The IJsselmeer was created by closing off part of the Zuiderzee. The Afsluitdijk was completed on 28 May 1932 and the official name change took place on 20 September 1932. The lake was named after the river IJssel, which flows into it. The part of the former Zuiderzee north of the Afsluitdijk is nowadays part of the Wadden Sea. The Zuiderzee originated in the beginning of the thirteenth century AD, after seawater flooded the Almaere during floods. (For more information about this, see: Zuiderzee origin). The Romans knew the Almaere under the name Flevo Lake. The Wadden Sea is the world’s largest continuous system of sand and mud flats that dry out during low tide. The area consists of most of the Danish protected Wadden Sea area, the Dutch part that is protected and the German National Wadden Sea Parks in Lower Saxony and Schleswig-Holstein. The Wadden Sea is not a very deep water area with a relatively flat coast. The habitat has tidal channels, seagrass meadows, mussel beds, sandbanks, tidal flats, salt marshes, beaches and dunes. A large number of plant and animal species are found, including marine mammals such as the common seal, the gray seal and the porpoise. It is also the breeding ground and the wintering area for 10 to 12 million birds per year. The area is one of the last remaining large-scale low tide ecosystems where natural processes continue to function. The Markermeer is named after the former island of Marken in the southwest of the lake, which is nowadays a peninsula connected to the hinterland. The name Markermeer is related to the earlier planned land reclamation of this area into the Markerwaard, which has since been removed. The remaining water area is now called Markermeer. The name Markermeer has only been used since around 1975. For this, the area was designated as the southwestern part of the IJsselmeer, or as (future) Markerwaard. This southwestern part of the IJsselmeer was separated by the construction of the Houtribdijk or Markerwaarddijk (the Enkhuizen-Lelystad road) in 1976. Since then it could be referred to as Markermeer.