For rivers, in general, it has become increasingly clear that inputs of nutrients via the headwaters exceed outputs to the estuary and the adjacent coastal sea, indicating retention and transformation during transport within the river basin. Previous research in the Scheldt system focused mainly on biogeochemical processes in the estuarine environment and much less so on the upper catchments of the basin, where enhanced production of macrophytes occurs. The huge production of biomass can cause water levels to rise, 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.
- Project description
The aim of this project is to understand:
- the factors controlling the appearance and development of macrophytes within a river basin;
- the impact of large macrophyte biomass on the carbon, nitrogen and phosphorus budget within the affected catchments;
- the effect of macrophytes on the nutrient export to the downstream zones;
- how macrophyte covered areas should be managed optimally, taking into account criteria such as flow capacity, amenity value, nutrient retention, biodiversity and ecological quality criteria.
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 utilisation 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.
- Interaction between the different partners
The UA will focus on the distribution of macrophytes in the river basin in relation to environmental factors. This information is needed for the development of models, which will be done by the ULB, and for the evaluation of various management options (UA, ULB). The three partners (UA, ULB and VUB) will try to estimate the effect of macrophytes on the carbon and nitrogen budgets in three ways. One of the ways is done by the VUB, the others are a combined effort. The results will be used also for the model development by the (ULB).
- Link with international programmes
- European Water Framework Directive.
- OMES: Research on the environmental consequences of the "Sigmaplan".
- SedNet: demand driven European Sediment Research Network (EU-5FP).
- EUROTROPH: Nutrients cycling and the trophic status of coastal ecosystems (EU-5FP).
- Expected results and/or products
The expected results will provide an improved insight into the nutrient dynamics of river systems and the ecology of macrophytes. Both the increased understanding of the system and the associated modeling exercise will be essential for incorporating the macrophyte problematic in the scope of integrated water management. The integration of the results into a model will make it possible to simulate the impact of different management options. Although the model will be in first instance a research tool, its results will be important to disseminate the knowledge gained towards all interested end-users. This will be done at a special workshop during which our research will be presented and where particular emphasis will be put result dissemination towards the management practice.
A leaflet will be produced describing the role of macrophytes, the possible impact of weed cutting on nutrient dynamics and the various management strategies that can be elaborated to optimize nutrient removal with minimal impact on the aquatic ecosystem. This leaflet will be distributed to all potentially interested water managers.