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Mussel beds are important biogenic structures in the intertidal and subtidal environment of the (Dutch) Wadden Sea. Mussel beds are partly able to stabilize the substrate of the Wadden Sea, to improve the water quality, to establish important habitats for many (benthic) species and to provide food for water birds and other organisms. Mussel beds also represent characteristic and scenic landscape units in an area of outstanding natural beauty: since June 2009, the Wadden Sea has been recognized as an important new UNESCO World Heritage Site.
In the Wadden Sea though, the area covered by stable mussel beds demonstrates a large variability from year to year and from location to location, in particular in the Western Wadden Sea. One can only speculate about the reasons and, so far different explanations have been offered. In general the stability of mussel beds is determined by a range of physical and ecological processes and conditions. Table 1 gives a summary of the main (a)biotic factors and conditions relevant for the stability of intertidal mussel beds. The interdisplinary research programme MOSSELWAD will be testing a number of hypothesis for the (lack of) stability of mussel beds and will consider aspects such as coverage, density and predation. The coastal research group of Utrecht University will focus on the physical/hydrodynamical processes and conditions that can play a decisive role in the development of stable mussel beds.
The main objective of the present study is to determine the impact of hydrodynamic processes in the Wadden Sea on the stability of mussel beds. In particular the study is initiated to:
- Determine the effect of waves, in combination with wind and tides in eroding mussel beds;
- Establish the hydrodynamic boundary conditions for the settlement and stable development of mussel beds;
- Evaluate the effect of mussel bed patterns in relation to stability;
- Understand the musselbed – sediment interactions, both in terms of erosion (resuspension of suspended matter) and biodeposition.
The research approach is based on a number of activities. First of all three major musselbeds will be selected in the Wadden Sea and each site will be instrumented with a large pole (top about 10 m above the surrounding tidal shoals and flats of the Wadden Sea) to make video-based observations of the mussel bed morphology (hourly and daily basis). The video data will also be used to observe the local hydrodynamic conditions at the measuring plots; this includes wave propagation (direction) and breaking (percentage of breaking waves, type of breakers) during more energetic conditions. At each site an Acoustic Doppler Velocity (ADV) meter is deployed to measure time-averaged and orbital velocities. Local, stand-alone pressure sensors are added to measure tidal water levels and wave characteristics.
At one more stable mussel bed, more detailed measurements will be performed to determine detailed flow and turbulence patterns, to measure wave propagation over mussel beds, to record the suspended load sediment transport and to measure detailed bed level changes due to erosion and deposition.
In the final phase of the project a third generation wave model (SWAN) will be used to explore the potential areas for the development of stable mussel beds in the Western Wadden Sea.
The project started in 2009 and the first year will be used to establish the infrastructure required to carry out the field measurements and observations.
Please contact Jasper Donker for more info on this project.