Biogeochemical impact of a dilution plume (Rhone River) on coastal sediments: comparison between a surface water survey (1996–2000) and sediment composition

The Rhone River is the major source of nutrients and particulate matter to the Mediterranean basin. Its influence on coastal sediments was investigated by searching for the presence of continental markers. The composition of coastal sediments situated in the area influenced by the Rhone River inputs was compared with surface water salinity time-series records (five years bi-monthly survey). Sediment samples were analysed for grain-size, total and organic carbon and nitrogen, pigments, trace metals, carbohydrates, protein and lipid content. Comparisons show that changes in sediments are associated with the mean extension of the dilution plume of the river. The organic carbon content of sediments does not exhibit spatial changes related to the variations in the overlying water column. A large amount of inorganic carbon is discharged into the sea by the river and results in dilution of the organic carbon in the deposited material. The CaCO3 fraction of the total carbon of sediments is influenced by the river inputs with lower values in front of the river mouth. Changes in the composition of organic matter are linked with the inputs of the Rhone River and associated with variations of concentrations of the chlorophyll b as a marker of the terrestrial origin of the organic matter. Chlorophyll b content of sediments showed statistically significant negative correlation with the five-year bi-monthly mean surface salinity values (slope ¼ 0:0233, R2 ¼ 0:9437, n ¼ 22, p < 0:001). Other parameters such as Mn content of sediments exhibit significant positive correlation with chlorophyll b and negative correlation with mean surface water salinity. Labile carbon fraction (carbohydrates, lipids and protein) shows significant correlation both with chlorophyll b and salinity. In the area influenced by the Rhone River dilution plume, there is enrichment in labile organic matter. The amounts of insoluble carbohydrates and phaeopigments suggest that this enrichment is mainly due to detritic organic matter of terrestrial origin. This twofold approach associating a long-term survey of the surface water with analysis of the sediments located beneath seems suitable for achieving a better knowledge of the environment at the sea–continent interface.