In order to understand the microbial processes controlling seasonal DOC accumulation in NW Mediterranean Sea surface waters, nutrient addition experiments were conducted. The dynamics of DOC as well as the response of heterotrophic bacteria in terms of abundance, activity, and community composition were investigated in October, January, March, and July of 2006-2007. During the summer stratified period, the addition of phosphorus (at 0.1 and 1 mu M final concentrations), as well as nitrogen and phosphorus (at 10 mu M N and 1 mu M P) yielded marked decreases in DOC concentrations. Bacterial DOC consumption was 5.6 mu M C per day in the low phosphorus treatment, 8.6 mu M C per day in the high phosphorus treatment, and 8 mM C per day in the nitrogen plus phosphorus treatment. Most, but not all experiments, showed a clear decrease in the DOC concentrations when amended with phosphorus, even when in-situ concentrations were low. The addition of phosphorus also enhanced bacterial production and caused changes in the community composition of the bacterioplankton. The phosphorus limitation of bacterial activity and the changes in the relative abundance of specific phylotypes upon relieving nutrient limitation might explain the low DOC consumption rates during the summer stratified period.