Numerous requirements must be met to protect and preserve natural resources so that the expansion of offshore wind energy capacity is as compatible with nature as possible. New wind farms may only be built once environmental compatibility studies have been carried out. These studies begin before a wind farm is built, continue to run during the construction phase and carry on into the initial years of operation. The aim is to identify any potential impact on marine mammals, birds, fish and other organisms at an early stage and find solutions. “We spend around two million euros a year on annual environmental monitoring measures for each offshore wind farm project,” explains Dr. Gotje von Leesen, who works at EnBW as an environmental manager for offshore wind and holds a doctorate in marine biology.
Wind farms have a positive impact on biodiversity
EnBW’s experience since commissioning Germany’s first commercial offshore wind farm – EnBW Baltic 1 – around 15 years ago and studies conducted by renowned research institutes as well as industry and environmental associations show that marine mammals avoid the area around the offshore wind farm while the foundations are being installed. “Once the wind farms have been built, however, we see many positive effects on wildlife and biodiversity in the sea,” says Dr. Gotje von Leesen. Fish stocks, for example, recover because the wind farms are off-limits to the fishing industry, while the wind turbine components lying below the water surface form artificial reefs, providing a habitat for all kinds of flora and fauna.
Porpoises enjoy the highest international protection measures during construction
The distribution of marine mammals such as protected porpoises and seals is monitored before, during and after the construction of an offshore wind turbine, supported by hydrophones that detect the calls and sounds of the animals.
The construction phase of offshore wind turbines, during which the foundations (monopiles) are driven into the seabed, represents a particular noise nuisance for the underwater world. Mammals such as porpoises and seals, but also fish such as cod, use sound to navigate, hunt, communicate or perceive enemies.
Set noise limits must not be exceeded during the construction of offshore wind farms. When driving piles into the seabed, for example, the average noise level at a distance of 750 meters must be no louder than 160 decibels. The permissible peak value is 190 decibels. This is the strictest noise control standard in the world, with measurements taken by independent institutes during construction and sent to the relevant authority for monitoring purposes.
Last year, EnBW used an innovative noise mitigation system developed especially for the company when installing the foundations for EnBW He Dreiht. It takes the form of a double-walled steel pipe through which air bubbles rise, attenuating the sound directly at the source.
In addition, a double bubble curtain was used, which mitigates noise using a simple but highly effective method. “Imagine it like a large garden hose with lots of holes lying on the seabed around the construction site, into which we blow air,” explains Dr. Gotje von Leesen. “Just like in a whirlpool, the air bubbles rise to the surface, changing the density of the water and breaking up the sound waves.” As a result, construction noise can no longer spread unhindered around the construction site because it is attenuated.
Offshore wind farms in operation in the German North Sea are havens for porpoises. These are the findings of a new long-term study conducted by the institute BioConsult SH and IBL Umweltplanung GmbH. The detection rates for porpoises are significantly higher within wind farms than in the surrounding area – an indication of possible reef and refuge effects. The analysis is based on data gathered over the course of 13 years across more than 60 stations in the German Bight in the North Sea.
In the future, however, it may also be possible to avoid the temporary disturbances by anchoring the foundations in the seabed. “New installation and noise control technologies are currently being researched in order to further reduce noise during the construction of foundations,” says Dr. Gotje von Leesen.
The impact on other organisms
Benthos
New species colonizing around the foundations
Benthos: new species colonizing around the foundations
Benthos, the organisms living in the sediment of the seabed, are studied by means of scrape samples, the evaluation of which enables precise identification of the species in the colonies. “We have observed population growth among many species in wind farms. On the foundations themselves, for example, mussels form layers several centimeters thick. An artificial reef is created,” explains EnBW expert Lars Stuible. The observations are consistent with the results of a research project undertaken by the Alfred Wegener Institute for Polar and Marine Research on the impact of offshore wind farms on benthic ecology. As early as 2013, the scientists reported that the steel and concrete foundations tend to enrich rather than restrict the underwater habitat. That’s because – just like shipwrecks – they represent an artificial “hard substrate” that enables the colonization of organisms hardly ever found in marine soft bottom habitats, if at all. The researchers demonstrated, for example, an increase in the crab population around one of the turbines examined as part of the project.
