Why do Danish fjords run out of oxygen?

Mainly because too many nutrients, above all nitrogen, wash into the sea; the weather then decides how bad a given year gets. The nutrients feed algae that sink and break down on the seabed, using up oxygen, and climate change adds to the effect1. Aarhus University's monitoring concludes that this over-fertilisation is what pushes oxygen depletion beyond natural levels, while the weather sets it off2. The Danish environment ministry estimates, as quoted by the national audit office Rigsrevisionen, that just over 70 percent of Denmark's nitrogen discharges come from fertilising farmland3.

Nitrogen inputs to the sea have fallen since 1990, mostly through lower losses from farming4. Yet the oxygen-depleted area in inner Danish waters has grown significantly since 2010, reaching just under 3,700 km² in late September 20255. In mid-September 2026 it was 1,650 km², probably partly because a dry year washed fewer nutrients off the land67. Researchers say nutrient inputs need to fall further8, and in 2024 Rigsrevisionen called the control of farmers' fertiliser use unsatisfactory9.