Offshore wind farms could become a breeding ground for invasive species

Research assistant Carl Baden from DTU Aqua inspects one of more than 700 underwater cameras the institute has set up to monitor life on rock reefs. Illustration: Casper Tybjerg

Mussels climbing up the foundations in dense clusters. Long undulating eelgrass around seaweed-covered rock formations, and fish swimming between the rocks. Life flourishes under the Øresund Bridge, and Denmark’s offshore wind farms may have the same positive effect on life in the sea. But it could also go the other way, biologists warn.

“This is still a new area, and our knowledge is therefore limited. But species diversity and habitats are changing throughout Danish waters. Not all places in the seabed are artificial reefs that help existing populations, and from a study in Belgium, we know that offshore wind turbine foundations attract non-native species,” says Karsten Dahl, head of section at the Department of Ecoscience at Aarhus University.

Seaweed—native as well as invasive—grows on hard surfaces such as rocks, including foundations of offshore wind turbines. Here it is on one of DTU Aqua’s stone reefs in Sønderborg Bay. Illustration: Casper Tybjerg

They examined foundations in the seabed a year after they were constructed and found as many as ten non-native species. And when we place not just one, but hundreds of foundations over large areas of the seabed, it does not only lead to the displacement of the naturally occurring populations of less visible animals that live buried in the seabed or on top of the sand and gravel substrate under the foundations—it carries a risk of non-native species that live on rocks on top of the seabed spreading via sea currents from foundations to foundations and from farm to farm with their eggs, larvae, and spores.

Karsten Dahl therefore believes that there is reason to fear that construction of very large parks in European waters will end up creating “a breeding ground for invasive species” and calls for thorough feasibility studies and a comprehensive risk analysis before the overall plan for the expansion of Denmark’s offshore wind farms is realized.

“Invasive species are something that needs to be taken seriously. They can threaten nature and the way we use it. This doesn’t mean that we shouldn’t have offshore wind turbines. We have to. But we need to minimize the risks, and that requires knowledge. If there’s a risk, it needs to be taken into account and we should, for example, strive to make the foundations less attractive, for example by using coating or paint that repels biological life,” he says.

Tailor-made solutions

Jon C. Svendsen, senior researcher at DTU Aqua, conducted a study on offshore wind turbine foundations two years ago. The study, funded by Vattenfall, concluded that the foundations have the potential to create life in the ocean as artificial reefs. Yet he sees the same dangers as Karsten Dahl.

“Our maritime life is fragile and the biodiversity crisis is as serious as the climate crisis. We have become much more aware of this in recent years, and several of the large energy companies are now increasing their ambitions in relation to nature. That's a good thing. We just have to remember that it’s a complex issue. There are very few quick solutions.”

Senior researcher Jon C. Svendsen says that the North Sea is different from the Kattegat, which is different from the Øresund, which is different from other sea areas further inland in the Baltic Sea.

“This means that new projects must be adapted to local conditions in order to promote biodiversity and have the desired effect. Perhaps offshore wind farms could target specific forms of biodiversity. Maybe they could help create habitats for fish. That could archived, among other things, by designing and placing the wind turbine foundations so that they help the marine life,” he says.

Such measures require thorough research, and someone needs to be willing to pay for it. When the conclusions of Jon C. Svendsen's and his colleagues’ study were published, he was hopeful. “It provides good opportunities for us to apply our knowledge of the artificial reefs to design foundations for offshore wind turbines that benefit marine life,” he said at the time, and the story was shared by the climate minister on Twitter.

Today, Jon C. Svendsen and his colleagues at DTU Aqua are still waiting for funding for the research that would help them acquire more knowledge for the design of offshore wind turbine foundations. That does not come as a surprise to Kjell Andersson, spokesman for the Swedish Coast and Sea Center (SCSC) and retired research engineer at Lund University. In the 1990s, he was involved in the construction of the Øresund Bridge as a consultant, and that project fortunately went well.

“Marine life returned quickly, and the bridge pillars came to function as artificial reefs, so Øresund today contains one of Europe’s largest contiguous mussel beds,” he says and emphasizes that this is not something one can take for granted.

“It requires tailor-made solutions and thorough feasibility studies, which they chose to pay for when the Øresund Bridge was built. But this is far from always the case,” Kjell Andersson says.

“Everyone wants to win, but no one wants to pay, and no one owns the seabed. Nor can we see it unless we dive underwater. This means that we humans have been able to prey on the seabed for far too long. We have trawled and dug up sand, gravel, and stone for all our constructions, so large parts of the seabed are today filled with algae. I don’t see offshore wind turbines as dangerous to the marine environment. But we could well consider placing them on the dead areas of the seabed. And then we should create a fund one needs to pay into when digging into the seabed. We could also take a single krone for each car driving over the bridges. That money could be used to restore lost nature. That is what is needed.”