Effect of macrophytes on C-N-P-Si fluxes - an integrated model approach.

Water quality and ecological integrity in estuaries of large rivers are determined by the input of organic matter and nutrients of these rivers. Organic matter is derived by dead aquatic vegetation and algae or imported from terrestrial sources like the fall of leaves or sewage water. In rivers organic matter is decomposed stepwise depending on environmental conditions. As well nutrients are degraded or assimilated by primary producers. Aquatic vegetation is crucial for these processes, because of their influence on matter transport and transformation. Nevertheless these effects have not fully been studied up to now. In the present study a mathematical model will be developed to investigate the complex relationships and to determine the impact of aquatic vegetation on the cycling of matter. Main hypothesis of the project is that aquatic vegetation decreases water flow and creates areas with increased transformation rates. Hence, the changed ratio between transport and transformation processes will affect local and catchment budgets of carbon and nutrients. To test the hypothesis, an integrated model will be developed gradually. Based on the hydrological flow conditions we will simulate simple biogeochemical relations, like the assimilation of nutrients by algae or the decomposition of organic matter in the water body first. After that a growth model for aquatic vegetation will be built-up and added to the model. Finally, degradation processes in the river sediment will be included. The model set-up will be complemented by water sampling and analyses every 2-4 weeks, the investigation of aquatic vegetation development, and the composition of the river sediments. Additionally, experimental approaches to determine growth rates of aquatic vegetation and decomposition rates of organic matter are planned. Thereby the knowledge about the role of aquatic vegetation in the cycling of carbon and nutrients of rivers can be significantly increased. Next to the simulation of the biogeochemical conditions with and without aquatic vegetation, the newly developed model enables us to simulate future environmental developments due to changed temperature or flow conditions and their impacts on matter cycling. The project will be conducted for a sub-catchment of the River Danube, where river sections with different aquatic vegetation densities are available. The set-up of an integrated model also combines the expertise of the two working groups submitting the project - WasserCluster Lunz, studying the effects of hydrological connectivity on carbon and nutrient cycling during the last decade and ECOBE (University of Antwerp) where the manifold roles of aquatic vegetation in riverine systems are explored - and enables its implementation.

In the 4-year project "Effect of macrophytes on C-N-P-Si fluxes - an integrated model approach" a team of scientists from the University of Antwerp, the University of Natural Resources and Life Sciences, Vienna and WasserCluster Lunz investigated the role of macrophytes, water plants, in river systems and potential effects of climate change on water plants. While water plants were intensely investigated in standing waters and shorelines, their role in running waters is less known. The key question was how the water plant development controls the river metabolism. The effects of changing environmental conditions and the factors related to climate change (changes in dissolved compounds, CO2 concentration and flow) might impact the development as well. Field investigations, laboratory experiments and a modelling approach was applied to investigate this question. As model plant Berula erecta was used and the main investigations took place in the River Fischa in Austria, a groundwater-fed lowland river. Water plants in the investigated river control the uptake of nutrients such as phosphorus, provide a major part of the oxygen produced and change environmental conditions by reducing flow and promoting fine sediment accumulation. These indirect effects facilitated microbial processes, such as nitrogen transformation by microbes, and had a positive effect on bacterial biomass in water. Thus, water plants are important components in lowland rivers controlling the gas balance and nutrient cycling. Based on flume experiments, we could show that several water plant parameters such as leaf area, growth form and nutrient content were affected by changes in concentrations of dissolved organic carbon, CO2 concentrations and changes in flow patterns. In the climate change scenarios, biomass and relative growth of the water plant B. erecta increased, while growth form changed especially in the climate change scenario with increased flow velocities. In summary, water plants are an important ecological component, also in lowland rivers, impacting metabolism and nutrient dynamics, and they are sensitive to climate change effects.