Modifications récentes de la végétation aquatique de l'étang du Vaccarès (Camargue, France) liées aux perturbations anthropiques

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Abstract
L'étang du Vaccarès est isolé des influences naturelles de la mer et du Rhône depuis la construction de digues au siècle dernier. Depuis 1950, des modifications de la salinité liées aux conditions climatiques et aux activités humaines ont entraîné des changements remarquables de sa végétation aquatique. Les transformations des herbiers ont pu être cartographiées grâce à des relevés effectués en 1963-64, 1974 et 1984. L'évolution saisonnière da la salinité, la bathymétrie, la nature du substrat et les qualités optiques de l'eau permettent d'interpréter les modifications de la végétation. Pendant la phase de dessalement, la végétation à Potamogeton pectinatus et Ruppia maritima évolue vers une végétation dulçaquicole à Myriophyllym spicatum et Phragmites communis. A partir de 1980, l'augmentation de la salinité a permis l'installation d'un herbier monospécifique à Zostera noltii. Par leur action sur la dispersion et la floculation des sédiments, les variations de salinité, ajoutées à l'action des vents fréquents en Camargue, influent sur la transparence de l'eau et la transmission de la lumière dans l'eau. Une meilleure transmission de la lumière serait une explication de la plus grande extension des herbiers pendant la phase de resalement du Vaccarès. L'avenir de cette végétation reste entièrement liée à la salinité dont les variations dépendent des conditions climatiques et des activités humaines. Since the construction of the dykes in the last century, the Vaccarès, which is the main waterbody on the Reserve Nationale de Camargue, has been cutoff from the natural influences of the sea and the Rhône. Since then, apart from the uncontrolled action of climatic conditions (rainfall and evaporation) water and salt balance have been basically controlled activities, whereas salinity has been and is still subjected to fundamental changes. Prior to 1950, it was very saline (up to 100 g.l-1) ; it became salty between 1950 and 1979 (mean salinity 6 g.l-1), then from 1980 its salinity increased again to reach values close to sea water as from 1984. Since 1950 these changes in salinity have divertly led to striking changes in the aquatic plant community, which was monitored and mapped in 1963-1964, 1974 and 1984. Salinity changes over this period, bathymetry substratum composition and optical quality of water can be used to interpret the evolution of the vegetation. During the desalination period the halophytic vegetation, including Potamogeton pectinatus and Ruppia maritima, tended to a freshwater type with Myriophyllum spicatum and Phragmites communis expanding whereas Ruppia maritima disappeared completely. As soon as 1980 as a consequence of the increase in salinity all these species disappeared and two years later they had been replaced by Zostera noltii. Salinity changes indirectly acted on water qualitv especially in its clarity. During the desalination period large amounts of muddy deposit on the substratum were defloculated and maintained in suspension by the turbulent action of very frequent winds (91 and 247 mg.l-1 TSM. yr-1 on average in 1965 and 1978). In 1978, the calculated compensation limit would have reached a depth of 0.6 m. Then the quantity of light transmitted was probably a limiting factor to the development of aquatic plant communities. Increasing salinity favoured flocculation of TSM (in 1984, the day after windy days, 12.8 mg.l-1 on average). The better transmission of the light through the water (with a compensation limit exceeding the depth of the waterbody) would explain a larger extension of the aquatic plant community during the last period. The future of this vegetation is closely connected with the variations in salinity depending in turn on climatic conditions and human activity.

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