water, with creeks and swamps of clear water. There was initial subsidence of the sediment (1.7 m), mostly in the first three months after filling, but this continued for 2.5 years.
Silt has accumulated on the sheltered side of the archipelago, which can be used for future maintenance. The islands are protected from stormy weather by the sandy edges. However, some measurements revealed that the sand can be lost laterally, causing a landward shift of the islands.
Biological development
Reed Phragmites communis was established manually through sowing, planting rhizomes and spreading grass clippings. Reed rooted rapidly and had a strengthening effect on the soil. The pioneer plants Marsh Ragwort Tephroseris palustris and Red Goosefoot Oxybasis rubra developed almost immediately on the shallow tidal flats. Geese were able to walk on the dried crust layer within a few weeks. Submerged aquatic vegetation developed after roughly one year, including pondweeds Potamogeton spp., stoneworts Chara spp . and Eurasian Watermilfoil Myriophyllum spicatum . In 2020 and 2021, four years after construction began, there were 43 and 47 species of breeding birds on the islands, respectively. Birds that breed on bare sand were the first to settle, for example Common Tern Sterna hirundo , Avocets Recurvirostra avosetta and Kentish Plover Anarhynchus alexandrinus (rare). Passerines and ducks were also recorded, including the first breeding pair of Long-tailed Ducks Clangula hyemalis in The Netherlands. The islands act as stepping stones between nearby marshland.
Nutrients released from the dredged material led to high densities of filamentous sulphur bacteria Thioloca spp. relative to the surrounding area. Their role in the food web is unclear.
Sources: KIMA (2022); Video: Marker Wadden – Positive impetus to the ecology of the Markermeer Lake (youtube.com/watch?v=3I0lJhZdUOc).
5. Other sources of information
Documents
Guidance on working with dredged material in coastal environments: Manning W., Scott C. & Leegwater E. (2021) Restoring Estuarine and Coastal Habitats with Dredged Sediments: A Handbook. Environment Agency: Bristol, UK. Available at: https://catchmentbasedapproach .org/learn/restoring-estuarine-and-coastal-habitats-with-dredged-sediment/ Guidance on habitat management for roosting shorebirds: Jackson M.V. & Straw P. (eds.) (2021) Coastal Hightide Shorebird Habitat Management Guidelines . Figshare. https://doi.org/ 10.6084/m9.figshare.16628560.v1 Review of factors affecting how birds use wetlands: Ma Z., Cai Y., Li B. & Chen J. (2010) Managing wetland habitats for waterbirds: an international perspective. Wetlands , 30 , 15 – 27. https://doi.org/10.1007/s13157-009-0001-6
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