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, oceanlc one was located further offshore over a depth of 2000 m. LVe found clear die1 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 bactenal activity were more than twice the daily average in the oceanic site, but they were less different in the other 2 sites. DOC chanyed daily by 15 pM (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 O/o in general, and lower than 2 "h in the oceanic station.

By directly measuring the size distribution of active (cells that took up and reduced the redox dye CTC, 5-cyano-2,3-ditolyl tetrazolium chloride) and inactive cells in a natural coastal bacterial community, we tested the hypothesis that the likelihood of a bacterium being active in marine plankton is a function of its size. The average size of an inactive bacterium was 0.055 pm3 while the average size of an active bacterium was 0.12 pm3. This average size was constant even after 3 d of incubation in dialysis bags placed in situ, which increased the percentage of active bacteria in the community from 6 to ca 43 %. The probability of a bacterium being active was a linear function of its size, from ca 5 % for cells of 0.01 l.1n13 to 100% for cells of the largest sizes. These results (1) support the hypothesis of Stevenson (1978, Microb Ecol 4:127-133) that very small bacteria are mainly dormant (inactive) while bigger bacteria are more likely to be active; (2) reconcile 2 apparently opposing observations, (a) commonly found higher specific activities in the larger size classes of bacterioplankton and (b) allometry regularities by which smaller unicellular organisms tend to have higher specific growth rates than larger organisms of similar metabolic mode; and (3) suggest that phagotrophic protists will preferentially crop the active portion of the bacterial community if they select their prey according to size.

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The role of losses in controlling phytoplankton bionlass and production off Hlanes Bay ( N W Ilediterranean) were examined, using In sltu dialysis cultures, during a period of very low nutrlent concentrat~on The ambient community experienced a negative net growth rate (-0.05 * 0.03 divisions d-l), compared to relatively high cornmunit). growth rates (0.78 * 0.07 dlv. d ' ) when metazoan grazers and physical losses, which appeared to bc negligible, were excluded, indicating that loss rates (0 83 dlv. d.') exceeded growth rates. This provided evidence that the blomass and production of the phytoplankton community was controlled by consumers, even at a t~m e of very low nutrlent concentratlons The comn~unity structure changed in response to the suppression of losses, leading to an enhanced growth of nano-and microphytoplankton, while the abundance of plcoplanktunic autotrophs tended to decline slightly. Yet, the biomass attained by nano-and microphytoplankton was slmilar despite large differences in t h e~r initial dens~ty, indicating the existence of a resource limitation of the biomass the different groups ]nay achieve. These results clearly provide e\.idence that the biomass and production of larger (>5 pm) phytoplankton was controlled top-down by metazoan consumers, which were excluded from the dialysis bags, while picophytoplankton biomass was likely to be controllcd by prot~sts This supports the notion that whether autotrophic production 1s channelled through metazoan herbivores or through the 'microbial loop' is closely dependent on the size (greater and smaller than about 5 pm, respectively) of the autotrophs, and that top-down control of phytoplankton production may occur even when nutrients are very scarce.

Growing evidence of inorganic nutrient limitation on oceanic bacteria suggests a global dependence of bacterial activity and production on rates of nutrient supply. The present study examined whether surface bacterial abundance is significantly related to water column stability, and whether bacterial activity and growth rate are related to the rate of diffusive supply of inorganic nutrients to the mixed layer in the Central Atlantic during 2 meridional cruises. The 2 cruises were run under very different oceanic conditions, with relatively low values of bacterial activity in spring 1995 and relatively higher values in fall 1995. We obtained depth-resolved data in the second cruise and found that the integrated value of bacterial production was also related to the rate of nutrient supply, while integrated particulate primary production and chlorophyll concentration were not. There was also no relationship between particulate primary production and bacterial production. The relationship between nutrient supply and integrated bacterial production was tested with data from a mesocosm experiment showing a good fit to the pattern obtained in the Atlantic. Average bacterial production was ~21% of primary production in the Central Atlantic, with values ranging between 5 and 100%, and higher values in the tropical areas. The demonstration of a direct relationship between nutrient supply and bacterial activity helps to explain a relatively large bacterial biomass as compared to phytoplankton biomass, a low bacterial growth efficiency, and a high bacterial carbon demand relative to contemporaneous primary production often measured in the open ocean, as well as the accumulation of dissolved organic carbon (DOC) observed in nutrient-limited oligotrophic seas.