Seasonal Evolution of Microplanktonic Communities in the Estuarine Front Ecosystem of the Rhône River Plume (North-western Mediterranean Sea)

Seasonal evolution of microplanktonic communities was examined throughout the water column of the Rhône River outflow plume. Because of the lack of tide, the water column in the plume area corresponds to a three-layered system: an upper freshwater layer (plume layer) overlying Mediterranean seawater (seawater layer) and an intermediate layer (frontal layer) corresponding to the discontinuity. The three layers displayed different compositions of planktonic community. Except for bacteria, which were more abundant in the plume water during spring and summer, microorganisms were most abundant in the frontal water. Seasonal analysis showed a succession of microbial populations: primary producers (phytoplankton, autoflagellates) peaked in spring while heterotrophic organisms (bacteria, heteroflagellates) were more abundant in summer. In winter, the primary producers were cyanobacteria. Bacterial density and production were dependent upon the combined influence of temperature variation and nutrient concentration. The highest values of bacterial production were observed in the frontal layer. They were about 25 μg Cl-1 day-1, that is, respectively, two and three times more than the bacterial production in the freshwater and the seawater layers. Mortality rates were highest in the plume water (0•51 to 0•57 day-1), and only 60-70% of the mortality was due to the grazing. In the plume layer, the growth rate was twice in the fall of that in spring (1•34 against 0•51 ). The average growth rate in the frontal layer (1•57 day-1) was highest of the three layers and was coupled with the lowest mortality rate (0•22 day-1). In the frontal layer, the flagellate carbon requirement was only one tenth of the bacterial carbon production.