Résumé : The bacterial utilization of organic matter was investigated during two cruises made in the north-western Black Sea in the summer of 1995 and in the winter-spring transition period (April-May 1997). Bacterial biomass and activities (enzymatic activity and production) as well as direct (dissolved and particulate organic carbon) and indirect (primary production) sources of organic carbon for bacteria were measured along high resolution salinity gradients and a spatial grid in the Danube-Black Sea mixing zone and in the Black Sea marine waters. Despite little change in bacterial biomass, bacterial activity displayed significant spatio-temporal variations. Bacterial production was maximal in the Danube-Black Sea mixing zone in July 1995. In this area, the source of organic matter for heterotrophic bacteria can be derived from autochtonous production and/or from allochtonous inputs. Total potential organic carbon sources were always higher in the Danube-Black Sea mixing zone than in the Black Sea marine waters, particularly due to high primary production and particulate organic carbon concentrations recorded in this mixing zone. Interestingly, dissolved organic carbon concentrations along the salinity gradients were uniform. A seasonal constant accumulation of DOC was, however, observed in the whole area investigated from 2000 mgC m -3 in April 1997 to 3000 mgC m -3 in July 1995. The relative contribution of the various sources of organic carbon to the bacterial carbon demand was analysed. In April 1997, the bacterial carbon demand could be satisfied only by the allochtonous sources of organic matter discharged by the Danube. In May 1997, the contribution of autochtonous phytoplankton-derived organic carbon was expected to increase due to the general enhancement of biological activity. In July 1995, direct sources of organic carbon were barely enough to satisfy the bacterial needs. The excess of dissolved organic carbon accumulated during the winter-spring period is assumed to support the high bacterial carbon demand of summer. Indirect evidence suggests that this delayed utilization of dissolved organic matter could be due to PO4 limitation of bacterial growth. © 2002 Elsevier Science Ltd. All rights reserved.