Stable isotope measurements of carbon were used to study the transfer of carbon from phytoplankton to bacteria during an experimental diatom bloom. A large-volume tank was filled with 900 liters of lo-pmfiltered seawater from Woods Hole Harbor, and N, P, and Si nutrients as well as 13C-enriched NaHCO, were added. This resulted in a high nutrient system with an isotope label of +85?& in the dissolved inorganic carbon pool. Algal growth depleted nutrients to near-zero concentrations midway through the 2-week experiment with little net dissolved organic C (DOC) accumulation; algal chlorophyll concentrations peaked at 38 pg liter-l near the time of nutrient depletion. Decline of the algal peak was associated with diatom sedimentation to the bottom of the tank and rapid accumulation of DOC. In 3 d, DOC increased from -130 to 250 PM C. The C : N ratio of this new DOC was >22 vs. 14 in background harbor water. 13C labeling showed that bacteria preferentially assimilated the newly produced DOC in both the rapid growth and postbloom phases of the experiment, while DOC measurements indicated that very little if any of the background DOC from Woods Hole Harbor was metabolized by bacteria during the experiment. Overall, the experiment showed rapid DOC release following algal blooms, selective use of newly produced DOC by bacteria, and that DOC released during blooms is not rapidly or completely turned over following algal blooms.Heterotrophic bacteria growth in aquatic ecosystems encounter a diverse suite of organic substrates of varying origin, chemical composition, and nutrient content. Some of these substrates are readily assimilated and incorporated into bacterial biomass, some are rapidly respired to CO2 and not incorporated into biomass (Ducklow et al. 1986), and others are extremely refractory and resistant to bacterial attack for thousands of years (Williams and Druffel 1987). Bulk dissolved organic C (DOC) measurements mostly reflect concentrations of refractory components. The ongoing slow metabolism of refractory substrates is unfortunately