Interpreting stable isotopes from macroinvertebrate foodwebs in saline wetlands

Abstract

We compared stable-isotope (δ13C and δ15N) and gut-content analyses of macroinvertebrate foodwebs in saline wetlands of the Laramie Basin, Wyoming, USA. Standard assumptions of stable-isotope fractionation between trophic levels (<1‰ for δ13C, mean of 3.4‰ for δ15N) suggested that zygopteran (damselfly) larvae consumed mainly amphipods. However, the guts of zygopterans contained no amphipods but rather a mix of chironomid larvae and zooplankton. In all wetlands the gut contents of zygopterans indicated that they were secondary consumers (trophic level 3), but enrichment of δ15N between zygopterans and their prey (Δδ15N) varied from 1 to 3.4‰ between wetlands. In other studies, such variation in Δδ15N has been interpreted to mean that food-chain length differed between aquatic systems. We review alternative interpretations of variable 15N enrichment, namely, varying C:N ratios in food, differential enrichment between consumer species, and habitat-specific variation of δ15N at the base of foodwebs. We also suggest that variation in the timing and rates of nitrogen cycling can affect measured Δδ15N both within and between foodwebs. For aquatic macroinvertebrates, we urge that stable isotopes be supplemented with independent observations to avoid incorrect conclusions about trophic pathways, trophic levels, and food-chain lengths in different ecosystems.