Relationship between phytoplankton efficiency and the proportion of bacterial production to primary production in the Mediterranean Sea

International audienceBiomass and production of phytoplankton and heterotrophic bacteria were measured during July 1995 along 3 onshore-offshore transects in the NW Mediterranean Sea (Gulf of Lions). The trophic status of the investigated stations varied from weakly oligotrophic to mesotrophic. Two distinct groups of stations were separated when examining the relationships between primary producers, bacteria and nutrients. The 2 groups of stations exhibited a different relationship between the ratio of integrated bacterial production to integrated primary production (IBP/IPP) and IPP. However, there was no longer a difference between the 2 groups when taking into account the phytoplankton efficiency (PE; i.e. IPP in mg C m-2 h-1 divided by integrated chlorophyll in mg m-2) instead of IPP. Indeed, the most striking feature was a highly significant power fit of IBP/IPP versus PE, for a larger set of data collected in the Mediterranean Sea despite large differences in location, season, physical structure, nutrient and biological status of the stations (IBP/IPP = 25.1PE-0.68). For a large range of PE (i.e. higher than 1.0 mg C mg-1 chl h-1) the ratio remained below 25%, but increased dramatically for lower values of PE. This relationship may help to explain the considerable variation (2 to 190%) in bacterial production/primary production ratio previously found in the world's oceans

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

International audienceThe proverbial blue colour of the Mediterranean reflects some of the most extreme oligo-trophic 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 bio-mass of benthic organisms in the region