Oxygen consumption is used to study the dynamics of dissolved organic carbon (DOC) utilization by bacteria. Preliminary incubation experiments examining oxygen consumption and bacterial growth demonstrated that a small labile fraction of the total DOC pool typically supports bacterial metabolism. Bacterial growth and respiration rates were frequently fastest within the first few hours of incubation experiments suggesting that the pool of organic matter used for growth was rapidly consumed. Comparisons of bacterial production and respiration with total DOC concentrations suggested that approxinlately 1 to 3 % of the pool supports bacterial growth. The presence of a small labile component of the DOC pool suggests a close coupl~ng between bacteria and sources of substrate. Bacterial coupling to phytoplankton was examined in mesocosm experiments in which phytoplankton were enriched with nutrients and bacterial production and oxygen consumption were followed over diel cycles. Bacterial production, oxygen consumption and the availability of organic matter were highest during daylight, when phytoplankton production was assumed to be greatest. The effect of a varying carbon supply over diel cycles on bacterial growth efficiency was examined. During the mesocosm experiment, growth efficiencies were greatest during daylight when substrate availability was greatest. At several estuarine sites, efficiencies varied markedly over daily and seasonal temporal scales. These results suggest that growth efficiency is an important consideration when estimating the bacterial role in aquatic carbon cycles.