Eutrophication: when waters are over-fertilised

In short: Eutrophication means that a body of water contains too many nutrients, above all nitrogen and phosphorus. Algae then grow in huge numbers, and the water may later run out of oxygen.

In more detail: The extra nutrients usually come from fertiliser, slurry and sewage. When the many algae die, they sink to the bottom and are broken down by bacteria, which use up a lot of oxygen in the process. In the worst case a lake “dies”.

In detail

• The term: The word comes from Greek and means roughly “well nourished”. Slow natural nutrient enrichment also exists, but humans speed it up greatly.
• Sources: Important sources are fertiliser washed off fields, slurry, untreated or poorly treated sewage and, in the past, phosphate-containing detergents.
• Algal bloom: With plenty of light and warmth, algae multiply explosively. The water turns green and cloudy, light barely reaches deeper layers, and water plants on the bottom die off.
• Lack of oxygen: Breaking down the dead algae uses up oxygen. Fish and other animals suffocate, and toxic substances such as hydrogen sulphide can form on the bottom.
• Blue-green algae: Over-fertilisation often lets cyanobacteria, also called blue-green algae, multiply. Some produce toxins, which is why bathing spots are sometimes closed.
• Dead zones: Seas are affected too. In parts of the Baltic Sea and off large river mouths there are oxygen-poor zones where hardly any life is possible.
• Countermeasures: Sewage works that remove phosphate, phosphate-free detergents, fertilising only as much as needed and green strips along banks reduce nutrient input.
• Recovery takes time: Nutrients settle in the mud and are released again for a long time. An over-fertilised lake can therefore often only recover after years.
• Connections: Nutrient cycles and ecosystems are closely linked (stickstoffkreislauf, nahrungskette-und-oekosystem).