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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 …