OCEANIC dissolved organic carbon (DOC) is one of the Earth's largest carbon reservoirs, but until recently its role in the carbon cycle has been neglected. New methodology1, however, has led to larger estimates of DOC concentrations and also to renewed interest in the biochemical lability of DOC2. Previous work found that the mean age of DOC in the surface ocean was > 1,000 years3. To examine the lability of DOC in greater detail, we have conducted experiments to estimate DOC turnover rates in the upper ocean. We directly observed rapid DOC turnover by bacterioplank-ton during the spring phytoplankton bloom in the North Atlantic ocean. Potential turnover rates, measured in 0.8-um filtered samples, ranged from 0.025 to 0.363 per day, and were consistent with bacterial biomass production and uptake of dissolved nitrogen (NH+4, NO-3 and urea). Our results indirectly suggest that cycling of dissolved organic nitrogen (DON) differs from that of DOC. The high estimates of DOC concentrations and turnover rates repeated here, if found to be general, would seem to demand changes in models4 of carbon cycling and of the ocean's role in buffering increases in atmospheric CO2.