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

Modifications to a conventional elemental analyzer-stable isotope ratio mass spectrometer system (EA-MS system) are described that allow the analysis of C-13 and N-15 in small samples (greater than or equal to1 mug N and 2 mug C). This system was used to analyze delta(13)C and delta(15)N of meiofaunal species from a coastal marsh using pooled samples of 5 to 60 individuals. In a conventional field survey, C-13 and N-15 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 C-13 and N-15 to 1 m(2) 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 C-13 and N-15 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.