Mariager Fjord, Denmark

The Mariager Fjord on the eastern coast of Denmark is about 40 km long and 2 km wide. Its central basin close to the city of Mariager is 30 m deep; the volume of the central part of the fjord is 1.37 x 108 m3 . Like the Black Sea the fjord has a permanently stratified sulphidic water column. This is due to a sill, which separates the fjord from the Baltic Sea. This sill is only about 2 m deep except of a ca. 4 m deep channel, which enables navigation on the fjord. Due to the sill water exchange between the deeper inner fjord and the Baltic Sea is limited. The average residence time of water in the upper layers (surface to 16 m) takes about 8 months. The upper water layers are affected by the input of freshwater from the terrestrial surrounding, the lower layers are build up from sea water. So there is a density gradient between a diluted surface salinity and deeper heavier layers with a higher salinity. A chemocline between the two layers exists. During the affection of predominantly westerly winds a netto eflux of surface water into the Baltic Sea increases a reflux of more saline water in the deeper layers of the water column back in the fjord. The density stratification of the fjord inhibits mixing and the decomposition of organic matter falling into the stratified water column quickly consumes all available oxygen. The system is anoxic below the chemocline. In the summer sometimes the stratification of the fjord achieves the surface waters. As a consequence major fish kills have been reported in the past. Whereas the water can be completely mixed up down the bottom of the fjord as a result of high wind velocities and high waving course in the winter months. We sample the fjord in collaboration with Arhus Amt, Denmark (special thanks to Captain Uffe Frisenette).