Researchers from the Royal Netherlands Institute for Sea Research (NIOZ), the University of Groningen, and Utrecht University have constructed artificial reefs made of pear-trees in the subtidal Dutch Wadden Sea in a novel experiment as part of the project Wadden Mosaic. The first surveys now show that the reefs are teeming with life. Seaweeds, barnacles, sea squirts, anemones, and bryozoans cover the tree reefs, while fish are five times more abundant on the reefs compared to the open sandy seafloor.

Today, the Wadden seafloor is predominantly sandy. Hard substrates such as driftwood, peat, and stones used to be common in the Wadden Sea and provided a foundation for reef-building species such as mussels and oysters. These reefs, in turn, provided an important habitat for numerous species of fish, crabs and shrimps. In recent centuries, these natural hard substrates have gradually disappeared due to the damming of rivers, as fishermen removed them, and as they disappeared under the moving sands.

Old pear trees
In the current experiment, researchers created three-metre-high reefs from cleared fruit trees, replacing driftwood that used to drift into the Wadden Sea from rivers in large quantities. “To our surprise, the tree reefs appear to have become true hotspots of biodiversity just six months after their placement. Bryozoans, mussels, anemones, and barnacles cover the wood of the higher parts of the reefs, while drifting seaweeds are mainly found at the base,” said Jon Dickson, researcher at NIOZ.

A nice place for larger animals
These algae and filter feeders, along with the tree reefs, provide a refuge and foraging ground for fish such as rocklings, gobies, whiting-pout and eels. Measurements show that, on average, there are five times more fish between the reefs compared to the adjacent sandy bottom. Moreover, more species are found on the reefs and the fish are larger. The reefs are also home to many more prawns, an important food source for several fish species. Greater numbers of seals were also observed around the reefs.

The results show that tree reefs effectively bring a stable three-dimensional structure to the water, which increases biodiversity. Natuurmonumenten and Stichting De Rijke Noordzee are both enthusiastic about these initial findings. Quirin Smeele of Natuurmonumenten says: “We are surprised by the effects the tree reefs are having after just a few months. In particular, the strong increase in fish is beyond expectations.” Christiaan van Sluis of the Rich North Sea adds: “These results may also make tree reefs an interesting recovery measure for the North Sea. Biodiversity restoration is badly needed there.”  

Results of observations and measurements in actual numbers of animals. Left: around and on the reefs. Right: the control measurements elsewhere in the Wadden Sea. Figure: Jon Dickson, NIOZ.