Detailed botanical surveys were carried out to monitor the population size of Apium repens in the Vörös-mocsár nature protection area (located in S Hungary, Császártöltés). We found a clear correlation between the water status and the land use type and management of the habitats and the size of the Apium repens population. Apium repens is a Nanocyperion-species, which simultaneously requires wet and open surfaces. Its population shrinks, becomes latent or disappears if its habitat dries out or any other radical transformation happens, like too much leaf litter, closure of the vegetation, or succession on the open peat surfaces. Proper management (water retention, extensive grazing, mowing, open peat surface) has a very positive effect on the populations in a short time, even within a growing season. The research was implemented within the framework of the National Biodiversity Monitoring System, in cooperation with the Kiskunság National Park Directorate.