Wetenschap - 23 maart 2017

Debris houses reefs

tekst:
Roelof Kleis

Drilling platforms, wind turbines and shipwrecks in the North Sea are generally seen as unnatural features which disturb the ecology. But it turns out that this debris from human activity also provides nurseries for lovely, species-rich reefs.

Frilled anemones cover the wreckage of the bow of the Biarritz, a Norwegian cargo ship which sank in the North Sea in 1940..
photos
Udo van Dongen

The bare sandy bed of the North Sea is not a place you expect to find reefs full of colourful marine life. But that was not always the case. Until roughly the end of the 19th century, the North Sea was more like one big reef than one big sandbank, says marine biologist Joop Coolen, who graduated with a PhD last week for a study of the biodiversity on reefs in the North Sea. Coolen shows me a Danish map from 1883 and points out a vast oyster bed to the north of the Wadden Sea. ‘A reef of 27,000 square kilometres. That is half of the Dutch part of the North Sea. But it’s all gone. The last oysters were fished there in the 1960s.’

But this gloomy picture needs some adjusting now too. The North Sea is not as bare and lifeless as it might sound. And not all human activities have been negative; the sea has received a ‘helping hand’ now and then too. Drilling platforms for oil and gas, wind turbines, buoys and shipwrecks are increasingly providing artificial environments for marine life to thrive in. Coolen studied these new reefs, the wealth of species on them, and the way these species colonize the sea.

Frilled anemones

As a reference point, Coolen takes the Borkumse Stenen, a large rocky expanse of sea to the north of the island of Schiermonnikoog. This is one of the few remaining stretches of natural reef in the North Sea. Coolen found as many as 193 different species there, including frilled anemones and reefs of sand mason worms. He then inventoried the species on the pillars of five gas platforms at various distances from the coast using video footage taken for technical inspections. This revealed the presence of 30 different species.

The distribution of the species on the pillars is interesting: the most species are found at average depths. Coolen; ‘Each species lives at a particular depth. Anemones, for instance, live much deeper than mussels. That is because of the waves. Many species cannot cope with being constantly battered by the water like that. The exceptions are species with a strong shell and species with a strong grip, such as barnacles.’ The species diversity increases up to a depth of 15 metres, and then it drops off. ‘At 20 metres all you find are frilled anemones. These are anemones which grow so fast they suffocate other species.’

Stepping stones

The question is how all these species colonize the North sea and what the role of drilling platforms and wind turbines is. One popular suggestion is that these ‘artworks’ facilitate the propagation of species by functioning as stepping stones. ‘A lot of species eject their eggs straight into the water, where they float around for a while, metamorphose into larvae and then attach themselves to a base,’ explains Coolen. ‘Sandy sea beds like that of the North Sea are not suitable for the larvae of reef species; there is nothing for them to hold on to. If the larva stage goes on longer than the time it takes to float from one platform to another, it survives.’ And so, goes the theory, species spread by hopping their way around the North Sea.

Coolen demonstrated this phenomenon rather elegantly using mussels. He collected a lot of mussels and used genetic techniques to map out the relationships between populations. He compared that map with a distribution model of particles which are transported by the prevailing currents in the North Sea. Coolen: ‘The current model matched the genetic data. So mussels make use of offshore installations to spread out through the North Sea. Further than would be possible naturally.’

‘Many species that grow on artificial bases are also found on natural reefs’

Japanese skeleton shrimp

Coolen’s stepping-stone evidence could have important implications for the question of whether disused drilling platforms and wind turbines should be dismantled. This is currently still compulsory but it costs billions and is, we now know, a retrograde step ecologically. Coolen would rather see these installations stay in place. He see their potential for extending natural reefs. ‘Many species that grow on artificial bases are also found on natural reefs such as the Borkumse Stenen. And for those natural habitats a duty of protection applies.’

The researcher would make one proviso, however. Drilling platforms and wind turbines provide stepping stones for exotic species such as the Japanese skeleton shrimp too. This creature thrives in tidal zones, and that includes installations out at sea, which are subject to tidal ebb and flow. ‘I think we should aim for as natural a situation as possible. Coastal zones out at sea are not natural.’

The emergence of these tidal zones can be prevented, says Coolen, by only leaving the underwater parts of platforms and wind turbines standing. ‘They are not natural either, of course, but at least they lead to an ecology which strongly resembles a natural one.’ He also suggests supporting the establishment of species on the pillars of drilling platforms and wind turbines by making them out of concrete rather than steel. Stone supports a greater biodiversity than steel, his study reveals. For existing installations he is thinking in terms of a coating of gravelly material.

Whether any of this will materialize has yet to be seen, but the first step has been taken. There are plans to leave several disused platforms standing for 15 years to facilitate further research.


27,000 wrecks

The North Sea is full of what is known as hard substrate, or artificial material that reefs can grow on. Such as the estimated 27,000 shipwrecks on the sea bed. ‘An awful lot of ships were sunk here, especially during the two world wars,’ says researcher Joop Coolen. ‘Wherever you dive, there is always a wreck within a radius of a few kilometres.’ There are also about 1400 oil and gas platforms and 1500 wind turbines in the North Sea, with hundreds more of the latter being added every year. Marine life attaches itself to these bases. Not in the form of the hard corals which live in warm tropical waters, though: the North Sea is only home to soft, leathery corals.



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