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 13 C and 15 N in small samples (1 g N and 2 g C). This system was used to analyze 13 C and 15 N of meiofaunal species from a coastal marsh using pooled samples of 5 to 60 individuals. In a conventional field survey, 13 C and 15 N 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 13 C and 15 N 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 13 C and 15 N 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.