Abstract :
[en] This PhD thesis is devoted to the study of the environmental impact of artificial marine structures, such as offshore wind farms (OWF), on the bottom deposition of organic and inorganic matter. This study aims to connect the local and regional scales using numerical modeling. Living on the foundations of turbines, filter feeders (like the blue mussel) filter suspended particulate matter (SPM) from the water column to produce fecal pellets, rich in carbon, that sink to the bottom. Thus, through biodeposition, fouling organisms living on wind turbines are able to modify the distribution of the carbon flux to the bottom, and the distribution of organic matter and the oxygen consumption in the sediment. Quantification of local effects at the scale of the Southern Bight of the North Sea (SBNS) is necessary within the framework of European strategies of the Blue growth and protection of biodiversity.
A hydrodynamic model of the SBNS coupled with a sediment and wave model (COAWST) has been developed. This model has been extended with a model representing the filtration and biodeposition by fouling filter feeders. The model covers the entire SBNS with a coarse resolution (CR) of 5 km while the Belgian coastal zone (BCZ) is covered by a high resolution (HR) model of 1 km. The HR model is forced at the borders by the CR model, while the results of the latter at the level of the BCZ are updated with the results of the HR model. Model simulations of hydrodynamic variables including tides and waves and sediments have been validated against existing data for the region. The impact of variability of atmospheric conditions and tidal regimes on variability of hydrodynamics has been analyzed. This validation effort has shown that the HR model has better performance for the simulation of residual currents, the extension of the river plume and the variability in response to atmospheric forcings and tidal regimes. However, the CR model has better performance for the simulation of overtidal constituents.
Regarding the impact of the biodeposition by fouling organisms on the flux of sedimentary particles, it has been shown that the impact of fouling fauna extends beyond the boundaries of the OWFs with an increase in the flux of sedimentary carbon (up to 50%) near (< 5 km) turbines and a slight decrease up to 30 km away. The model has then been used to provide recommendations on the design of a new OWF planned in a new concession area (NCA), located near a site of the Natura 2000 network according to the Marine Spatial Plan 2020. In particular, the impact of the number and position of turbines on the carbon flux to the sediment has been simulated. Placing the NCA turbine foundations over the protected gravel beds of the Natura 2000 network would result in a drastic increase (> 50%) in carbon deposition on them; while if turbines were placed at least 2 km away, the increase in carbon deposition would only be about 10%. The number of turbines plays a marginal role in altering the biodeposition flux.
