Vegetation ecology of flood plain and tidal marsh vegetation and flora One of the major questions within the OMES project is whether tidal marshes have a significant impact on the fluxes of carbon and nitrogen in the estuarine ecosystem. Since ca. 7 % of the brackish part and ca. 22 % of the fresh water part of the Zeeschelde-estuary is occupied by tidal marshes the expectation is justified that important interchanges of material between the upper intertidal area (tidal marshes) and the lower intertidal (mudflats) and pelagic part of the ecosystem take place. Above that the percentage covered by interidal area can easily be enlarge to ca. 30 % when controlled inundation areas are incorporated in the estuary by more frequent flooding or by introduction of reduced controled tide. Major hydraulic changes within this enlarged ecosystem can be expected to have a major impact on the performance of the vegetation and, on the other hand, vegetation changes might effect interchange characteristics with the estuary. Therefore a thorough vegetation analysis of tidal marshes and flood plains is needed and the vegetation determining environmental parameters have to be detected. Since flood plain vegetation and tidal marsh vegetation are very distinct and determined by different environmental factors, two more or less separate studies were carried out. Based on several hundreds of relevÈs vegetation was classified in different phytocoena for both environments. All relevÈ quadrats were made permanent to allow time series of vegetation succession in the future. Environmental parameters measured or estimated for all relevÈs were the type of management, general moisture characteristics and general vegetation features. For all tidal relevÈs flooding dynamics were calculated (flooding frequency, height and duration). Based on the vegetation analysis (23 fytocoena in the flood plains, 16 on the fresh water and brackish tidal marshes) a selection of ca. 100 relevÈs was further investigated for a whole range of chemical and physical soil and hydrological characters. Analysis of both vegetation and environmental data allowed us to separate between relevant and irrelevant vegetation determining factors. Among others salinity, flooding dynamics and groundwater characteristics are most important for tidal marsh vegetation, management, nutrient availibility, presence of peat in the soil, ground water quality and fluctuation rates are most important for flood plain vegetation. Final aim is to develop a statistical model in which the response variables are vegetation type, species presence and/or species performance and the explanatory variables are the above mentioned relevant environmental factors. General techniques used for modelling are multiple regression and/or multiple logistic regression, with which the chance of presence of a species or vegetation type is estimated. Auto-ecological research is focused on two key-stone species of the intertidal zone, i.e. Schoenoplectus tabernaemontani and Phragmites australis. Above and below ground biomass production, leaf area and seasonal C-, N- and P-translocation within the plant are studied. A third key-stone species, Salix spp., will be incorporated within the auto-ecological research within soon. The former Institute for Nature Conservation was a partner in this project