The mostly oligotrophic character of the Mediterranean Sea is altered drastically in areas receiving the outflow from large rivers. The Gulf of Lions, receiving discharges from the Rhone River, has nutrient and phytoplankton concentration much higher than the adjacent open northwestern Mediterranean Sea. A surface layer of freshwater, with thickness that varies with the meteorological conditions between 2 and 40 m, overlies the deeper open seawater; this is advected onto the shelf and influences an area that covers the eastern half of the Gulf of Lions. Most of the waters affected by the river discharges show property relationships indicating conservative behaviour, with very little or no loss of nutrients through phytoplankton uptake, particularly in winter. Phytoplankton populations in winter are sparse, with maximum densities just above and below the boundary between the fresh- and seawater. Diatoms are the main group of organisms, although dinoflagellates, coccolithophorids and cyanobacteria are abundant. Small heterotrophs (cilliates, tintinnids, etc.) are also abundant and are positively correlated with the diatoms. A water balance model, linking the river discharge to the advective fluxes of water and nutrients, is proposed. The primary productivity supported by such fluxes is estimated.
