Coral reefs in the tropics are fascinating because of their colorful fish and the variety of organisms that live in and on them. Few people know that there are reefs in the seas at cooler latitudes. They are also real hotspots of biological diversity – and, like their counterparts in warmer climes, are unfortunately also affected by loss. But Dutch marine biologists have developed an approach to regrowing reef habitats. Fruit trees play a central role, because Jon Dickson and his colleagues from the Royal Netherlands Institute for Marine Research NIOZ on the island of Texel wanted to test the historical ecological role of wood in the Wadden Sea as a substrate for new settlements and as a nucleus for surviving reefs.
Before humans intervened in the river systems, huge amounts of driftwood ended up in the estuaries and the marine environment. This also became a settlement site and habitat for shellfish and other sessile marine organisms. “The damming of rivers and the deforestation of their banks over the last few centuries has ended the flow of timber into estuaries, severely limiting the availability of this natural hard substrate for marine reef communities,” explains Dickson.
For their study, recently published in “Frontiers in MarineScience,” the scientists used old pear trees, short-stemmed, 2.5 to 3.5 meter tall trees with densely grown branches and twigs that are strong enough to withstand the tidal currents to withstand in the Wadden Sea. They come from a fruit tree plantation in which 25 to 35 year old trees that had exceeded their economic lifespan were felled.
Sunk with concrete blocks
Last year, the team built 32 pyramid-like structures out of six trees each. At their locations in the so-called Eijerlandse Gat between Texel and Vlieland, they cast the trunks in concrete blocks and sank them into the seabed in four different places so that they are still three meters under water even at low tide.
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“Within six months, the tree reefs were covered with an abundance of sessile animals and algae and supported more fish than the surrounding control areas,” said Dickson. In total, after this relatively short period of time, the researchers found 15 species of organisms near the pear tree reefs, including barnacles, polyps, seaweed, starfish and sea lettuce.
They also caught six species of fish and four species of crustaceans, while they only found two species of fish in the sandy control sites 200 meters away. In general, there were five times more fish in the area around the Birnbaum reefs than in more distant Wadden Sea areas. This shows, Dickson said, that initial colonization of natural tree reefs occurs quickly and that rebuilding communities through active restoration is possible.
Organisms colonize reefs because these structures dampen currents and waves, providing sensitive animals and their reproductive stages with a quiet, protected and stable habitat that is also a feeding and breeding site. However, these reef types have declined worldwide in recent centuries. Land reclamation destroyed underwater habitats, offshore structures destroyed soil, and mussel beds were dredged to extract shell limestone.
In the late 19th and early 20th centuries, the southern North Sea lost around 25,000 square kilometers of oyster reefs due to overfishing. In the 1950s, the shrimp fishery with its bottom trawls destroyed the sand coral reefs, glued together by tubeworms made of sand and mucus.
More recently, climate change has also taken its toll on the few remaining reefs. Increasingly strong storms and the associated stronger waves tear the reef-building organisms away from their substrate.
New reefs can no longer naturally build up in the North Sea and the Wadden Sea. Because they need a hard surface to start with. But this hardly exists in the North Sea with its sand and mud bottoms, except near Helgoland, the Borkum Reef and the Sylter Outer Reef. Stone fishing once dredged up rocks and boulders from the seabed, left behind from the Ice Age glaciers. This was used to fortify banks and build breakwaters. Many hard substrates were also buried because changed water currents after large-scale diking buried old reefs under quicksand.
With their experiment, the researchers were able to show that fruit trees sunk into the sea are an extremely cost-effective and effective way to restore reefs and promote local marine life diversity.
Other artificial reefs
Scientists at Wageningen University led by Benoit Berges and Marcel Rozemeijer have investigated whether non-natural foundations are also suitable for the settlement of reef communities, for example at offshore wind turbines in the North Sea. The foundations of the wind turbines are also hard substrates. For three years, the team has been researching how the habitats develop on four artificial reefs next to the Borssele 1 and 2 wind farms north of the Belgian seaport of Zeebrugge at depths of 14 to 38 meters. Here, concrete tubes serve as shelter for cod and lobsters.
Dickson sees this critically. He writes in an email: “While we can create new reefs, a ‘mature’ reef has intrinsic value, both because of its biological composition and because the communities there have adapted to their immediate environment. Wind turbines only have a lifespan of 20 to 25 years.
But with their pear trees, the Texel researchers want to create biodegradable reefs on a large scale that can be left alone for longer than just a few decades so that oysters, mussels and other sessile organisms can settle there. “If the wood is gone, we’re hoping that so many mussels have settled there that they’ve become a reef,” says Dickson.
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