Do you remember? Beaches flooded with bryophytes, researchers are now diving into

Two years ago, the beaches in North Holland and Friesland were suddenly covered in a thick layer of bryophytes. And that’s not normal at all. “It is ecologically unique,” says Reinier Nauta from Wageningen University & Research in Den Helder. And so he led two studies on the animals. The second investigation will be completed this month.

Mossy animals on the beach at Bergen aan Zee in 2021 – NH News / Jurgen van den Bos

Not much is known about bryophytes. A whole colony of the animals looks like a piece of moss, hence the name. “They are very small, they live in our garden. They are there, but they don’t have a big impact,” says Nauta. So he was also surprised when the Dutch beaches were suddenly full of the animals.

“It is quite special to experience,” says Nauta. “It triggered something like: how is this possible, how is this happening?”

“The nice thing about bryozoans, they’re a very large group of organisms, but they get very little attention,” says Nauta. “I’m not a bryozoan expert myself, but a seaweed researcher.”


Although the small organisms do not do much harm in principle, they cause difficulties for fishing. The huge mass of creep means that fishing nets fill faster and also require more maintenance. The municipalities of Ameland and Katwijk therefore commissioned research into the animals.

Nauta: “The biggest obstacle in the fishing industry is off the Dutch coast from Hoek van Holland to Den Helder. Further out to sea it becomes less. Fishermen suffer from slammed nets due to pits full of bryozoans. As a result, catches and production falling.”

“Very little was known about them before this research. Until recently they had no influence at all in the Netherlands, but they were more common in the Mediterranean. It’s intriguing. How is it possible that these extreme masses move largely passively, around the bottom?”

The first study from WUR showed that the bryophytes are mainly found between Hoek van Holland and Den Helder –

A possible explanation for the bryozoan’s spontaneous arrival is the water temperature. Nauta: “The marine system is temperature-controlled: below 12 degrees everything goes into winter mode, above 14 degrees you get spring flowering and development. A change in temperature, even just a tenth of a degree, can have an effect.”

It has previously been speculated that the wind turbines on the sea were a possible reason for the bryozoans’ arrival. “It is now clear that it does not come from there,” says Nauta.

“We see climate change anyway: we see warm water fish coming here more and more. To be another good example the seahorseswhich you barely saw in 2018. You now see them washed up by the dozens.”

A carpet of bryophytes on the Wadden at Den Oever in 2021 – NH News / Laura Gremmee

This increase has only a limited impact on the North Sea ecosystem. Nauta: “Species will always benefit and be negatively affected. A positive example is the Zuiderzee snail. It was thought to be extinct for a long time, but that species eats bryozoans. So now we see them more often.”

The negative impact primarily applies to animals that live in the soil, such as shellfish. They pump fresh water from the sea. Nauta: “If you live in the ground and you suddenly get a roof of garbage on your head. They depend on that water, it’s not possible with such a roof of bryophytes.”


Seen from above, birds on the beach have the same problem. For example, sand runners have a hard time hacking through the blanket of bryozoans to get to their food.

It remains to be seen what the long-term effects will be. Nauta: “It’s primarily a question of how it will develop. We’ve seen bryozoans en masse in the last 2.5 years, but it’s very local. Birds are mobile, so they can move easily.”

The report is expected to be delivered to customers at the end of January and published a month later.

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