The Markermeer (700 square kilometres) was split off from the IJsselmeer in 1976, when the Houtribdijk dam between Enkhuizen and Lelystad was completed.
Both lakes used to be part of the Zuiderzee, a North Sea inlet dammed off in 1932. Today, the Markermeer has lost most of its natural shores to land reclamation and flood protection measures, and is cut off from the sea and rivers by dams and dikes. In many places a thick layer of silt has accumulated on the lake floor, where it suffocates bottom-dwelling organisms. Moreover, the water of the lake is often extremely turbid due to sediments being churned up by the waves. This reduces the amount of light reaching lower depths and inhibits the growth of algae and water plants. As a result, fish and bird populations have declined dramatically.
Natuurmonumenten has initiated the Marker Wadden project to address these problems. We will construct islands with natural shores, and trap the silt to decrease water turbidity. Suspended sediments will settle on the lee side of the islands. This will greatly improve habitat quality for endangered plants and animals. By restoring the Markermeer we will create a robust ecosystem in the heart of the Netherlands, which will play a key role in the national (and international) ecological network. Moreover, the lake will become much more attractive for water sports and nature lovers. The new nature islands of the Marker Wadden will be accessible to the public.
During the last twenty years, the Markermeer ecosystem has deteriorated dramatically. Research has shown that multiple factors have contributed to this decline. The fact that the Markermeer is no longer flushed by the sea and rivers has resulted in significant siltation of the lake. The thick layer of mud on the lake floor is suffocating bottom-dwelling organisms. Turbidity from suspended sediments has greatly reduced primary production by algae and water plants. Moreover, the lake has barely any natural shores or shallow littoral zones. As a result, many water plants, fish and molluscs such as the zebra mussel are struggling to survive.
The decrease in primary production and reduced invertebrate and fish populations in the Markermeer have led to a food shortage for many water birds. Since the 1980s, birds such as the common pochard, tufted duck and common goldeneye have declined by more than 75%!
One of the main problems of the Markermeer is the thick layer of silt covering the lake floor.
The silt problem in the Markermeer is partly due to the construction of the dam between Lelystad and Enkhuizen. This dam was completed in 1976. The plan at that time was to convert the lake into polder land. However, land reclamation kept being postponed, and in 2006 the Dutch government abandoned the plan altogether. By then the Markermeer had been cut off from the IJsselmeer for more than thirty years. The separation of the two lakes had a major impact on the water quality and ecology of the Markermeer. Before the dam was built, fine sediments were flushed out from the Markermeer by currents transporting the silt to deeper parts of the IJsselmeer. This no longer being the case, a suffocating blanket of silt has now accumulated on the bottom of the Markermeer. Moreover, as the lake is relatively shallow (only 2-4 m deep), waves keep churning up the sediments, leading to high turbidity even at moderate wind speeds.