The significant changes of hydrological regime were observed during seven years (1992-1998). The changes concerned mainly the number of days when the groundwater and floodwater levels were high and their timing. The first two years were relatively dry and the floods occurred only in spring - at the beginning of growing season. In the next two years the floods started in winter and lasted until the beginning of summer. In addition, during the last three years of the permanent plot was flooded once again - at the end of summer. The changes of moisture condition were followed by changes in species composition of vegetation. The number and cover of annual and biennial species declined. On the contrary, the abundance of perennials, especially of helophytes, gradually increased. Between-year species turnover increased rapidly after the long-term flooding of the permanent plot. Species turnover was subsequently influenced more by disappearance than by immigration of species. The number of species in permanent plot was reduced from 24 to 14. The increasingly brief duration of the terrestrial ecophase led to a change of vegetation within the permanent plot. A meadow sward, closely related to the communities of the alliance Molinion and Potentillion anserinae (class Molinio-Arrhenatheretea), changed, due to exchange of dominants and also of many accessory species, to marsh vegetation syntaxonomically close to the association Glycerietum maximae of the alliance Phragmition (class Phragmiti-Magnocaricetea).