Small-sample methods for δ13C and δ15N analysis of the diets of marsh meiofaunal species using natural-abundance and tracer-addition isotope techniques

Abstract

Modifications to a conventional elemental analyzer-stable isotope ratio mass spectrometer system (EA-MS system) are described that allow the analysis of 13C and 15N in small samples (≥1 μg N and 2 μg C). This system was used to analyze δ13C and δ15N of meiofaunal species from a coastal marsh using pooled samples of 5 to 60 individuals. In a conventional field survey, 13C and 15N isotope values indicated that (1) harpacticoid copepod species, nematode species, and ostracods differentially exploited phytoplankton detritus, Spartina alterniflora detritus, and benthic microalgae, and (2) all taxa showed a general shift toward S. alterniflora isotope values in winter relative to summer. In a field experiment, benthic microalgae were labeled in situ by addition of 13C and 15N to 1 m2 sediment plots. Two nematode species with apparently similar primary food resources (S. alterniflora detritus) based on natural isotopic values differed dramatically in their uptake of 13C and 15N in labeled plots, indicating differences in feeding strategies that were not indicated by natural isotope values. A combination of natural-abundance isotope surveys and isotope-addition experiments appears to be a powerful approach for investigating both average patterns and interspecific variability in resource exploitation.