Harmony at sea: Nurturing marine ecosystems alongside sustainable offshore wind energy

The critical role of offshore wind energy in achieving net-zero climate targets is widely recognized. Across the globe, electricity production from offshore wind farms has surged, driven by urgency to mitigate climate change, technological advancements, favorable government policies, and increased market investment. According to BloombergNEF, as of the end of 2023, 73 gigawatts (GW) of offshore wind capacity had been installed worldwide, with Europe contributing 32GW to this total. BloombergNEF’s Q1 2024 forecast expects a tripling in offshore wind operational capacity by the end of this decade, reaching 256GW by 2030 and 742GW by 2040.

Increasing offshore wind installation is essential for the energy transition. However, the scale and pace of the planned projects raises questions about their impacts on marine ecosystems. Offshore wind farms take up space on the seabed, on the water’s surface, and in the air – all natural habitats for wildlife. The installation, operation, maintenance, and decommissioning of offshore wind farms can impact different parts of the marine ecosystem. Offshore wind energy development could influence marine habitats, modifying population of marine life and altering natural marine processes and, in turn, affecting the provision of ecosystem services. This may entail both positive and negative impacts.

On the positive side, offshore wind energy can play a significant role in protecting marine ecosystems by contributing to the reduction of carbon dioxide emissions and mitigating the harmful effects of global warming. The increasing frequency and intensity of marine heat waves, leading to extensive coral bleaching and reef degradation, exemplify the catastrophic impact of global warming on marine life. Coral reefs are extraordinary hotspots of marine biodiversity, and their degradation results in significant loss of marine life. Beyond their global benefits, offshore wind farms may also support marine life locally by serving as artificial reefs, providing spawning grounds, and acting as feeding stations that attract diverse marine species. The reef effect can lead to the clustering of marine life around the sturdy structures of offshore wind farms, boosting the population of certain species and improving food availability in and around the turbines. This, in turn, may attract birds and other species that rely on this food source. Moreover, wind turbines can also provide resting spots for birds.

On the other hand, offshore wind projects may have unintended effects on marine ecosystems, which are detailed in the subsequent sections. The ecological risks associated with the negative impacts of offshore wind farms can differ based on geographical factors. These risks are contingent upon the environmental characteristics and vulnerability of the impacted areas, making the identification of potential impacts case-specific. To evaluate impacts of offshore wind farms on marine life, it’s crucial to have sufficient baseline information on marine wildlife distribution, abundance, and trends within potentially effect areas. Long-term monitoring programs are also essential to address uncertainties about the magnitude and extent of the long-term impacts of offshore wind projects.

The installation phase could potentially impact a range of species, such as seabirds, marine mammals, sea turtles, and various fish species, from bottom-dwelling to open-water species. The vessels used to install offshore wind farms could pose a risk of collision with marine fauna including harbor porpoises and turtles, potentially causing injuries or fatalities. The presence of these vessels at wind farm sites could disturb marine habitats, and the noise pollution produced during operation may displace marine life, including seabirds, from preferred feeding grounds, which could lead to a decrease in local food availability. Regulations and restrictions on vessel noise, speed, and routing can help mitigate these risks, particularly in sensitive areas or during specific periods, such as near breeding colonies of vulnerable species. Other underwater sounds produced during installation processes, especially pile driving, drilling, and dredging, also present potential risks to marine species. These activities can displace animals from natural foraging and breeding grounds and cause temporary hearing loss and disorientation, particularly in marine mammals and predatory fish.

A variety of technologies, including small and large bubble curtains, hydro sound dampers, and noise mitigation screens, are deployed to mitigate such impacts during installation. In addition, techniques like the soft start of pile driving and the use of pingers, which send acoustic warning signals to marine mammals and fish, have become standard practices. It is encouraging to observe that, alongside existing technologies, ongoing research and innovation strive to further reduce noise disturbances. More transparent and stringent regulatory requirements could provide greater incentives for further development of advanced techniques to enhance the protection of marine life.

Beyond noise pollution, the construction process, such as cable laying, could alter and disturb the natural habitat conditions. These alterations could impact benthic species – those living on, above, or below the seafloor – the most. Using construction methods that lessen turbidity and sediment suspension and selecting the optimal route for cable installation could mitigate the environmental risks.

Offshore wind farms may attract or repel marine species during operation. Large seabirds like guillemots, for instance, tend to exhibit strong avoidance behavior toward these structures, and some birds, especially gannets, have shown increased avoidance during their breeding season. Birds’ avoidance behaviors could restrict their foraging zones, potentially affecting species’ breeding success, especially within wind farm areas. For bats and especially birds, there is also risk of collisions and fatalities with turbines. This risk depends on different factors, including the flight height of different species, the duration of their flight over water, their activities during nocturnal flight, and the rotor height and sweep zone of the turbines. Measuring mortality rates of avian species from collision with offshore wind turbines is challenging due to the difficulty of locating carcasses at sea. However, some research, such as one study conducted by Vattenfall, indicates that the risk of birds colliding with offshore wind structures during daylight is minimal, highlighting the birds’ ability to perform effective micro-avoidance maneuvers. There is still room for further research to identify the actual collision risk under different conditions.

