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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.

In the NE Atlantic during May 1990, a period of high aggregate abundance, amorphous aggregates contained substantially higher concentrations of bacteria, cyanobacteria and flagellates than the surrounding seawater. Those from 45 to 55 m, at the aggregate maximum just below the seasonal thermocline, contained 2.1 to 25.4 X 10' bacteria, 1.0 to 4.7 X 10' cyanobacteria and 1.3 to 33.0 X 106 flagellates ml-' aggregate. Leucine incorporation rates by bacteria attached to aggregates ranged from 12 to 206 X 10-*' m01 cell-' d-' The concentration of bacteria associated with faecal pellets was generally higher than the concentration on the amorphous aggregates. Bacteria attached to aggregates were equivalent to 10 and 14 % of free-living bacterial carbon integrated through the water column to 100 and 300 m, respectively. This could rise to 25 and 34% if the different carbon content of attached and free-living bacteria was taken into account. However, this study coincided with the maximum marine snow concentration measured 150 km southwest, so at other times when marine snow concentrations are lower, the proportion of attached bacteria will be less. The contribution of attached bacteria to total bacterial production in the top 100 and 300 m ranges between 1.8 and 3.4 %. The contribution of free-living bacterial carbon to suspended POC (particulate organic carbon) was between 25 and 33% and after correcting for their retention on the glass fibre filters, this contribution could be 28 to 40%. Flux studies during 1989 and 1990 indicated that a smaller proportion of POC flux (9 %) and bacterial carbon flux (10%) reached 3100 m than mass flux (25 and 35%), indicating there are processes involved which preferentially utilise or reduce the POC and bacterial components. Bacterial detachment from sinking particles could contribute 2.4 % of the integrated bacterial biomass per annum. The fraction of sink~ng POC lost between 150 and 3100 m may be an important carbon source to the mid/deep-water bacterial population, capable of supplying around 90% of the bacterial carbon demand during April to July 1989 at 47" N, 20°W. In the deeper waters, between 600 and 3100 m, there was sufficient depth-dissipated sinking POC to potentially supply the carbon demand of the bacteria. However, above 600 m an additional source of organic carbon is required to support their growth.

Urcmema sp. of ca 12 X 5 p m and Euplotes sp. ca 20 X 10 pm were isolated from surface waters of the English Channel. The rapidly motile Uronerna sp. has a relative growth rate of 3.32 d-' and responds rapidly to the presence of bacterial food with a doubling time of only 5.01 h. Its mortality rate is 0.327 d-' and mortality time is therefore short at 50.9 h once the bacterial food resource has become h i t i n g. Uronema sp. therefore appears to be adapted to exploit transitory patches when bacterial prey abundance exceeds a concentration of ca 6 X 106 cells ml-'. In contrast, Euplotes sp. had a slower relative growth rate of 1.31 d-' and a doubling time of ca. 12.7 h, implying a slower response to peaks in bacterial food supply. The mortality rate of 0.023 d-' is considerably lower than In Uronema and mortality time is as much as 723 h. This suggests that, relative to Uronerna, the slower moving Euplotes has a more persistent strategy w h c h under the conditions of our experiment favours a stable e q u h b n u m wlth its food supply. Grazing activities of these 2 ciliates have an important influence on abundance and size-class structure of their bacterial prey. In the presence of both Uronema and Euplotes, there is some evidence of an initial removal of large rods and squat rods, followed by removal of the smaller cocci before the initial population of bacteria is consumed. This is followed by the appearance of a secondary bactenal assemblage which is associated only with the development of the grazlng chates and is dominated by small thin rods. Gross growth efficiency of Uronema and Euplotes is ca 27 and 19 '10 respectively. Hence as much as 73 to 81 % of the carbon ingested with the bactenal food is dissipated through respiration and excretion. Should this occur in natural euphotic waters this process may supplement the DOM release by primary producers. Extracellular DOM production by the ciliates may therefore be of major significance in the maintenance of the secondary population of bacteria, and emphasizes the close interdependence of ciliate predators and bacterial prey in rnicroheterotrophic food webs.

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