Interpreting stable isotopes from macroinvertebrate foodwebs in saline wetlands
E. Andrew Hart and James R. Lovvorn
Limnol. Oceanogr., 47(2), 2002, 580–584

We compared stable-isotope (¹³C and ¹⁵N) 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 ¹³C, mean of 3.4‰ for ¹⁵N) 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 ¹⁵N between zygopterans and their prey (¹⁵N) varied from 1 to 3.4‰ between wetlands. In other studies, such variation in ¹⁵N 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 ¹⁵N at the base of foodwebs. We also suggest that variation in the timing and rates of nitrogen cycling can affect measured ¹⁵N 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.