Holocene development of the Arresø area, north-east Sjælland, Denmark

The Arresø area in north-east Sjælland, Denmark, was deglaciated about 18,000 to 16,000 years ago. In the Holocene it was probably a land area until it was transgressed by the sea c. 8500 years BP. During a first marine phase the area housed a species-rich marine fauna that included the oyster Ostrea edulis, the salinity and water temperatures were higher than at present, and there was a wide connection to the Kattegat sea. At about 6500 years BP there was a short-lived lake or brackish-water phase, but marine conditions were soon re-established with a fauna less diverse than before, and both salinity and water temperatures decreased. The present lake Arresø became isolated from the sea about 2500 years BP. The transition from brackish water to fresh water was rapid; the lake developed from shallow alkaline waters to deeper more acidic waters and finally to eutrophic waters.