Life, death and revival of the hypertrophic Mariager Fjord, Denmark

The bottom water in the Danish Mariager Fjord is normally anoxic but for 2 weeks in 1997, the water column became anoxic to the surface. The fjord is about 40 km long and mostly less than 2 km wide. The inner 17 km of the fjo rd is 10-30 m deep and the water in the deep basis at Mariager is nearly pe rmanently anoxic below 15 m. The outer part consists of tidal. plains with water depths around I m bordering a narrow, winding, 6-m deep channel held open by the tide. The Inner Fjord is presently hypertrophic with a primary production in the range of 800-900 g C m(-2) year(-1). Under normal environ mental conditions, the mussel Mytilus edulis covers the slopes and banks be tween 1 and 8-10 m depths. In 1997, the summer in Denmark was unusually warm, sunny and calm. The wind induced very little mixing of the surface water and the input of oxygen fr om the air was therefore very limited. At some point, the limited oxygen in put could not meet the mussels' and other organisms' oxygen demand. The Inn er Fjord became anoxic and sulphidic to the very surface for a distance of 20 km. The aerobic flora and fauna died and added to the oxygen demand. The general conditions and special events resulting in the total anoxia are il lustrated and discussed. The revival of the fjord ecosystem is commented up on, based on the first results of the ongoing investigations. (C) 2000 Else vier Science B.V. All rights reserved.