Cross frontal variability in hydrological and biological structures observed in a river plume area (Rhone mouth, NW Mediterranean Sea)

In the Rhone River plume area, NH4+, DFAA, seston particles, bacteria, flagellates and mesozooplankton were studied over 24 hours at a fixed station. Variations of both abiotic and biotic variables demonstrated rapid changes in vertical structure, from stratified towards mixed. This evolution was related to hydrodynamical changes caused by wind. Strong variability resulting from hydrological changes was apparent in surface water where the plume edge crossed the station as evident by sharp increase in particle volume (20-25 mu m ESD range), NH4+ (from 9 to 12 mu M) and DFAA (from 1 to 3 mu M) concentrations. This frontal ecosystem favored fast growing organisms such as bacteria of which densities reached 19 and production 0.22-0.26 mu g C l(-1) h(-1), and flagellates which showed densities of 1.5 to 2 mu g C l(-1). Among DFAA, the relative concentration (mole %) of serine increased from 13.3% in the plume water to 44-45% in the front system. The biomass and feeding activity of zooplankton also increased at the plume boundary.