Use of a Stable Carbon Isotope Addition to Trace Bacterial Carbon through a Stream Food Web

The use of bacterial carbon by stream invertebrates was assessed by dripping 13C as sodium acetate into a headwater spring at Coweeta Hydrologic Laboratory for three weeks during August 1992. The addition raised the δ 13C value of dissolved organic carbon from approximately -26‰ to approximately 100‰. Coarse particulate organic matter, fine particulate organic matter (FPOM), and 14 taxa of animals were analyzed by mass spectrometer before and after the 13C addition. Pre-addition sample δ 13C ranged from -36‰ to -22‰; post-addition samples ranged from -35‰ to 129‰. Predators contained less 13C label than collectors, shredders, and scrapers. Shredders were not uniformly labeled, suggesting low use of bacterial carbon. Stenonema (Heptageniidae), a biofilm scraper, was the most highly labeled taxon (up to 128‰), even though biofilm δ 13C was -16‰. Chironomids and copepods were clearly labeled and had a higher δ 13C than the FPOM, suggesting preferential assimilation of bacterial carbon relative to FPOM. Although adults and larvae of Optioservus (Elmidae) are believed to be scrapers, the adults were more labeled than the larvae, indicating greater dependence on bacterial carbon. Gut analyses of Optioservus corroborated the stable isotope results: adult guts contained mostly detritus whereas larval guts contained a high proportion of diatoms. This technique is useful for determining the relative differences in bactivory by an assemblage of stream animals.