Seasonal variation of bacterial production, respiration and growth efficiency in the open NW Mediterranean Sea

Bacterial growth efficiency (BGE) can have a major influence on planktonic food web structure and function; however, its controlling factors are poorly understood. Here, we report results from an annual study aimed at examining bacterial respiration (BR), production (BP), biomass (BB) and BGE in the upper 130 in of the DYFAMED station (JGOFS site, France) in the NW Mediterranean Sea. From the pre-bloom period to early summer, and with a negative gradient from sub-surface to 130 in, bacterial parameters seem to follow each other, i.e. maximum respiration in February, maximum production in March and maximum biomass in April. BGE ranged from 0.1 to 43 % throughout the study period and was highest during the spring bloom and during the fall. These data clearly showed the wide range of BGE that can be experienced at 1 site on an annual timescale in the surface mixed layer and suggest that several factors (concentrations of inorganic nutrients and DOC) control BGE at this site. Our results also suggested that this oligotrophic system is net heterotrophic.