Modifications physico-chimiques dans l'étang de Thau de 1965 à 2000

On the French Mediterranean coast, the Thau Lagoon is the main area for mussel and oyster culture, with 1200 hectars devoted to shellfish culture. Some important modifications have been brought to this stretch of water as : digging of a new canal between the open sea and the lagoon, reorganization of the shellfish culture area, free boarding of the North shore. Many physico-chemical data (temperature, oxygen, salinity, pH, phosphorus, nitrate, nitrits) are available, from 1963 till 2000. This study concerns essentially the long trend evolution for temperature, salinity and phosphorus. The main results are the following : Temperature: real trend of increase of water temperature (2°C) between 1975 and 2000. This increase is directly in relation with the increase of air temperature. Salinity : trend is also at the increase but it is due not only to the increase of temperature but also to the decrease of rain falls in the South of France. Phosphorus : shows a very clear tendency to decrease from 1975 to 1999. This is mainly due to the improoving of the sanitation of he vicinity of the lagoon (depuration station, treating lagoon ....). The methods used in this study did not allow to show the impact of the reorganisation of the shellfish culture area or to proove the influence of the new aperture between sea and lagoon.