Freshwater inflow from the River Arc to the Etang de Berre (French Mediterranean coast) : hydrology, physical and chemical characteristics) . The influence of the Arc River on the ecology of Etang de Berre (a brackish lake located an the French Mediterranean coast) , of which it is the main tributary , has been studied during two years. The flow and some physical and chemical characteristics of the Arc were estimated. This river is featured by a mediterranean regime attenuated by karstic pondération and irrigation surplus. The average flow reaches 5 m3.s-1, but rare floods can enormously exceed this value. The total annual input of the Arc into the Etang de Berre amounts to about 160. 106 m3, respectively more than 200.106 m3 in 1977 and 260.106 m3 in 1978. The seston weight is very variable, but moderate , with an average of 41 mg.l-1, viz. 12-13000t seston input per year into Etang de Berre. The dissolved oxygen usually does not reach saturation level, with an average content of 6,9 ml.l-1. The values of pH, around 8, are comparatively high for freshwater. The contents of inorganic phosphorus and nitrogen, principally dominated by human activities in Arc basin area, are extremely abundant, compared with other tributaries of the Etang due Berre. Phosphate concentration fluctuates around an average value of 13,7 μatg.l-1 and progressively increases from winter to autumn, nitrates and nitrites, which show many variations and a summer maximum , are exceptionally abundant, with averages of 225 μatg.l-1 nitrates and 10,9 μatg.l-1 nitrites. On another hand silicate concentrations (65,6 μatg.l-1 on an average) are not particularly high because the Arc basin is calcareous and silicium is not an anthropogenic element. Annual quantities of nutrient elements, transported into the Etang de Berre by the Arc River are approximately as follows : 70 t P-PO4, 642 t N-NO3, 34 t N-NO2 and 400 t Si from silicates. These quantities of nitrogenous compounds and phosphates respectively almost equal to three-fold and 3-4 fold the average actual content of the Etang de Berre. This enrichment with elements that frequently limit primary production, constitutes an essential factor of the fertility of the Etang de Berre.