Macrophyte patches as biogeochemical hotspots: impact on river water quality?
1. Problem
Macrophytes play an important role in the structural biodiversity in aquatic ecosystems. Being primary producers, they are a matter of life and death for many organisms. Even on a ecosystem level, they take a central role, but the processes here involved are not yet well known. Though, a good knowledge is crucial to be able to take correct management decisions concerning the improvement of our fresh water ecosystems. On top of this, the presence of macrophytes has an even greater influence on the hydraulics. Macrophytes act as ecological engineers and have therefore a direct influence on stream velocity patterns and on sedimentation and erosion patterns. Changes in these patterns have immediate consequences on biodiversity and geomorphology.

2. Objectives
I want to test the main concept of macrophytes being biogeochemical hotspots. After all, there are strong indications that the processes in the sediments underneath macrophyte patches can have greater impact on the water quality than the typically studied pelagic processes. To test this hypothesis, three questions are postulated:
1) Are macrophyte patches biogeochemical hotspots and at what quantity?
2) Which is the maximal length and width a patch can have under certain circumstances?
3) What is theoretically the maximal surface patches can have in a river stretch, given certain circumstances (and what is the total effect of these patches on the water quality, regarding question 1)?

3. Methodology
Question 1) will be answered by gathering field data. The organic material from selected patches will be characterized and processes such as denitrification and silica transformation are followed up. All these data will be merged with patterns of stream velocity and sedimentation and erosion in and around the patches. Afterwards, results are analyzed with a diagenetic model and statistically tested.
Question 2) will be answered by placing in situ flumes around patches in rivers. In these flumes, the patch growth limiting factors such as stream velocity and erosion-sedimentation will be quantified. Additionally, a great number of patches throughout the country will be measured to verify field flume data.
Question 3) will be answered with the Delft-3D model. Data from question 1) will calibrate the model, data from question 2) will validate the model. With this model, I want to estimate the impact of macrophyte patches on the water quality of larger parts of rivers (e.g. 100-1000 m).