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 C-13 as sodium acetate into a headwater spring at Coweeta Hydrologic Laboratory for three weeks during August 1992. The addition raised the delta(13)C value of dissolved organic carbon from approximately -26 parts per thousand to approximately 100 parts per thousand. Coarse particulate organic matter, fine particulate organic matter (FPOM), and 14 taxa of animals were analyzed by mass spectrometer before and after the C-13 addition. Pre-addition sample delta(13)C ranged from -36 parts per thousand to -22 parts per thousand; post-addition samples ranged from -35 parts per thousand to 129 parts per thousand. Predators contained less C-13 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 parts per thousand), even though biofilm delta(13)C was -16 parts per thousand. Chironomids and copepods were clearly labeled and had a higher delta(13)C 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.