Since the early 1980s, more or less widespread hypoxia has been observed nearly every autumn in Danish coastal waters. The causes of hypoxia in Denmark are related to increased nutrient loads and natural physical factors (e.g. weather) affecting the timing and duration of hypoxia.
The most severe and widespread open waters hypoxia occurred in 2002 when a combination of unusually high winter precipitation and unusually calm, warm weather in late summer and autumn. Hypoxic events in Danish estuaries are not in phase with the open waters; the most severe hypoxic episode in estuaries was reported in 1997 when parts of the Mariager Fjord turned completely anoxic. Roskilde Fjord has been particularly negative affected by the coastal Kattegat hypoxia.
The response of marine benthic macrofaunal communities to seasonal hypoxia in coastal Denmark is a collapse or mass mortality.