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.

Linking foraging strategies of marine calanoid copepods to patterns of nitrogen stable isotope signatures in a mesocosm study

“…An alternative explanation for the stronger enrichment of C. hamatus δ 15 N compared with that of P. elongatus may be that trophic fractionation in the former species is relatively stronger, i.e. that the same diet results in a higher δ 15 N. Despite reports of species-specific δ 15 N enrichment (Macko et al 1982, Hart & Lovvorn 2002, it is common practice in stable isotope studies to consider trophic fractionation to be independent of species per se ('same diet -same enrichment'). A violation of the assumption in our study would imply that slope values are indicative of species-specific enrichment rather than of true differences in diet composition.…”

High intraspecific variability in carbon and nitrogen stable isotope ratios of lake chironomid larvae

“…Selection of three individuals (cf. Hart and Lovvorn 2002) from the Esthwaite sample potentially results in ␦ 13 C of Ϫ48.6 Ϯ 29.6‰ and ␦ 15 N of 0.2 Ϯ 7.6‰ (by mass balance of the minimum, maximum, and closest to median value for C. plumosus larvae). Such standard deviations would negate their use as a satisfactory baseline.…”

“…In principle, the isotope ratios of a good baseline indicator organism should provide a better timeintegrated isotopic perspective and be unaffected by small fluctuations in basal resource values. Thus, in a recent study to elucidate macroinvertebrate food webs in saline wetlands, Hart and Lovvorn (2002) used Chironominae and Orthocladinae as baseline indicator organisms because they were assumed to integrate the most available algal and detrital foods. This assumption would certainly not be valid if a particular chironomid species was selected to represent the base of the Esthwaite food web, and the problem would be exacerbated if no distinction was made between species.…”

Algal vs. Macrophyte Inputs to Food Webs of Inland Saline Wetlands

“…Use of stable isotopes to identify carbon sources and food web pathways is typically based on the assumptions that ␦ 13 C changes by Ͻ1‰ and ␦ 15 N by a mean of ϳ3.4‰ per trophic level (Fry 1991, Neill andCornwell 1992 and for different trophic levels (Vander Zanden andRasmussen 2001, Hart andLovvorn 2002). Moreover, the mean of more than one food with different isotope values can erroneously indicate consumption of yet another item that is not eaten.…”

“…Beyond this trend, few patterns in ␦ 15 N were evident and we chose not to interpret results further. Effects of varying food quality (C:N ratios, Adams and Sterner 2000), relative turnover rates at different trophic levels, and the seasonal change of ␦ 15 N at the base of foodwebs rendered assumptions about trophic-level 15 N enrichment unreliable (Hart and Lovvorn 2002). That is, ␦ 15 N could have been enriched by anywhere from 0 to 3.4‰ (or an even wider range) with each trophic level, and we had no basis for assuming a particular value.…”