Relationship between primary producers and bacteria in an oligotrophic sea- the Mediterranean and biogeochemical implications

The proverbial blue colour of the Mediterranean reflects some of the most e xtreme oligotrophic waters in the world. Sea-surface Sea-viewing Wide Field -of-view Sensor (SeaWiFS) satellite data show the relatively clear, pigment poor, surface waters of the Mediterranean with a generally increasing olig otrophy 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. Howe ver, the relationship between bacterial production and primary production i n the east and west are significantly different. While bacterial production is directly proportional to primary production in the east, in the west it increases as approximately the square root of primary production. This sug gests that the bacteria in the west are relatively decoupled from local con temporaneous 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 a ccumulation of DOG. In addition, the constant proportionality between bacte rial and primary production of 0.22, which, if all primary products are res pired, gives an estimated geometric mean bacteria growth efficiency of 22 % (95 % confidence limits of 17 and 29 %) for data in the eastern Mediterran ean. Our data suggest that the degree of bacteria-phytoplankton coupling ha s an important effect on apparent trends between bacterial and phytoplankto n production in high frequency data. The combination of low primary product ion 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 regio n.