The proverbial blue colour of the Mediterranean reflects some of the most extreme oligotrophic waters in the world. Sea-surface Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satelhte data show the relatively clear, pigment poor, surface waters of the Mediterranean with a generally increasing oligotrophy eastward, apparent even from space. Integrated over depth, however, the east and west Mediterranean show similar amounts of phytoplankton and bacterial biomass. By contrast, primary production and bacterial production are 2 to 3 times lower in the eastern Mediterranean than in the west. However, the relationship between bacterial production and primary production in the east and west are significantly different. While bacterial production is hrectly proportional to primary production in the east, in the west it increases as approximately the square root of primary production. This suggests that the bacteria in the west are relatively decoupled from local contemporaneous primary production. In contrast, the gradient of close to 1 in the log bacterial production versus log primary production relationship in the east suggests less temporal decoupling and, therefore, less seasonal accumulation of DOC. In addition, the constant proportionahty between bacterial and primary production of 0.22, whlch, if all primary products are respired, gives an estimated geometric mean bacteria growth efficiency of 22% (95% confidence limits of 17 and 29%) for data in the eastern Mediterranean. Our data suggest that the degree of bacteria-phytoplankton coupling has an important effect on apparent trends between bacterial and phytoplankton production in high frequency data. The combination of low primary production and bacterial dominance of secondary production in the east is also of significance as it could account for the low fisheries production, the low vertical flux of material and low biomass of benthic organisms in the region.

This work is a study of plankton food web structure and carbon flow in March and September 1997 in the Aegean Sea, area of outflow of Black Sea waters in the Mediterranean Sea. Biomass and production of autotrophs were measured by size fraction as well as bacterial biomass and production; furthermore, we studied heterotrophic nanoflagellates (HNAN), ciliates and mesozooplankton biomass, copepod production and grazing impact on phytoplankton. The obtained low values of nutrients and plankton biomass and production confirmed the oligotrophic character of this region. Despite the fact that there was no significant horizontal variability in the spatial distribution of nutrients throughout the study area, the planktonic biomass and production revealed a gradual decrease from the Northeast Aegean (NEA) towards the South Aegean (SA). In the Northeast Aegean, a large part of the fixed carbon was channelled through the microbial food web towards copepods; in contrast there was a low transfer of energy in the South Aegean where the multivorous food web was developed. Throughout the study area, almost 60 -70% of autotrophic biomass and primary production was performed by cells < 3 Am.

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The seasonal patterns of nitrification, denitrificat~on and dissimilatory ammonium production (DAP) rates were studied in the sediment of 2 stations in the Thau lagoon (south of France). The stat~on ZA was located within the shellfish farming zone and the station B was the reference site. A marked effect of shellfish farming on bacterial activities was observed. Spatial differences were associated with discrepancies in the organlc content and the reduction state of sediments, i.e. highest reductive processes (denltrificatlon and DAP) were noted In shellfish farming area, whereas the oxidative process (nitrification) was predominant outside the farming zone. At both stations, the DAP activity increased in September (autumn) concomitant with an increase of the C/N ratio in the sediment due to the sedimentation of the summer phytoplanktonic production. Nitrification and denitrification rates exhibited maxima in November (winter) corresponding to dissolved inorganic nitrogen inputs from the surrounding land. In the shellfish farming site, 98% of nitrate was reduced to NH,+ and 2 % to NzO, showing that the most of the NO3-was reduced to ammonium and remained available for the ecosystem.

Particulate material collected in sediment traps at 80 m depth in coastal northwestern Mediterranean waters, and fresh fecal pellets of copepods harvested in the surrounding water, were examined for the presence and activity of ammonia-oxidizing and methanogenic bacteria. Results provide the first evidence for the coexistence of living and active cells of methanogenic and nitrifying bacteria in either fresh zooplankton fecal pellets or large settling particles containing numerous large size fecal pellets. In addition, the presence of these 2 bacterial types in both copepods and the fecal pellets produced by them suggests that the bacteria probably originate from the digestive tract of zooplankton, most likely as ingested species for nitrifiers and enteric species for methanogens.

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The cultural bacterial strains of two sediment samples, i.e., 260 strains, were submitted to numerical taxonomy to determine "ecological profiles." From these profiles several calculations of bacterial diversity were done with increasing number of strains (between 10 and 130). Studying 20-30 strains was sufficient to obtain a diversity of bacterial community.Number of tests could be reduced from 62 to 30 without any influence on bacterial diversity. Similarity between studied tests was shown by using numerical taxonomy.

We examined inorganic nutrients and microbial assemblages in the RhBne River plume during January 1987. When wind speed is low the plume forms a transient layer of high nutrient-low salinity water overlaying the Mediterranean Sea which has very low nutrient concentrations. The vertical thickness of the plume was on the order of 100 cm, and during one horizontal transect surface density decreased 2-fold within 1 km. Ammonium, nitrate, and phosphate concentrations were at least 10-fold h~g h e r in the thin plume compared with Sea values. Bacterial production (thymidine and leucine incorporation) was higher by about 2-fold at plume-sea boundaries. Although bacterial biomass and production peaked with chlorophyll at plume-sea boundaries, the correlation between these bacterial parameters and chlorophyll concentrations was low. The ratio of bacterial biomass to phytoplankton biomass was greater than 1 for low salinity waters. Our results suggest that microbial loop relationships had been disrupted in the RhBne h v e r plume because of the input of allochthonous carbon and because of rapid changes in growth conditions caused by mixing. In addition to affecting physico-chemical parameters, mixing appears to have diluted heterotrophic flagellate abundance which allowed increases in autotroph abundance and bacterial production.