River Discharge and Wind Influence Upon Particulate Transfer at the Land–Ocean Interaction: Case Study of the Rhone River Plume

Drifters were released at the Rhone River outlet (north-western Mediterranean sea) to follow the mixing of the river plume with the marine underlying water, and to sample suspended matter during its transfer to the open Sea. Different hydrological and meteorological situations were encountered during these trackings. During the first trajectory, salinity values and river flow measurements indicate that a flood runoff occurred as an impulse with high initial velocity. The maximum transfer was completed in a day and a progressive decrease followed. Two further trajectories characterize the spreading of the river flow under steady north–north-westerly wind conditions. During the last trajectory, the role of the Rhone River flow was secondary and the buoyant river plume reacted quasi-instantaneously to a wind shift from the north to the west. Under these different environmental conditions, great variations were observed in distribution and fate of particulate matter. The decrease in salinity induced by the spreading of fresh water into the marine environment interrupted primary production and prevented its restarting whilst the riverine influence was dominant. During the spreading of runoff, a rapid concentration decrease in total suspended matter (TSM) and particulate organic carbon (POC) occurred in the vicinity of the river mouth. This implies exportation of suspended matter down through the water column. At the peak flood, the TSM concentration was high, and as much as 60% of the initial concentration of TSM was transferred down through the water column after 1 h of drifting. At the end of runoff, the TSM concentration was lower, and about 30% was exported downward. In both cases, TSM or POC concentrations remained stable after 2 h of spreading of the plume. However, the influence of wind forcing induced redispersion in the upper part of the plume of organo-mineral aggregates temporarily accumulated at the halocline.