Fish
Cod proliferation
Fish: cod proliferation
Fish stocks in the North Sea and Baltic Sea have been declining since the 1980s. Many of them are overfished. There are Marine Protected Areas in the North Sea and Baltic Sea where fishing is prohibited so that fish populations can recover. Conservationists, however, feel that these are still too small and are making the case for more marine sanctuaries and more consistent restrictions on fishing.
Since wind farms are off-limits to shipping and fishing, they have a similar effect on the fish world as the Marine Protected Areas. In other words, the fish can thrive undisturbed here.
Rocks deposited around the wind turbines to protect the foundations create an “artificial reef” effect. Besides serving as a spawning ground for the cod, these structures around the foundations can also be a habitat for rich food sources – a key factor in the recovery and survival of marine populations. In addition, the findings of the study reveal that the newly created habitats also benefit other species.
Offshore wind farms can be sanctuaries for cod in the North Sea.
As part of the two-year project “Offshore wind farms in the context of ecosystem-based marine spatial management,” scientists from the Thünen Institute of Sea Fisheries proved in 2020 that cod also feels comfortable living in wind farms: “The cod aggregate around the turbines. Plankton samples in combination with drift models indicate spawning activities in the wind farm area,” as stated in the study’s final report. Plankton refers to the organisms that drift and float in the water. So-called drift models are forecasting methods used to simulate the drift trajectory of any objects, substances or resources drifting at sea. Scientists at the federal research institute also analyzed the stomach contents of cod from wind farms as part of the study – with the finding “that the diet of the organisms from the wind farm is much more varied than that of the organisms caught outside the wind farm.”
Birds
Unbothered by the turbines
Birds: unbothered by the turbines
“In terms of the avifauna, we are primarily talking about passage migrants and migratory birds,” explains EnBW expert Lars Stuible. “The seagull is an example of a passage migrant, while the crane is a migratory bird.” For biomonitoring purposes, aircraft and ships must record the bird species and the numbers present at several observation points in a given area. This is done by taking photos, observing the birds and listening to their calls. “The flight altitude is also particularly interesting,” says Stuible.
The studies do not bear out the preconceived idea of the “bird chopper at sea.” “We can see that wind farms are certainly obstacles, but the birds fly over them. This is also evidenced by the fact that we do not find any dead birds on our foundations or see them floating in the water. We would notice it because we are there every day,” explains Stuible. Large flocks of migratory birds avoid the EnBW Baltic 1 and EnBW Baltic 2 turbines, as has been observed for a number of years. “That is why we don’t have to undertake any temporary shutdowns there – with the aid of radar equipment, for example, to detect the migrating birds.”
Studies conducted by the Federal Maritime and Hydrographic Agency (BSH) confirm that the risk of collision for seabirds is low and that migrating bird species avoid wind farms. In 2018, the Offshore Renewables Joint Industry Programme (ORJIP) published findings from the world’s most comprehensive and technically complex study of seabird behavior and the risk of collision with offshore wind farms, concluding that the birds’ evasive behavior is greater than often assumed. Over the course of the two-year study period, the scientists observed just six collisions in a wind farm off the coast of Great Britain – this equates to 0.05 percent of all flight movements recorded there.
Wind farms are becoming protective spaces
By way of a pleasing summary, for all the ways of protecting marine life where any potential impact can be determined by means of comprehensive biomonitoring measures, there is no evidence of negative effects associated with the construction and operation of EnBW’s offshore wind turbines. “That is why EnBW Baltic 1 and EnBW Baltic 2, our two wind farms that have been in operation for many years, have not yet been subject to any additional conditions,” summarizes Lars Stuible, who works as an operations manager for EnBW’s Baltic Sea wind farms.
The fact that new, undisturbed ecosystems are created in wind farms by “artificial reefs” and fishing exclusion zones is a welcome side effect. The abundant food supply in the new, modified habitats attracts porpoises, seals and fish – turning the wind farms into protective spaces that serve biodiversity.