Boulders bring new life

The final turbine for the Hollandse Kust Zuid wind farm was installed last summer. This offshore wind farm between Scheveningen and Zandvoort consists of 139 wind turbines and officially opened in September 2023. It’s one of seven offshore wind farms that the government wants to have up and running by 2030. Collectively, these wind farms are expected to supply 21 gigawatts of energy. That’s 16 per cent of the total energy demand in the Netherlands and 75 per cent of current electricity consumption.

Porpoises and seals

Offshore wind plays an important role in the transition from fossil fuels to renewable energy. However, one of the prerequisites for the new wind farms is that their construction takes ecology and biodiversity into account. “The Netherlands is one of the few countries to impose such requirements for the construction of wind farms, because we consider this an opportunity for environmental restoration,” says marine biologist Oscar Bos of Wageningen Marine Research.

Offshore wind farms can have both positive and negative effects on marine life. “Last year, I was lying on the beach near Noordwijk when I suddenly heard a heavy thumping noise. In the distance, I could see ships, and wind turbines under construction. That’s when I realised this was the sound of pile driving. I know from studies that pile driving is an incredibly noisy and energy-intensive process, but I didn't know you could hear it on the beach. Underwater, sound carries even further than through the air, so you can imagine how disturbing this would be to animals like porpoises and seals.”

And the wind turbines are disruptive to wildlife above the water too. “Swimming guillemots and razorbills, for example, are afraid of the wind farms,” says Bos. “They prefer to avoid them. And the turbine blades can cause problems for flying gannets. Cormorants, on the other hand, thrive in the vicinity of wind farms. They use the poles as a base from which to catch fish.”

Nature-inclusive construction

Researchers, construction companies and wind farm developers are working on nature-inclusive designs for wind farms, so that the developments can offer a greater range of ecosystem functions such as shelters, additional habitats or spawning grounds. In practice, this means designing standard elements for wind farm construction, such as rock armour or cable protection, in such a way that they positively encourage natural biodiversity. At the Hollandse Kust Zuid wind farm, for example, artificial reefs made out of rocks have been built in several locations on the seabed directly next to the rock armour, providing places for fish and other species to hide and establish themselves.

Bos and his colleagues hope to determine which nature-inclusive measures have the best cost-benefit ratio. As part of the KOBINE project – a collaboration between Wageningen Marine Research, De Rijke Noordzee, Waardenburg Ecology, Witteveen+Bos, and Vattenfall – they’re conducting research on biodiversity and construction costs for a number of nature-inclusive designs.

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One important factor for marine biodiversity is the type of foundation used for wind turbines. Rock armour is installed around the base of the piles in order to keep the sand around the turbines from washing away. Bos has noticed that this appears to have beneficial effects on biodiversity. “In July 2023, as part of the KOBINE project, we used an underwater drone at the Hollandse Kust Zuid wind farm to make sonar and video recordings of the rock armour. We saw that it was teeming with life, with different species of anemones, starfish, mussels and large North Sea crabs living and sheltering there. Butterfish also hide among the rocks, and schools of fish like pouting and horse mackerel swim around. They, in turn, are sources of food for predatory fish like cod, and for seabirds and marine mammals such as seals.”

Disappointment

In the summer of 2022, Bos discovered first-hand that research doesn’t always deliver the results that one hopes for. As part of the BENSO project, an experiment had been set up on Dogger Bank – a large, relatively shallow sandbank in the middle of the North Sea – to see which type of reef would be best for biodiversity. Ecologists from Waardenburg Ecology had placed a number of artificial reefs there near a shipwreck, using a mix of small and larger rocks. A year later, in September 2022, Bos dived down to see what had come of it.

“I descended down the anchor rope to the bottom of the blue water with my buddy to start taking samples of the reefs. I had imagined that they would look beautifully overgrown, full of anemones, soft coral, and with schools of fish, just like the shipwrecks we dive to. It would have been a bonus to spot the eggs of the small-spotted catshark, or an octopus. So we were really taken aback and disappointed when all we saw at the bottom was a few remnants of the experiment. The whole lot had ended up buried under the sand, and all we could find was a few large stones and some labels.”

Following the failed experiment on Dogger Bank, Bos and his colleagues turned their attention to more promising sites, such as existing artificial reefs. Four hours away from Den Helder by boat, there’s a large concrete base of a former oil drilling platform, surrounded by a mix of large and small rocks. “We went on a dive there last July. That artificial reef is similar to what’s being built in wind farms and neatly shows how habitat like this can develop over the course of a few decades.”

Oyster reefs

Wageningen researchers are also experimenting with restoring natural oyster reefs in and around wind farms. A hundred years ago, about 35,000 km2 of the international North Sea floor was covered with oysters. Overfishing and diseases have led to the complete disappearance of those oyster beds, leaving a sandy seabed. “Restoring these oyster beds would have a positive impact on biodiversity,” says Bos. “Since they filter water, oyster beds have a beneficial effect on water quality. They also provide a habitat for fish, crabs, lobsters, shrimp and many other species. Oyster shells provide a surface for all kinds of small prey species to settle. These species then serve as food for fish.”

Several oyster restoration projects are currently underway, with encouraging results. “The projects in the Voordelta and Borkum Stones nature reserves and in a number of offshore wind farms show that flat oysters can do just fine on the North Sea floor, even if they originate from Norway or Ireland. True reintroduction will require scaling up, preferably with Dutch oysters,” says Bos.

Neighbouring countries are showing increasing interest in this research, even though no other countries require wind farms to deliver biodiversity improvements. “Other countries are also working on oyster restoration, but they often do it close to the coast. The nature-inclusive design of rock armour for wind farms is relatively new. Just last year, I gave presentations in the UK and Germany about our research and the approach we’re taking in the Netherlands. It got a really enthusiastic response.”