.