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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.