Macrophytes and nutrient dynamics: process and field studies in the upper reaches of river basins - MANUDYN II. (FASE 1 en Fase 2)

In general, the turbidity of our rivers has significantly decreased since more waste water treatment plants (WWTPs) are operational. The increased light availability enables the germination and subsequently the growth of macrophytes. Huge amounts of biomass develop as concentrations of inorganic nutrients (NH4+, NO2-, NO3- and PO43-) remain high. These are partly originating from intensive agriculture, but also from domestic and industrial activities as a result of our densely populated areas. The presence of macrophytes alters the hydraulics of these rivers in such way that water discharge is hampered and flooding risk increases (Sand-Jensen, 1998; Stephan & Gutknecht, 2002; Trepel et al., 2003). Mowing macrophytes is therefore frequently applied in management strategies to avoid flooding in urban areas.
The Manudyn I project focused on the role of macrophytes in nutrient cycling in the Nete catchment and results clearly indicated that macrophytes do have an impact on the nutrient balance in rivers. Additionally, it was demonstrated that macrophytes also incorporate metals, such as copper, from the sediments, thereby playing an important role as natural sediment decontaminators. However the mechanisms underlying the macrophyte-nutrient interactions are not well understood yet. The uptake mechanisms and nutrient sources (sediment or surface water), allocation inside the plant and the eventual release of nutrients and metals to the water are unclear. Are nutrients and metals mostly trapped inside macrophyte biomass? Or do macrophytes act as a transit compartment pumping nutrients and metals from the sediments and releasing them to the water? And what happens when they decay in late autumn? What is the role of temperature and light intensity on growth and decomposition? Furthermore, the flume experiments in the Manudyn I project revealed that a clear difference exists in nutrient take up behaviour between different macrophyte species.
Therefore, the Manudyn II project will mainly focus on process studies. The objective is to describe and understand the uptake, allocation and possible release of nutrients and metals related to the growth and decomposition of some highly abundant river macrophytes. The results will be used to develop new models describing the processes at different experimental scales and refine models that were developed within the scope of the first Manudyn project.

This will be done by executing several work packages. The first work package tackles small scale experiments, i.e. at the level of a single macrophyte specimen. The second work package deals with experiments at the level of a macrophyte patch and the third one will undertake field experiments with different macrophyte patches.
In the fourth work package, modelling approaches will be applied at the different scales.

One of the aims of the first work package is to find out uptake/decay rates of nutrients for some macrophyte species and filamentous algae as a function of light intensity, temperature and NH4+ and NO3- concentrations. O2 and CO2 production or consumption will be assessed with a photo bioreactor system. These short term experiments will be conducted at different timings along the growth season, based on isotope dilution methods. Growth, decay and concentration of N, P and C in the macrophytes will be studied via long term growth and decomposition experiments, lasting several months, with monitoring of light intensity, temperature, NH4+ and NO3- concentrations and also influence of bacteria and fungi.
Furthermore, the exchange of nutrients between macrophyte roots and sediment interstitial water concerning nutrients will be investigated by separating sediment and roots from the surface water and aboveground biomass. This will be carried out with artificial sediment as well as with real sediment. To resolve the strictly macrophyte-related nutrient and element processing, one must also get insight in the epibenthic exchange between sediment (without macrophytes) and water column which will be done via sediment core experiments. In addition, the interaction between plant, sediment and water will also be tested in situ by means of in situ cores. The former three process studies will be carried out using isotopic techniques.
Some additional core experiments are executed to detect the relationship between copper and several macrophyte species. Copper polluted sediments with macrophytes will be compared with non-polluted sediments with macrophytes and the same will be done without macrophytes. Copper will be followed up in all compartments; the pore water of the sediment, the surface water and in the biomass.

In the second work package, the effect of variable hydrodynamic conditions will be investigated using stable isotope techniques. Using a large flume tank, different stream velocities will be enforced to test nutrient uptake and O2/CO2 production or consumption at different locations within a macrophyte patches with different densities. Secondly, the breaking strength of some macrophyte species will be determined.

In the third work package, nutrient uptake will be investigated in situ in two river stretches containing macrophyte patches (one stretch in the Aa and one in the Semois). Two types of a field flume will be built in each river stretch: one type of flume accelerates the discharge in the stream section and the other type slackens river flow. Isotopic techniques will be applied to detect possible differences in uptake patterns between the two types of flumes and between various locations within the patches.

The modelling work package serves two main goals; a tool for data or experiment analysis and development of dynamic mechanistic macrophyte models, culminating in a flexible management tool to answer questions regarding the role of macrophyte dynamics on water quality in streams.
Models will be developed at different scales, going from macrophyte growing models of different species to models in function of ruling environmental conditions (light, nutrients) via the behaviour in patches to integration at the ecosystem level. The ecosystem model of a stream with macrophytes might serve as a tool to study scenarios for macrophyte removal and its effect on nutrient retention/removal and to estimate the ecosystem functioning of new systems after restoration projects.