Abstract
Offshore wind farms (OWFs) are an important source of renewable energy accounting for 2.3% of the European Union's electricity demand. Yet their impact on the environment needs to be assessed. Here, we couple a hydrodynamic (including tides and waves) and sediment transport model with a description of the organic carbon and mineral particle dynamics in the water column and sediments. The model is applied to the Belgian Coastal Zone (BCZ) where OWFs currently occupy 7% of its surface area which is estimated to double in the next 5 years. The impact of OWFs on the environment is represented through the filtration of the water column and fecal pellets production by the blue mussel, the dominant fouling organism. Our model simulations show that the impact of biodeposition on the mud particle sedimentation and on sediment composition is small compared to the fluxes associated with tidal deposition and resuspension and the lateral inputs. In contrast, the total organic carbon (TOC) flux to the sediment is significantly altered inside the OWF perimeters and TOC deposition is increased up to 50% in an area 5 km around the monopiles. Further away, the TOC flux to the bottom decreases with a notable effect up to 30 km away. The major changes are found along the direction of the main residual current and tidal ellipse's major axis. In addition, sub-mesoscale gyres act as retention areas with increased carbon deposition. A future OWF in the BCZ will be located close to gravel beds in a Natura 2000 area, considered as vulnerable habitats and biodiversity hotspots. The different scenarios for this OWF, varying in turbine number and positioning, are compared in terms of impact on the carbon and mineral particle deposition flux in the BCZ and, particularly, to these gravel beds. The scenarios show that the number of turbines has only a slight impact on the TOC deposition flux, unlike their positioning that significantly alters the TOC flux to the gravel beds. The TOC deposition flux exceeds 50%, when the turbines are placed next to the gravel beds; while a limited increase is simulated, when the turbines are located the farthest possible from them.
Highlights
The European Union (EU) heavily invests in the development of renewable energy, and offshore wind farms (OWFs) are an important contributor
These organisms feed on the suspended particulate matter (SPM) in the water column, which they partially expel in the form of fecal pellets, further referred as FPs (Callier et al, 2006)
Areas of statistically significant changes of the simulated annual average of the total organic carbon (TOC) deposition fluxes are identified using a t-test modified for auto-correlated time series
Summary
The European Union (EU) heavily invests in the development of renewable energy, and offshore wind farms (OWFs) are an important contributor. The underwater parts of turbine foundations provide a substrate for fouling species, with the most abundant in the North Sea being the blue mussel Mytilus edulis, the amphipod Jassa herdmani and the plumose anemone Metridium senile (Krone et al, 2013; De Mesel et al, 2015) These organisms feed on the suspended particulate matter (SPM) in the water column, which they partially expel in the form of fecal pellets, further referred as FPs (Callier et al, 2006). Remote sensing (Vanhellemont and Ruddick, 2014) and in-situ measurements (Baeye and Fettweis, 2015) have shown SPM plumes over a kilometer in length in the wake of the turbines Their exact origin is unknown, possibly it is a combination of biodeposited and resuspended materials with dynamically trapped SPM (Forster, 2018), due to the splitting of the current around the foundation and reconvergence downstream, (i.e., the island effect). These plumes show that the impact of OWFs is not local and may affect a much larger area than just the OWF perimeter
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