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

Biomass 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.