Offshore Windfarm Footprint of Sediment Organic Matter Mineralization Processes

Abstract

Offshore windfarms (OWFs) offer part of the solution for the energy transition which is urgently needed to mitigate effects of climate change. Marine life has rapidly exploited the new habitat offered by windfarm structures, resulting in increased opportunities for filter- and suspension feeding organisms. In this study, we investigated the effects of organic matter (OM) deposition in the form of fecal pellets expelled by filtering epifauna in OWFs, on mineralization processes in the sediment. OM deposition fluxes produced in a 3D hydrodynamic model of the Southern Bight of the North Sea were used as input in a model of early diagenesis. Two scenarios of OWF development in the Belgian Part of the North Sea (BPNS) and its surrounding waters were calculated and compared to a no-OWF baseline simulation. The first including constructed OWFs as of 2021, the second containing additional planned OWFs by 2026. Our results show increased total mineralization rates within OWFs (27–30%) in correspondence with increased deposition of reactive organic carbon (OC) encapsulated in the OM. This leads to a buildup of OC in the upper sediment layers (increase by ∼10%) and an increase of anoxic mineralization processes. Similarly, denitrification rates within the OWFs increased, depending on the scenario, by 2–3%. Effects were not limited to the OWF itself: clear changes were noticed in sediments outside of the OWFs, which were mostly opposite to the “within-OWF” effects. This contrast generated relatively small changes when averaging values over the full modeling domain, however, certain changes, such as for example the increased storage of OC in sediments, may be of significant value for national / regional carbon management inventories. Our results add to expectations of ecosystem-wide effects of windfarms in the marine environments, which need to be researched further given the rapid rate of expansion of OWFs.

Sedimentary carbon on the continental shelf: Emerging capabilities and research priorities for Blue Carbon

“…Physical disturbance of the seabed which resuspends near-surface sediments into the oxic water column will recycle labile organic carbon into the ocean and potentially impact the atmospheric carbon system (Sala et al, 2021), although it can also simply re-distribute and redeposit sediments (Rijnsdorp et al, 2021). Seabed disturbance impacts the benthic faunal community, which plays a key role in the biogeochemical processes determining what fraction of deposited carbon is recycled (remineralized) or buried (stored) (de Borger et al, 2021a;de Borger et al, 2021b).…”

“…Bottom trawling is the most spatially significant anthropogenic process physically disturbing shelf sediments (Oberle, et al, 2016b;Rijnsdorp et al, 2021), while wind farms have been shown to locally increase organic matter deposition (and recycling) because of faunal colonization of their structure (de Borger et al, 2021a). Other activities, including pipe and cable laying, aggregate extraction, and dredge disposal, are also expected to impact organic matter storage and deposition, although their impacts are more spatially restricted.…”

A first estimate of the effect of offshore wind farms on sedimentary organic carbon stocks in the Southern North Sea

“…This new epifaunal colonization leads to a significant transfer of organic carbon (OC) in the form of fecal pellets and biomass sinking from the structures onto the sediments surrounding the wind turbines (Krone et al, 2013;De Borger et al, 2021;Mavraki et al, 2022). Using a 3D hydrodynamic model developed by Ivanov et al (2021); De Borger et al (2021) found that this leads to a 10-11% increase in OC accumulation in the upper 10 cm of the sediment and an increase in anoxic mineralization processes in the vicinity of the OWFs. As a result, the OWFs installed on the sediments of the Belgian Part of the North Sea became local carbon sinks compared to the baseline scenario (areas without OWFs) and have increased the total amount of OC in the upper 10 cm of the sediment by 28,715 tons (De Borger et al, 2021).…”

Artificial hard-substrate colonisation in the offshore Hywind Scotland Pilot Park

“…The effects on local benthic habitats during installation works and operations of offshore wind farms (OWFs) are of a complex nature and extend both below and above the surface of the sea. Previous studies have shown that OWFs can impact areas through the introduction and spread of alien species (De Mesel et al, 2015;Wilhelmsson and Malm, 2008), affect organic matter deposition (De Borger et al, 2021) and carbon assimilation (Mavraki et al, 2020), and alter community structures (Coates et al, 2014;Hutchison et al, 2020;Wilhelmsson and Malm, 2008) through the loss of soft-sediment habitats and the subsequent introduction of artificial hard-bottom substrates. The newly created habitat is usually larger than the lost habitat (Wilson and Elliott, 2009).…”