Trophic relationships among Southern Ocean copepods and krill: Some uses and limitations of a stable isotope approach

Abstract

The use of stable isotopes to study food webs has increased rapidly, but there are still some uncertainties in their application. We examined the ␦ 15 N and ␦ 13 C values of Antarctic euphausiids and copepods from the Polar Front, Lazarev Sea, and Marguerite Bay against their foodweb baseline of particulate organic matter (POM). Interpretations of trophic level were helped by comparison with other approaches and by calibration experiments with Euphausia superba fed known diets. Results for well-known mesozooplankters (e.g., Calanoides acutus and Metridia gerlachei) were internally consistent and corresponded to those derived from independent methods. This gave confidence in the isotope approach for copepods and probably larval euphausiids. Among the dominant yet poorly known species, it suggested mainly herbivory for Rhincalanus gigas but omnivory for Calanus simillimus and furcilia larvae of Thysanoessa spp. and Euphausia frigida. The ␦ 15N values of adult copepods were up to 3‰ higher than those of early copepodites, pointing to ontogenetic shifts in diet. In the Lazarev Sea in autumn, the isotopic signals of E. superba larvae suggested pelagic, mainly herbivorous, feeding rather than feeding within the ice. In contrast to the mesozooplankton, some anomalous results for postlarval krill species indicated problems with this method for micronekton. The experiments showed that postlarval E. superba did not equilibriate with a new diet within 30 d. We suggest that the slower turnover of these larger species, partly in combination with their ability to migrate, has confounded trophic effects with those of a temporally/spatially changing food-web baseline. Interpretations of food sources of micronekton could be helped by analyzing their molts or fecal pellets, which responded faster to a new diet.

Trophic position of coexisting krill species: a stable isotope approach

“…This was explained by the adults shifting to a more detritivorous diet containing carcasses of the copepod Neocalanus cristatus. Krill larvae are found to be herbivorous and to some extent omnivorous depending on season and species (Daly 1990, Frazer 1996, Meyer et al 2002, Schmidt et al 2003, supporting the general observation of an ontogenetic increase in trophic position. The paradox of the krill Krill have a prey-size spectrum that spans several trophic levels, as indicated by the trophic overlap in stable isotopes documented here.…”

“…C caused by lipids (Schmidt et al 2003), as C:N values of all species analysed were near the lower limits of the ranges reported in the literature (Schmidt et al 2003, Kiørboe 2013) and thus indicated low lipid content.…”

“…González 1992, Karlson & Båmstedt 1994, Schmidt et al 2003. However, this method only provides snapshot information of recently ingested prey and is biased towards prey with an exoskeleton, such as larger zooplankton, and thus underestimates or ignores soft-bodied prey (Båmstedt et al 2000).…”

Trophic-level interpretation based on d15N values: implications of tissue-specific fractionation and amino acid composition

“…Problems are partly caused by ecosystem complexities such as multiple food sources or variable growth and turnover rates in relation to an isotopically changing food-web baseline (O'Reilly et al 2002, Schmidt et al 2003. There are, however, further problems related to the fundamental principles of the method.…”

Comparison of isotopic turnover dynamics in two different muscles of a coral reef fish during the settlement phase

“…3F) sustains the hypothesis of a high level of activity and lipid loss in this muscle complex compared with the unchanged C/N ratio during treatment 1 ( Fig. 3C; Schmidt et al 2003, Logan et al 2008). …”