18 pages, 12 figures, 3 tables.Primary producers must respond to the diel changes in light availability. Therefore, detection of diel cycles in bacterial activity would imply tight coupling between the production of photosynthetic dissolved organic carbon (DOC) and its consumption by bacteria. Absence of diel cycles, on the contrary, would indicate that bacteria depend largely upon allochthonous organic carbon and that bacteria are not tightly dependent on photosynthetically produced autochthonous carbon. In 1993 and 1994 we sampled 3 sites in the NW Mediterranean Sea several times a day, and measured several microbial parameters as well as the vertical profiles of DOC along the diel cycle. The sites were selected so that one was on the continental shelf and, thus, was more influenced by coastal runoff; a second one was over the shelf slope and a third, oceanic one was located further offshore over a depth of 2000 m. We found clear diel cycles in bacterial total and specific activity always in the oceanic stations and sometimes in the shelf slope stations. Diel changes were detected as changes in both DNA and protein synthesis rates. These diel cycles were accompanied by diel changes in the distribution of total DOC, and by diel changes in the proportion of bacteria containing visible nucleoids. Noon estimates of bacterial activity were more than twice the daily average in the oceanic site, but they were less different in the other 2 sites. DOC changed daily by 15 µM (5 to 15% of the total stock). For bacterial activity to explain the diel changes in DOC concentration, bacteria should have growth efficiencies lower than 10% in general, and lower than 2% in the oceanic station.This work was supported\ud by grants AMB 94-1019 and 94-0853 from CICYT, PB91-075 from DGICYT and MAS2-CT93-0063 and MAS3-\ud CT95-0016 grants from the EU to the ICM and to A. Hagströjm.\ud M.D.D was supported by a fellowshii from the EU MAS2-CT93-0065 project.Peer reviewe

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