Since 2016, the Netherlands has used 37.2 million tonnes of lakebed sediment to build seven wildlife islands covering 1,300 hectares

There is a stretch of water in the Netherlands where an unusual experiment in nature recovery has been unfolding since 2016. In Markermeer, engineers used sand and mud dredged from the lakebed to create Marker Wadden, an archipelago of seven artificial islands and surrounding underwater habitats. The project was the subject of Aisling Irwin’s 2023 Nature feature Rewilding the planet, which followed the emerging landscape and the scientists studying whether the new habitat could improve conditions in a troubled lake.

Markermeer has a history shaped heavily by engineering. It was once part of the Zuiderzee, a marine estuary connected to the North Sea, until the 32-kilometre Afsluitdijk was completed in 1932, turning the former Zuiderzee into the freshwater IJsselmeer. Between 1963 and 1976, the 26-kilometre Houtribdijk was built across the IJsselmeer, creating the separate Markermeer basin. With limited river inputs, few natural transitions between land and water, and a bottom rich in fine sediment, the shallow lake developed difficult conditions for aquatic life. Wind repeatedly stirred the silt, reducing water clarity and limiting the light available to plants.

That sediment eventually became part of the solution proposed by conservationist Roel Posthoorn. Rather than attempting to return Markermeer to its former marine state, Natuurmonumenten and government agencies pursued a different approach. By dredging material from the lakebed and incorporating it into sheltered islands, shallow zones and sediment-trapping areas, they aimed to reduce the amount of fine silt repeatedly stirred into the water while creating habitats that the engineered lake lacked. Construction of the first island began in 2016. The project eventually developed into seven nature islands and an associated underwater landscape covering about 1,300 hectares.


Building islands from mud

The construction was straightforward in principle but challenging in practice. Engineers built each island by gradually raising a ring of sand from the lakebed and filling it with muddy water. The clay, silt and peat then settled inside the sandy enclosure, creating the foundations for the new habitats. The project used about 37.2 million tonnes of material, almost all dredged from the lake floor.

The deposited sediment did not always behave as predicted. As water drained away and the mud compacted, the newly formed land shrank more than expected, requiring additional material. The islands have also continued to settle into the soft lakebed, making their long-term development something researchers have had to monitor rather than treating construction as a finished process.

The ecological aim was to introduce shallow, sheltered conditions and gradual transitions between land and water that were largely missing from the engineered lake. A 2021 study by Casper van Leeuwen, Liesbeth Bakker and colleagues examined whether the combination of shallow water, varied depths and sheltered areas could improve ecological conditions and support a more diverse food web. The researchers found that the developing landscape provided a range of habitats that differed from those available in the open lake.

Birds were among the first animals to use the new landscape. Terns arrived while the islands were still largely bare. Researchers fitted transmitters to some of the birds and found that they flew to feeding grounds in the North Sea, about 50 kilometres from Marker Wadden, before returning to the islands to rest. As vegetation developed, the range of bird species using the archipelago expanded.

By 2021, monitoring had recorded 47 breeding bird species, while more than 60,000 migratory birds had used the islands for food or shelter during the year to July. Greater flamingos and Eurasian spoonbills were among the visitors. Gull-billed terns also nested there, while a long-tailed duck, a species that normally breeds in Arctic regions, was recorded on the archipelago.

A new underwater habitat

Fish also began using the developing shoreline habitats. Monitoring at Marker Wadden in 2018 and 2019 recorded 19 fish species in the surveyed areas, while researchers observed spawning and schools of larvae in vegetated shallows. These observations showed that the new habitats were being used by fish at different stages of their life cycles, although the monitoring did not establish that Marker Wadden had increased fish populations throughout the wider lake.

The developing aquatic food web included phytoplankton, zooplankton, insects, spiders and molluscs. Separate research published in 2023 found that phytoplankton quantity and quality inside the archipelago were higher than in comparable areas of the surrounding lake. The researchers linked these differences to greater nutrient availability and found corresponding increases in zooplankton biomass.

The islands have also required active management. Willow trees have spread into areas intended to develop as reedbeds, prompting volunteers to remove young trees. The problem is partly linked to Markermeer’s managed water cycle, which does not reproduce the seasonal water-level pattern that favours reed growth. This is one of the practical difficulties of allowing a deliberately constructed landscape to develop into a functioning wetland.

Erosion has brought another challenge. One of the northern beaches disappeared completely in 2020 and was subsequently replenished with fresh sand. Research into the islands’ sandy margins has also documented continuing sediment movement, meaning that some parts of the landscape require intervention even as other areas are allowed to change naturally.

Some effects extend beyond the islands. Monitoring has reported clearer water for several kilometres downwind of Marker Wadden, while researchers have also found changes in nutrient and biological conditions within the archipelago. However, scientists have differed over the scale and significance of effects beyond the project area. The separate phytoplankton and zooplankton study provides evidence of biological changes within Marker Wadden itself, but these findings do not establish that the entire lake has undergone an ecological recovery.

That distinction matters because Markermeer covers roughly 70,000 hectares, making the 1,300-hectare Marker Wadden project only a small part of the lake. The 1,300-hectare figure also includes the surrounding underwater landscape, rather than representing 1,300 hectares of newly emerged land. The research so far shows clear ecological changes around the islands, but it remains too early to determine whether those changes will translate into substantial improvements across Markermeer as a whole. Longer-term monitoring will be needed to establish how persistent the effects are and how far they extend.

The project also raises a broader question about what restoration means in a heavily engineered landscape. Marker Wadden is not a recreation of a former freshwater archipelago. Instead, it is a deliberately constructed landscape designed to introduce habitat diversity into a modified lake. That has made it an important example in discussions about rewilding and nature-based restoration, where people establish the physical conditions but leave natural processes to determine much of what develops afterwards.

Marker Wadden has therefore become less a story about returning Markermeer to an earlier state and more an experiment in creating new ecological space. The islands have attracted breeding and migratory birds, provided sheltered waters used by fish, and developed food webs that differ from those in the open lake. Their long-term influence on Markermeer remains uncertain, but the project has shown that dredged sediment can be transformed into a changing landscape capable of supporting wildlife in a lake whose shores and waters have been extensively engineered.