Offshore Wind Farm Footprint on Organic and Mineral Particle Flux to the Bottom

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.

Offshore Windfarm Footprint of Sediment Organic Matter Mineralization Processes

“…The response of the sediment metabolism to alterations in carbon deposition induced by OWFs was represented by a dynamic model of early diagenesis (OMEXDIA, Figure 2; Soetaert et al, 1996a,b). The diagenetic model was forced by the carbon deposition estimated by Ivanov et al (2021). This carbon deposition was estimated under different scenarios of OWF development (and without, see section "Study Area and Scenarios"), by implementing a representation of the filtration and feces production processes associated to monopile fouling 2021) outlined in purple, and future developments (operational ± 2026) in orange, with letters corresponding to concession zone characteristics in Table 3.…”

“…The COAWST model was solved with a horizontal resolution of 5 km, downscaled to 1 km over the BPNS using a two-way nesting procedure (Ivanov et al, 2020). The ROMS module was configured and validated for the modeling domain by Ivanov et al (2020), whereas implementation and validation and the CCSTM and SWAN modules were performed by Ivanov et al (2021). For clarity, fundamentals of the CCSTM and SWAN modules are noted below, while for details we refer to Ivanov et al, in press.…”

“…Filtration of OM by the blue mussel M. edulis on turbine foundations was implemented in grid cells containing OWFs as a conversion of silt, and the two classes of OC (FDOC, SDOC), to fecal silt, fecal fast, and fecal slow degrading matter (Ivanov et al, 2021). These fecal substances have exactly the same characteristics as the "non-fecal" forms, except for the sinking speed, which is 1,000 times higher (18 mm s −1 ) to account for the increased size and mass of the fecal pellets (Callier et al, 2006).…”

“…In this research, we model the potential effects of increased carbon deposition related to filtration by fouling fauna in OWFs on benthic mineralization processes. To do so, we used output generated by Ivanov et al (2021) who implemented a model describing filtration by blue mussels on turbine substructures in a hydrodynamic model of the Southern North Sea, including the full BPNS and parts of the bordering French and Dutch coastal zones (Figure 1; Ivanov et al, 2020). In biomass, M. edulis is by far the dominant species present on turbine foundations (Joschko et al, 2008;Krone et al, 2013).…”

“…In biomass, M. edulis is by far the dominant species present on turbine foundations (Joschko et al, 2008;Krone et al, 2013). Whereas alterations to sediment and OM transport to sediments from the water column under different scenarios of turbine placement in a new concession area are evaluated in an accompanying paper (Ivanov et al, 2021), we used the OM deposition fluxes as input for a model of sediment biogeochemistry (OMEXDIA, Soetaert et al, 1996a), to investigate potential changes to mineralization pathways in the sediment. This was done for three cases: the first containing the current OWFs present in the model domain, the second a likely future scenario starting form 2026 when additional OWFs have been constructed and fully colonized in planned concession zones in the BPNS, and the Dunkirk OWF in the French coastal zone (Figure 1), and a third case containing no OWFs as a point of reference.…”