Challenges using stable isotopes for estimating trophic levels in marine amphipods

摘要

In marine food web studies, stable isotopes of nitrogen (δ~(15)N) and carbon (δ~(13)C) are widely used to estimate organisms' trophic levels (TL) and carbon sources, respectively. For smaller organisms, whole specimens are commonly analyzed. However, this "bulk method" involves several pitfalls since different tissues may fractionate stable isotopes differently. We compared the δ~(15)N and δ~(13)C values of exoskeleton versus soft tissue, in relation to whole specimens, of three common Arctic amphipods in Svalbard waters: the benthic Anonyx nugax, the sympagic (ice-associated) Gammarus wilkitzkii and the pelagic Themisto libellula. The δ~(15)N values of the exo-skeletons were significantly lower than those of the soft tissues for A. nugax (10.5 ± 0.7‰ vs. 15.7 ± 0.7‰), G. wilkitzkii (3.3 ± 0.3‰ vs. 8.3 ± 0.4‰) and T. libellula (8.6 ± 0.3‰ vs.10.8 ± 0.3%o). The differences in δ~(13)C values between exoskeletons and soft tissues were insignificant, except for A. nugax (-21.2 ± 0.2‰ vs. -20.3 ± 0.2‰, respectively). The δ~(15)N values of whole organisms were between those of the exoskeletons and the soft tissues, being similarly enriched in ~(15)N as the exoskeletons (except G. wilkitzkii) and depleted in ~(15)N by 1.2-3.7‰ compared to the soft tissues. The δ~(15)N-derived TLs of the soft tissues agreed best with the known feeding preferences of the three amphipods, which suggest a potential underestimation of 0.5-1.0 TL when stable isotope analyses are performed on whole crustaceans with thick exoskeletons. The insignificant or small differences in δ~(13)C values among exoskeletons, soft tissues and whole specimens, however, suggest low probability for misinterpretations of crustaceans' primary carbon source in marine ecosystems with distinctly different δ~(13)C-carbon sources.