Automated analysis of DIC and DOC in water samples for stable isotopes – a new technique.

R.R. Doucett and B.A. Hungate. Department of Biology, Northern Arizona University, Box 5640, Flagstaff, Arizona, 86011

Most food-web studies now routinely incorporate stable-isotope-ratios in efforts to describe food-source and trophic relations among invertebrate and fish communities. Few studies, however, include isotope data on dissolved inorganic carbon (DIC) or dissolved organic carbon (DOC), because of the increased effort needed to prepare and analyze these samples. Here, we present a new automated technique to measure stable-carbon-isotope-ratios in DIC and DOC simultaneously from the same water sample. Using a commercially-available TOC analyzer interfaced to an isotope-ratio mass spectrometer, a typical water sample (1 to 20 ppm C) requires little volume (< 40 ml) and time (< 10 min) to measure both stable-isotope and concentration data. Preliminary results will be presented from samples collected along longitudinal gradients in two streams in northern Arizona (Fossil Creek and Oak Creek). The relative ease of this new technique should allow researchers to incorporate DIC and DOC isotope data in future food-web and nutrient-cycling studies.