Modifications du régime hydrique et salin des étangs du système de Vaccarès (Camargue, France) liées aux perturbations anthropiques des cinquante dernières années

Abstract

The central lagoons of the Camargue (Vaccarès and "Etangs inférieurs") extend over a large area (about 110 km2) but are shallow (1 m deep on average). They constitute a hydraulic unit in which water levels and salinities vary in space and time according to climate, the action of the sea and, above all, man's activities.During the last fifty years, the lagoon system has experienced several major perturbations which have had important ecological consequences:- prior to 1950, water balance depended essentially upon the cycle of rainfall and evaporation. The seasonal range in salinity (high on average) and water levels (on average low) was very great;- from 1950-1978 there was a massive input of fresh water each summer resulting from the cultivation of rice. This intensified the effect of high rainfall by raising mean water levels, lowering the salinity of the lagoon system (4 g/l at mid-1978) and dampening fluctuations in the level of salt and water variables;- from 1979-1985 there was an increased input of sea water (due to a weakening of the sea dike and commercialisation of lagoon fisheries) and a decrease in the input of fresh water (due to a succession of dry years and a decline in rice cultivation). This maintained the high water levels but raised the salinity of the system (35 g/l at mid-1984). Salt and water variables fluctuated very little;- in 1986, 1987 and 1988, heavy rainfall raised water levels in winter and lowered the salinity of the lagoons.Changes in salinity over several decades result from fluctuations in the lagoon system's stock of dissolved salts. This paper quantifies this variation over the past 50 years. Dissolved salts fell from three million tonnes before rice cultivation to 600,000 tonnes in 1978, increased to over three million tonnes between 1979 and 1985, and then declined again. Decreases are caused by loss of salts when lagoons overflow into the sea, increases by inflow of sea water and salt-laden groundwater. © Gauthier-Villars, 1992.