Impact of phytoplankton and bacterial production on nutrient and DOM uptake in the Rhone river plume (NW Mediterranean)

From April 1998 to April 1999, 4 cruises were conducted to investigate the evolution of physical and biological characteristics of the Rhone River plume. Particular emphasis was placed on the relationships between phytoplanktonic or bacterial production and environmental conditions. For most of the year, distributions of nutrients, phytoplankton and bacterial activities were conservative along the salinity gradient, indicating the dominance of physical dilution processes. In April 1999, nutrient-salinity relationships showed strong deviations from the conservative mixing line which were associated with a phytoplankton bloom. Deviations from the conservative mixing line were used to estimate the relative contributions of physical and biological processes in the removal of riverine derived material. During the phytoplankton bloom, there was a specific organisation of the microbial populations and their activities at intermediate salinity. Maximal productions were associated with maxima in ammonium and phosphate uptake rates for bacteria, but with maxima in nitrate uptake rates for phytoplankton. The relative accumulation of dissolved organic carbon (DOC) and nitrogen (DON) at high salinity, and the seasonal variability in their distribution, resulted in a potential competition between bacteria and phytoplankton for key mineral nutrients highlighted by an increase in the DOC:DON ratio up to 10. This study contributes to our understanding of the relationship between phytoplankton, bacteria and dissolved organic matter and helps to elucidate the factors controlling the productivity of the system. In particular, the impact of wind and riverine output need to be considered alongside seasonal variability in the chemical and hydrodynamic environment.