Marker Wadden is an archipelago of nature islands in the center of the Netherlands, created with excess sediment form the bottom of the lake. The islands have been extensively researched over the past 5 years, on the themes:
Did water quality
of the lake improve?
Which birds did find Marker Wadden?
Why did we see an explosion of sulphur bacteria?
Is better food
Is Marker Wadden
a stepping stone?
What is the role of
Did Marker Wadden
stay put during
Why build with
How much soft
sediment is collected
in the silt channel?
a fish paradise?
Why build with soft sediments?
Sludge - thin sediment - is abundant on the Markermeer lakebed. This sediment is not very valuable when it comes to regular construction. However, it has good potential when it comes to making natural islands from silt. Vegetation grows well on it due to the high organic content (thus nutrient-rich) and the variation in composition creates variation in height, which is exactly what is needed for nature development. Subsidence of the silt layer appears to be difficult to predict because of the interaction between variations in composition, layer thickness, water level management, and fill layers. Therefore, management and maintenance must take place in an adaptive manner.
Marker Wadden: a fish paradise?
The dynamics of windward shores and the habitat diversity of the nutrient-rich land-water transitions - the marsh fleawort and reed beds, and the adjacent fields of pondweed - are attractive to young fish. In addition, deeper, sheltered water features - such as the harbour and deep sand extraction pits around Marker Wadden - fulfil an important role as refugia for both small and large fish.
From dream to reality
Natuurmonumenten dreamed of a nature area in the Markermeer and applied for the Postcode Lottery’s Droomfonds (dream fund). It is unique that the initiative for such a large hydraulic project as Marker Wadden was taken by a nature organisation. The eventual cooperation between Natuurmonumenten and Rijkswaterstaat was a new and successful form of public-private partnership.
Did Marker Wadden stay put during the storms?
Yes, the characteristic sand edges around Marker Wadden protect the silt-rich islands behind them, even in heavier storm conditions. Sand can be lost locally as a result of spatial differences in the supply and removal of sand throughout the year. The presence of currents plays an important role here, particularly on the North beach.
Is Marker Wadden a stepping stone?
Marker Wadden acts as a stepping stone for other marshland areas in the vicinity. The birds of Marker Wadden use a large area. Birds of various species commute between breeding, resting, and foraging areas in different parts of the Markermeer system and even beyond. This spatial component contributes to the biodiversity of the region.
Which birds did find Marker Wadden?
Marker Wadden was almost immediately found en masse by birds. In just a few years, a diverse breeding bird community has emerged on Marker Wadden, with 43 species in 2020 and 47 in 2021. The first nesting pairs to settle on the islands were those that nest on bare sand and among the low pioneer vegetation: common tern, little tern, avocet, fur-billed plover, little ringed plover and, at a somewhat later stage, the black-headed gull. Rare species such as the black tern, the lesser black-backed gull, the common scoter, and the corncrake have also been observed. On a small scale, a marsh bird community is beginning to form, with species such as the bearded reedling and water rail. Recently, the calling bittern has been heard on the main island.
Why did we see an explosion of sulphur bacteria?
Large mats of sulphurous bacteria have been found on the seabed around Marker Wadden. The filamentous bacteria were known, though not in such huge quantities. The construction of the islands caused plumes of sediment, and it was there that the growth of the sulphur bacteria exploded. The bacteria form mats on the bottom that have a large share in the primary production (via chemosynthesis, so without light) of the system and, on the other hand, have an effect on the rest of the bottom life and the degree to which the sediment is disturbed. Sulphur bacteria are not a new food source for fish; they have not been found in fish stomachs.
Is better food available now?
In the Markermeer, not all algae are available as food for mussels or water fleas. Some of the algae clump together with other floating matter to form a kind of flake, which means the algae are trapped and therefore less suitable as food. Marker Wadden provides clear water on the lee side of the islands. In these calm waters, we see that no flocks are formed, and the algae are more available to water fleas and mussels, which in turn feed fish and birds.
What is the role of volunteers within research?
In recent years, KIMA has made grateful use of data collection by volunteers on Marker Wadden, in addition to the basis of regular, standardised monitoring. We saw enthusiastic volunteers helping with flora and fauna monitoring. In the future, there will be opportunities to continue involving volunteers, schoolchildren, and visitors in data collection at Marker Wadden.
How much soft sediment is collected in the silt channel?
The silt channel has done its job; there is a supply of silt present and the channel silts up to approximately 40 cm per year. Silt has also accumulated around Marker Wadden in wells and on the lee side. The KIMA study shows that approximately 250 kilotons of silt are captured each year in the silt channel, the extraction wells and the other lee zones around Marker Wadden and, according to the calculations, this has a significant impact on the local silt balance and turbidity of the Markermeer. The collected silt can be used for future maintenance of the current islands.
Did water quality of the lake improve?
The effects of Marker Wadden on the water quality of the Markermeer are tentative; there is a clear zone on the lee side of the islands. The total effect on the clarity of the lake cannot yet be indicated due to the activities that cause the silt to be stirred up. This windblown silt is not only negative, but also creates new gradients in turbidity (positive for fish) and the release of nutrients from the nutrient-rich soil.
WHY MARKER WADDEN?
Marker Wadden was constructed to reverse the decline of the biodiversity. The turbid water of the shallow lake affects the foodweb at the expense of fish and birds in the area. Marker Wadden is created using soft sediments from the bottom of the lake, decreasing the turbidity and creating new habitats under and above water to strengthen nature values.
APPLICATION OF BUILDING WITH NATURE
In addition to scientific and technical knowledge, the project has generated knowledge about the realization of Building with Nature. Both public and private parties can apply this knowledge, for example in large lakes, dammed rivers and estuaries. Up to now the research has generated three doctoral theses, 10 scientific articles and 46 research reports.
1 FIELD STATION
visitors per year
Only one island accessible to the public
A harbour for 60 boats
hectares of new nature
around the islands
moths, beetles and flies and midges have each been recorded with over
Size and location of Marker Wadden contribute to development of a biodiversity hotspot in an area that was low in production and diversity before.
Non-breeding birds find Marker Wadden in tens of thousands. Up to:
Movements of birds between Marker Wadden and nearby areas (Trintelzand, Oostvaardersplassen) have also been recorded.
Marker Wadden is a stepping stone and will enhance biodiversity of the whole area.
A community of marshland birds is developing with 49 pairs of bearded reedlings in 2021 and records of sedge warblers, waterrails and bittern.
While numbers of breeding species of pioneering habitats are likely to decline, the desired marshland community is developing.
A diverse breeding bird community has emerged on Marker Wadden, with 43 species in 2020 and 47 in 2021, including rare species like:
Marker Wadden does make an important contribution to the national Natura 2000 targets.
of new spawning habitat for 14 FISH SPECIES so far, while 26 SPECIES frequently visit the islands. Key is the mosaic of newly created shallow vegetated shore zones.
By offering habitat of types underrepresented in Markermeer, Marker Wadden contributes to fish production and to a more diverse fish community in the lake.
building with nature
Many fish also use 25 m deep sandpits and migrate up to over 100 km between the lake systems of IJsselmeer and Markermeer.
Fish use Marker Wadden as part of a large network of habitats in the entire IJsselmeer area.
cubic meters sand, clay and silt is used for building the islands.
kilotons of silt
captured annually in the silt channel, the extraction pits and the in lee zones around Marker Wadden. This sediment mass is equivalent with about 1 million m3, i.e. about 100 truckloads per day.
Maintenance of the islands will be necessary. The collected silt will be a source.