June 24, 2024

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Once again the seaweed is gaining a foothold in a ravine.  How did scientists get this important plant on Grienderwaard again

Once again the seaweed is gaining a foothold in a ravine. How did scientists get this important plant on Grienderwaard again

Don’t expect a swaying, grassy plain. At low tide, seaweed pollen lies flat in the last part of the seawater at Grienderwaard. This is a tidal plain to the northeast of Griend, out of sight of the inhabited world. Seaweed leaves indicate the direction in which the seawater was previously drained.

“Try not to tread on plants,” says Finnish researcher Max Gravings of the University of Groningen, who planted this 20-by-20-meter section of the test himself last year. The fact that annuals then self-sow so generously that the box is full again this year can be called a success. The fact that so much of the seaweed has grown outside the specified mass is nothing short of promising. It appears that factories are expanding in their space on their own. Seed buds are already filled with seeds.

If you step carefully through the pollen, the baby shrimp will fly away. Copepods, guppies, and mud snails find shelter in seagrass. “Snails in particular do well. And then you have to ask yourself: Are they here because there is seaweed or have they already been there and their chance of survival is much higher now?”

Organic Builder

All that life illustrates the importance of seaweed, which has been recorded as a “vital builder.” It is a plant that puts the environment in its hand. By inhibiting the stroke wave and current, it causes the solid particles to fall off. This makes the water clear, which makes it grow better again”, explains RUG coastal ecologist Professor Tjisse van der Heide.

Seagrass beds act as nurseries for sea creatures, contribute to coastal protection, store carbon, and also serve as a habitat for pigeons, swans, and brent geese. It does not hurt that the latter eat seaweed in the fall. The seed has already sprouted and the plants will die anyway.

Seaweed is not an herb, but rather related to pondweed. There are two types: small and large weeds, the latter being peculiar enough in one type that lives under water and the other that prefers to dry out. The variety is underwater perennial and spreads across hills. Desiccants put all their energy into flowering and seed formation, because they do not survive winters on the surface of the water. Small seaweed in the middle. It is perennial, dries up and works with suckers.

winning formula

Within the “Keying to Seagrass Recovery” project, researchers and specialists have been searching since 2013 for the winning formula for a self-sufficient dry seagrass combination. Natuurmonumenten is in charge and works with the University of Groningen, Radboud University, the Fieldwork Company and Waterproof bv. On Wednesday they organized a trip for stakeholders and the press to show the serious progress that has been made.

On the Grienderwaard, the area containing seaweed has grown significantly in recent years, from 30 hectares in 2018 to 100 (2019), 164 (2020) to 276 hectares now. That equates to 200,000 plants, RUG scientist Laura Govers estimates. Tjisse van der Heide thinks it’s too early to cheer. . But this is an important first step toward recovery on a serious scale. We are no longer talking about square metres, but about hundreds of hectares.”


What is growing now is nowhere near what it was a century ago, when large parts of the Wadden Sea were littered with seaweed, especially submerged species. This was the time when the coastal people would go out into the sea with machetes to harvest them with sandals. Using dried seaweed, you can fill mattresses, cover roofs, and even build levees.

With the closure of Afsluitdijk in 1932 and an outbreak of seaweed disease, seaweed almost completely disappeared. Except for a few stray plants, submerged seaweeds are no longer present in the entire Wadden Sea. The tidal type is still found in the German and Danish valleys.

New success must come from afar. Project leader Quirin Smeele of Natuurmonumenten says that in 2017 he was about to pull the plug on the project. The €200,000 invested annually, in particular by the Wadden Fund, mainly resulted in disappointments. The old method of sowing did not work, in which jute bags with german seed sprouts were laid on a stone and floated in supposedly promising places. Also, trials of new seeding methods off the coast near Uithuizen did not produce lasting results.

large compartments

Laura Govers, who has been involved in restoration and research work on Griend, accidentally came across beak ruppia east of that island, a plant that thrives in a seaweed environment. It called the seaweed researchers, after which it was decided to give another chance to experiments on this plain. The water is calm on Lee Island and drains relatively slowly at low tide.

Experiments in recent years have shown the conditions in which seaweed grows best. Large boxes, where sowing is done more densely, work best. Small lumps quickly fill with sand, which causes the plants to dry out. In 400 square meters of areas, this occurs only along the edges, so that more water remains in the middle where the plants grow more vigorously.

The seed is injected into the bottom of the mud using a dam syringe where a mixture of sludge and seeds is placed. This German seed is harvested in late summer, after which it spends the winter in a refrigerated trailer. In it, phytophthora disease can be cured. Copper sulfate cannot be used in the valley. Sowing in March is a particularly cold and labor-intensive work, especially when a test section of an entire hectare has been completed, as was the case this spring.

There are a lot of studies going on to buy time and land. Jannes Heusinkveld is working on a pushcart with dozens of dam nozzles so you can sow faster. An experimental nursery for its own seaweed seed is being operated in Groningen. New plant experiments are taking place in places where tufts of dry seaweed still grow: in the Rottumerplaat and on the sandy bank of the Hond en Paap in Ems. There are also trials with small and submerged seaweeds.


The “Keying to Seagrass Recovery” project will end next year. Natuurmonumenten transfers it to the Rijkswaterstaat, Principal Administrator of the Wadden Sea. “This is where it belongs. I often had the feeling we were planting seeds in Rijkswaterstaat’s backyard,” says Quirin Smeele.

Now that a viable method is available, Rijkswaterstaat can work on expanding the seagrass area. The administrator is also obligated to do so. According to the strict guidelines of the Water Framework Directive, there should be 10,000 hectares of large and small seagrass in the Wadden Sea by 2027.

Marine water quality consultant Raven Cammenga, who was also present during the trip, cannot say how Rijkswaterstaat wants to tackle this. Since such a multi-million project would have to be tendered according to European rules, it is uncertain whether the current seaweed alliance can continue its work.

Coastal ecologist Tjisse van der Heide is not yet reassured. “There is a danger that one will pass without seeing and doing something. It takes a long breath. In America, there is a successful project underway in the Chesapeake Bay, but this has been in the works for decades. We get it here.”

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