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 fjord is 10–30 m deep and the water in the deep basis at Mariager is nearly permanently anoxic below 15 m. The outer part consists of tidal plains with water depths around 1 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 environmental conditions, the mussel Mytilus edulis covers the slopes and banks between 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 from the air was therefore very limited. At some point, the limited oxygen input could not meet the musselsʹ and other organismsʹ oxygen demand. The Inner 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 illustrated and discussed. The revival of the fjord ecosystem is commented upon, based on the first results of the ongoing investigations.