A notable example of ongoing efforts to identify, monitor, and mitigate impacts is the Dutch offshore wind market. Driven by regulatory obligations in tender requirements, developers who win Dutch offshore wind auctions are investing in various pertinent projects. Measures used to further minimize the risk of bird-turbine collisions include the temporary shutdown or curtailment of turbine operations during mass migration events or specific weather conditions. Developers are also improving turbine visibility and using deterrent devices. Night lighting on offshore wind farm structures can also attract bats and migrating land birds, particularly during adverse weather, leading to physical harm or death. Other practices include reducing illumination levels, adjusting the color spectrum of lighting, using deflectors, and avoiding unnecessary lighting to reduce the risk of nocturnal collisions.

Offshore wind farms are becoming one of the constant sources of low-frequency noise underwater. Noise produced by operating turbines could potentially disturb marine mammals, turtles, and other aquatic organisms. Consequences of the underwater noise pollution could include wildlife displacement, temporary suffering from sound masking, and hearing loss. Sound masking occurs when human-generated noises interfere with marine animals’ ability to perceive other critical natural sounds. It is reported that the underwater noise emitted by individual wind turbines (up to approximately 7MW) is relatively low compared to the noise from cargo ships. However, wind turbines are distinct in that they are static sources, thereby contributing more consistently to the local soundscape. There is also research indicating minimal or negligible negative effects on mammals from the low-intensity and low-frequency noises emitted by smaller offshore wind turbines (1.4MW), but the turbines that are now being installed are 10 times bigger than those included in the study. It is also important to recognize that wind turbines are not installed individually but as part of large wind farms. Therefore, further research is needed to understand whether noise from larger turbines and farms could have different impacts on underwater species, particularly in regions with low background noise.

Apart from the turbines’ potential impacts, offshore grid components such as submarine cables that emit electromagnetic fields (EMFs) into the marine environment could also have potential impacts on marine ecosystems. The EMFs emitted from export cables are higher than those from inter-array cables and the wind turbines themselves. Submarine cables do not generally constitute a physical barrier to the movement and migration of marine species, but they can influence the navigational abilities and hunting behaviors of species such as elasmobranchs (rays, skates, sharks), which rely on electromagnetic-receptive sensory systems for orientation. Subsea cables could also intersect with egg laying, mating, and nursery sites, especially for benthic elasmobranchs. Overall, uncertainties remain regarding the impacts of subsea power cables on the various life stages of marine species, highlighting the need for further research to bridge the knowledge gap.

The traffic of service and maintenance vessels could increase the risk of collisions, particularly with marine mammals and sea turtles. As in the construction phase, the application of vessel regulations and restrictions on factors like noise, speed, and routing in sensitive habitats, especially during crucial life stages of at-risk marine species, can help reduce the risk of collisions. Throughout their operational lifespan, it is also possible – but uncommon – for damaged parts of offshore wind turbines to fall into the sea. Apart from the complexity and expenses of retrieving such fallen parts, fallen pieces of turbines could also pose safety concerns and disturb marine life. Even during planned maintenance, turbine parts might accidentally be dropped into the sea, resulting indebris like fiberglass fragments dispersing in the water. Compared to the volume of marine litter, particularly plastic debris that humans have dumped into the ocean, offshore wind debris might seem insignificant. However, it is always better to prevent a problem rather than seek solutions after it has escalated.

Offshore wind farms are a relatively recent development, and so far developers have primarily focused on refining installation and maintenance methods and on operational efficiency. Nonetheless, wind turbines have a finite operational lifespan, usually around 25 years, after which the farms are expected to be decommissioned, unless a lifetime extension adds a few more years of service. According to a TGS market report, a total of 132MW of offshore wind capacity has been decommissioned worldwide to date. Although this volume is not significant yet, the rapid acceleration of offshore wind farm installations since 2010 means that the industry must increasingly address the issue of future decommissioning, especially from 2035 onward. Despite the potential for substantial costs, logistical challenges, and environmental consequences, the topic of decommissioning has not yet received much consideration compared to installation and operation. Decommissioning represents a considerable segment of any project and ought to be considered from the design phase. Neglecting this could lead to exacerbated economic costs and environmental impacts.

Decommissioning operations could pose similar impacts and risks to those seen in the installation and operational phases, including collisions with vessels, noise pollution, and contamination releases into the seawater. One effective mitigation measure to minimize these risks is to avoid decommissioning during periods when species are most vulnerable.

Theoretically, decommissioning an offshore wind farm involves the removal of all its components, including turbines, foundations, transition pieces, as well as export and inter-array subsea cables, meteorological masts, offshore substations, and any existing scour protection materials. Ideally this process should also include restoring the seabed to its original condition. However, since decommissioning is a relatively new field, it currently lacks comprehensive legislation governing the extent and scale of such an operation. Consequently, the options available vary by country and may include complete removal of installations, including the potential reuse, recycling, and disposal of their parts, or leaving parts of the structures in place after operations have ceased.

