Abstract
Scenarios from PBL1 , aiming for 2050, foresee the construction of 12 to 60 GW of marine offshore wind capacity in the Dutch part of the North Sea. Neighbouring countries have comparable plans, possibly cumulating to several hundred GW of offshore wind farm capacity in the southern North Sea. Such massive deployment of offshore renewable wind energy devices may have effects on the wind, wave, current, sediment and water quality properties of the North Sea, which have knock-on effects on the North Sea ecology. Various recent studies point to offshore wind farm effects that transcend local boundaries and may have regional or even system-wide impacts.
Rijkswaterstaat, part of the Dutch ministry of Infrastructure and Environment, in concert with the Dutch ministry of Economic Affairs and Climate Policy, asked Deltares to develop a scoping study to the possible system effects of the large-scale development of offshore wind farms on the southern North Sea. This study adds to the currently implemented research programme (Wozep: the Dutch Governmental Offshore Wind Ecological Programme, 2016-2023) to the effects of offshore wind farms.
Based on available literature and expert judgement, the current report probes these possible regional and system-wide effects with a main focus on the physical and chemical properties of the southern North Sea, i.e. the meteorological conditions, waves and currents, suspended matter and nutrient concentrations, and seabed habitat changes. Based on these impacts, the likely consequences on the primary production, zooplankton and benthos are described. The possible knock-on effects of such changes on higher trophic levels (e.g. fish, birds, marine mammals) are also likely but are not treated in this report.
The following possible effects have been identified and prioritised according to their risks:
- Large-scale development of OWF may lead to (as yet poorly quantified) effects on the wind (and therefore waves) on the North Sea. There likely is a limit to how fast the atmosphere can replenish the energy that the OWF have harvested, either through transport from higher levels or from the area surrounding the OWF (with more OWF, less energy available there).
- The impact of wakes (wind shadows) on wave generation may be significant, and impact may still be present near the coast, e.g. with respect to density driven transport of suspended matter and nutrients in coastal areas directly influenced by riveroutflow.
- Tidal current blockage may have repercussions for tidal dynamics in the southern North Sea.
- Enhanced vertical mixing of the water column may lead to (local/regional and/or temporal) destratification and resuspension of SPM and nutrients and concurrent shifts in light climate.
- Feeding activities from epistructural fauna on the OWF foundations may significantly decrease phytoplankton densities around wind farms affecting in turn zooplankton densities.
- The “stepping-stone” effect of the OWF (increase of spatial distribution of hard-substrate species) may be serious and lead to genetic homogenisation and to the spread of species beyond their natural boundaries.
Directions for first steps to resolving major knowledge gaps are given, consisting of targeted combinations of remote and field measurements, experiments and major modelling exercises.