In coastal waters, nitrogen and phosphorus fuel large, often harmful, algal blooms. When the algae die, sink to the seabed and decompose, oxygen is consumed. This oxygen consumption is often so high that the oxygen supply through downward mixing of surface water is insufficient. The deeper water runs out of oxygen, and the further decay of the algae releases hydrogen sulphide - making the situation worse.
Climate change leads to a further deterioration: the warmer the seawater, the less oxygen dissolves in it. Vertical mixing will also become more difficult, because warmer water is lighter than cold water and, therefore, will float on the surface. Increased discharge of river water, which often occurs in warmer climates, can increase the contrast in density between (fresher) surface and (saltier) deeper water. Both changes enhance the vertical stratification of the water that is naturally present and prevent the supply of oxygen to the seafloor.
This causes persistent problems because recovery from low oxygen in coastal areas is often slow. In the case of the Gulf of Mexico dead zone, it has turned out to be particularly difficult to reduce the nutrient loads in the Mississippi river. But even when this is successful, the problem is not necessarily directly solved.
Almost forty years ago, around 1980, similar efforts to reduce the nutrient supply began in the Baltic Sea. But the water quality has hardly improved. This is partly due to the long retention time of nutrients in the sea. Phosphate, for example, disappears by burial in the seabed through a relatively slow process.