Eutrophication refers to excessive nutrients and minerals such as nitrogen and phosphate in a body of water. Often caused by runoff from land into water, this excess in nutrients leads to algae blooms, in turn depleting oxygen levels in the water, which can result in structural changes to the ecosystem, including the death of marine life. Often referred to as ‘dead zones’ due to the depletion of water quality and fish species in affected areas, eutrophication is becoming increasingly problematic around the world. 

How is eutrophication related to our use of animals for food and fibre?

Crops grown for animal feed 

The animals that humans raise for food are primarily grown in intensive farming operations, including factory farms and feedlots. These animals are fed crops like soy and corn; in fact, as much as 36% of crop calories worldwide are fed to livestock [ref]. 

Between the 1960s and 1990s there was a steep upsurgence in the use of synthetic nitrogen and phosphorus fertilizers, which studies have found are often applied to crops in excess [ref]. This excess in nutrients is lost through surface run-off, leaching into groundwater and through volatilization, which refers to nitrogen vaporizing in the atmosphere in the form of ammonia [ref]. A percentage of volatilized ammonia returns to waterways through atmospheric deposition. Phosphorus binds to soil, and is generally lost through soil erosion on agricultural land [ref].

Essentially, the crops grown to feed the animals that people eat and use for clothing contributes to eutrophication of water. 

Animal manure from CAFOS

The industrialisation of farming animals has seen operations intensify; globally approximately 70 billion land animals are raised for food and fibre each year. Unsurprisingly, this massive number of animals produce huge amounts of waste. Animal waste is used as fertiliser for plant crops, stacked in factory-farms and feedlots, or stored in large lagoons on farming properties. Across the US, farm animals produce three times as much waste as humans [ref].

This manure enters waterways through runoff and leaching, contributing hugely to eutrophication.


Fish farming is becoming increasingly problematic with regards to eutrophication, generating a considerable amount of phosphorus and nitrogen from unconsumed food, excrement and other organic waste [ref]. Fish farms are mostly situated in nets and pens in enclosed bays, posing risks to surrounding ecosystems due to nutrient waste being directly discharged into surrounding waters. From 1960 to 2015 fish farming increased 50-fold, producing over 100 million tonnes of fish each year [ref]