Gullmarsfjord has a maximum depth of 118m with stagnant bottom water which is usually renewed with oxygen-rich deep water every spring. Hypoxia in Gullmarsfjord is episodic with a clear seasonal trend. Hypoxia evolves in autumnn and winter. It has caused mass mortality for instance in the winter of 1979-1980.

In 1997-1998 the hypoxic period was longer than ever before reported due to failed bottom water exchange in 1997. Since the water exchange didn't occur, the dissolved oxygen content continued to decrease. The fauna died at depths below 100m and was significantly reduced in depths between 80-100m. In 1998 the oxygen content slightly increased in January and a major bottom water renewal took place again in the spring.