Small-scale variability in the coupling/uncoupling of bacteria, phytoplankton and organic carbon fluxes along the continental margin of the Gulf of Lions, Northwestern Mediterranean Sea

International audienceA High Frequency Flux (HFF) experiment was conducted during spring 1997 on the continental slope of the Gulf of Lions (Northwestern Mediterranean Sea) with the aim of examining the dynamical and biological processes controlling particle transfer in this margin environment. Within this general framework, a special attention was paid to short temporal and small spatial variations of phytoplankton and bacterial production through six hydrological and biological surveys performed during a 7-week period at nine sampling stations located on a 10 Â 20-mile grid. Downward fluxes of particulate organic carbon at each station were measured by traps deployed at 240 m depth. The f-ratio and the ratio of integrated bacterial to primary production (IBP/IPP ratio), computed as indexes of biological export for each survey and station, did not provide a clear, unambiguous understanding of the importance of biological processes in the cycling of carbon in the upper water column. However, the data collected allowed to draw up carbon budgets for the different phases of the experiment. The comparison of primary production with measured and estimated organic carbon removal terms (sinking, cycling through the microbial food web, grazing by ciliates and metazoans) showed that a balance was never reached between fluxes of production and removal of organic carbon during the course of the experiment. The system shifted from an initial situation of 'missing' carbon (removal > production) to one of 'excess' carbon (removal < production). Factors such as horizontal advection of carbon into and out of the experimental area and accumulation of dissolved organic carbon (dissolved biological pump) are invoked to explain the observed imbalances. A sensitivity test of the budget to the variations of the different parameters involved showed that bacterial growth efficiency was the most important factor affecting the budget.