For rivers, in general, it has become increasingly clear that inputs via the headwaters exceed outputs to the estuary and the adjacent coastal sea. For nutrients, there is strong evidence for retention and transformation during transport from the small headwaters to the coastal sea. For the Scheldt system, previous research has focused mainly on biogeochemical processes in the estuarine environment and much less so on the upper catchments of the basin. A specific recent feature in the upper catchments of the Scheldt basin, especially in the Nete basin, is the enhanced development of macrophytes during summer, which is apparently linked with improvement of the water quality. The huge production of biomass can cause water levels to rise, even to the brink of flooding, but on the other hand it can act as a biological filter with respect to dissolved and particulate matter. The other important group of primary producers, phytoplankton, usually dominates in zones with different characteristics compared to those where macrophytes thrive. Both groups can attain quantitatively equivalent and important yearly productions. It is the aim of this project to understand and quantify the factors controlling the appearance and development of macrophytes and to quantify the effect of macrophytes on the nutrient export to the downstream zones.The questions will be tackled by (1) a detailed macrophyte inventory within the Nete river basin and the analysis of physical and chemical controlling factors, leading to the development of a predictive model of macrophyte occurrence; (2) a set of mass balance experiments and of process studies, performed on an appropriate river reach situated along the Aa river (a tributary of the Kleine Nete river) or in the laboratory. The temporal evolution of growth and biomass of macrophyte communities will be determined over the full growth season. Flume experiments under controlled conditions will focus on nutrient retention by macrophytes and nutrient delivery by the sediment, to assess influence of e.g. stream velocity and macrophyte density. These studies will be complemented with nutrient mass balance studies in the field, for situations with and without macrophyte presence. The decomposition process of macrophytes will be followed and the contribution of macrophyte detritus to the suspended organic matter pool followed. Further information on nutrient utilization during synthesis of and on the subsequent fate of the organic matter will be followed via stable isotope composition of the essential inorganic and organic substrates.Results of the project will be put together to develop a 1D transport-reaction model of the experimental Aa river reach in order to verify the coherence of the various approaches and to test the validity of the process formulations and rates. In parallel, an available modeling package will be set-up for the Kleine Nete river system and used as a predictive tool to assess the effect of various management options on the nutrient retention within the system and on nutrient export to the downstream zones. Besides the construction of predictive instruments for the quantitative assessment of nutrient fluxes the project results will also support decision making for basin management, especially concerning macrophyte development, based on criteria such as flood protection, amenity value and biodiversity preservation.