Current knowledge and key gaps in understanding of offshore wind farm impacts on the physical marine environment

Abstract Offshore wind energy production is on the rise globally, projected to occupy significant areas in shallow shelf seas and moving into deeper waters as floating turbine technology is becoming more mature. However, knowledge about the potential impact of wind farms on the physical oceanography and lower trophic organisms is still severely limited. In this review, we assess the current state of knowledge on the effects and impacts of offshore wind farms on regional and local hydrography and circulation, nutrient distribution, phytoplankton and primary production, and sediment load in the water column during the operational phase of the wind farms and identify critical knowledge gaps. The body of literature on the topic has grown rapidly over the last years, but most studies focus on wind farms in relatively shallow (<60 m water depth) and mainly unstratified or seasonally stratified shelf seas, predominantly on the northern European shelf and around China. In situ observations are scarce, leading to heavy reliance on numerical models. As floating wind farms have become operational only very recently, few studies focus directly on their specific impacts. There is general understanding of local impacts on ocean physics, e.g. on turbulence, mixing and stratification due to flow past turbine foundations, or the potential of wind wake impacts on surface currents, up- and downwelling. Consequences for phytoplankton and primary production are much less clear and both physical and biogeochemical impacts on regional scales remain uncertain. There is a critical need for observational data for validation and targeted impact studies. Particularly characteristics and temporal and spatial scales of circulation and hydrographic changes and their effects and impacts on primary producers, vertical flux, and pelagic-benthic coupling are little understood especially in stratified and deep shelf regions. Given the rapidly accelerating growth of the offshore wind farm industry and expansion into deeper seas using floating technology, addressing these knowledge gaps is crucial for reliable environmental impact assessments and sustainable development of this still relatively new energy sector.