Recently, the implementation of the European Water Framework dDirective resulted in restored water transparencies of small rivers that restored the germination and growth capcities of macrophytes. However huge amounts of biomass develop as concentrations of nutrients (NH4, NO3- and PO4 3-) from agricultural and domestic origin remain high in densely pomulated watersheds. As water discharge is hampered and flooding risks increas, mowing of macrophytes is frequently applied in management strategies, However little is known about the ecological consequences this mowin has on the nutrient transfer to downstream ecosystems. Within this large framework, the objective of this project are to describe and understand the uptake, allocation and possible release of N-nutrients during the growth of some highly abundant river macrophytes. The results will be integrated in a general project aiming at the development of new models describing the growth of macrophytes in upstream catchments of European rivers (MANUDYN II project). At first, small scale in-vitro experiments, i.e. at the level of a single macrophyte specimen will be performed using 15N tracers to determine the uptake-release rates of N for typical macrophyte species as a function of light intensity, temperature and NH4+ and NO3- concentrations. Furthermore, experiments at the level of a macrophyte patch will be performed using flume incubations in order to study the interaction between plant nutrient uptake and flow velocity.