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Biogeochemical processes in Rhone diluted mesoscale structures

Naudin Jean-Jacques

Depending on wind conditions, on the density gradient induced by the Rhone River input and on the intensity of mixing processes, diluted water masses may accumulate along the coast (landward wind) or be transferred to the coastal zone (seaward wind). In that case diluted water masses may 1) detach from the river plume proper to form lens or filament as shown by satellite images, and 2) interact with the shelf currents or the mesoscale circulation at the shelf break. Simulations of the salinity field through a 3D-hydrodynamic model were consistent with field measurements.
These diluted structures, have a larger volume than the river plume and are transferred all over the Gulf of Lions whatever the season. During their offshore transfer, a higher content in dissolved and particulate compounds than in the surrounding marine environment is maintained over long distances and the microbial diversity leads to differential processes in C and N assimilation and regeneration. Within the river plume, both autotrophic and heterotrophic communities contribute to preferential consumption in ammonium at salinity lower than 30, whereas large phytoplankton develop at higher salinity due to the nitrate stock still available. Within detached diluted structures, the confined ecosystem is dominated with picoplankton using nitrogenous compounds locally regenerated.
Due to the dominance of microbial recycling processes, the impact of these 3D mesoscale structures on the carbon cycle and the associated biogenic fluxes, as well as on the transfer of particulate and dissolved material must be accounted for. Besides the "classical" biogeochemical approach on the role of the photic zone microbial food web, further investigations on photo-biological and chemical processes as well as on interactions between biological activity and pollutants associated to particulate compounds have been performed during the May 2006 BiopRhofi cruise on board "Le Suroît". All these parameters will contribute to calibrate and validate coupled biogeochemical - dynamical models to estimate the role played by river plumes and associated structures in transforming river-born compounds.