In typical decommissioning, structures above the seabed are removed, while the decision to remove or leave behind materials beneath the seabed, such as foundation piles and subsea cables, is left to developers and legislation. Abandoning aged offshore structures is not exclusive to offshore wind projects. It also appears to be a common practice among marine industrial users. The seafloor has already been littered with buried and abandoned pipelines, cables, and other aged oil and gas structures. In the context of offshore wind projects, both the removal and retention of buried components can have potential positive or negative environmental impacts. These choices can also affect the economics, technical design, and operation of future projects. In cases where complete removal is not a legal requirement, developers typically cut the foundations to a specified depth below the seabed, leaving the rest in place. While this method is more cost-effective and may cause less disruption to the ecosystem compared to complete removal, leaving parts of the foundation below the mud line might restrict developers’ options for long-term repowering development in designated wind farm areas. The complexity could increase considering that achieving offshore wind energy targets necessitates the installation of numerous larger fixed-bottom turbines with larger foundations that occupy more seabed area in shallower waters.

Subsea cables that could last for 40 years (longer than the typical lifetime of an offshore wind turbine) are usually buried at depths exceeding one meter beneath the seabed. Subsea cable removal can be partial or complete. However, to date, complete removal has not been economically viable due to the high costs of extraction and the market’s lack of recognition for the economic and environmental advantages of cable recycling. Additionally, there could be an environmental argument to be made against the complete removal of cables, aimed at minimizing the damage and disruption to the seabed habitat due to the cables’ extensive length. It is also argued that removing cables could destroy the habitat formed around and on the cable protection structures. While these arguments may be valid, another environmental consideration is that if the cables are left behind, future subsea cables, which could otherwise be laid along already existing routes, will need to find new seafloor routes. Removing old cables and reusing existing routes and protection structures could minimize habitat disruption and may even be cost-effective, as it could reduce construction costs.

Optimal positioning of wind farm locations is one of the most effective strategies to minimize potential harm to the ecosystem, especially to minimize the risk of collision with avian species. A key consideration for the optimal siting of offshore wind farms is identifying areas where migratory birds and bats frequently pass through and avoiding placement of the wind farms in those routes. In addition, thorough habitat impact assessments in advance are crucial to identify the impacts of wind farms on marine wildlife nesting sites, feeding grounds, and breeding areas. Under certain conditions, countries that operate their site investigation and allocation under a state-led system might have an advantage compared to countries that use a developer-led system. The primary reason is that a state-led system tends to adhere to more consistent and integrated procedures and standards, which could result in more thorough and careful risk assessments. Additionally, state authorities are likely to have better access to the necessary databases (regarding, for example, geological and environmental information) for conducting investigations. This approach may reduce variation in decision-making during the investigation and review process. However, if government authorities are understaffed or do not employ efficient procedures for site investigation, the state-led system will become a bottleneck. In such instances, developers with better equipment and more experience could perform more efficiently, thereby expediting the site identification and investigation process.

Non-price criteria are gaining momentum in European offshore wind auctions. Biodiversity conservation, and even more so, achieving a biodiversity net gain, which involves increasing the overall biodiversity value of development sites to ensure enhancement rather than depletion, is becoming a fundamental requirement for offshore wind development, particularly in northwest Europe including the Netherlands, Germany, and Denmark. The Dutch government has played a pioneering role in establishing environmentally oriented, non-price criteria for offshore wind tenders. These ecological criteria require developers to concentrate on minimizing the potential negative effects of offshore wind energy on the environment as much as possible. Developers must prove that they will implement measures to mitigate adverse ecological effects on local birds and marine mammals. Additionally, they must demonstrate efforts to bolster and restore underwater nature, marine ecosystems, and the diversity of native marine species. A further stipulation is the contribution to knowledge, research, and innovation aimed at lessening ecological detriments and augmenting positive impacts. These requirements have effectively encouraged offshore wind developers to invest in developing and applying innovative solutions that minimize the environmental footprint of their projects. These initial measures are commendable and must advance swiftly to establish offshore wind development as a pioneering and sustainable energy production model for other industries to emulate.

Offshore wind energy is essential for tackling the climate crisis and protecting marine ecosystems from adverse global warming consequences, but careful planning and monitoring are needed to mitigate and minimize potential unwanted impacts to marine ecosystems. Like any human-made development, offshore wind farms can have both negative and positive effects on the ecosystem. The creation of a sustainable energy system requires the consideration of economic, social, and environmental factors. It is encouraging to observe that stakeholders in the offshore wind market are increasingly recognizing the significance of protecting marine ecosystems. This is primarily evident in the recent wind tenders in northwest Europe, which have incorporated non-price criteria, including the conservation of marine biodiversity. However, there is still substantial scope for enhancing our knowledge about offshore wind farms’ impacts and for developing methods to mitigate and minimize the unwanted effects.