In developed countries, heavy use of animal manure and commercial fertilisers are the main contributors to eutrophication, which runs off from fields into creeks and bays. Fossil fuels also release nitrogen into the atmosphere.

This process has increased because of the rise in intensive agricultural practices, industrial activities and population growth, which all emit large amounts of nitrogen and phosphorus that settle into our air, soil and water. You might also like: 10 of the Most Endangered Species in the Ocean What Causes Eutrophication? Additionally, waters that are shallow and clear enough to allow light to reach the bottom can support primary producers such as phytoplankton, algae and seagrasses that release oxygen during photosynthesis. This is because shallow waters tend to be well-mixed by winds and tides. Shallow waters are less likely to stratify than deep waters, and so are less likely to develop hypoxic conditions. Human activities mainly cause these excess nutrients to be washed into the ocean, which is why dead zones are often located near inhabited coastlines. If stratification of the water column (when water masses with different properties form layers that prevent water mixing) occurs, these waters will remain oxygen poor. When the algae die and sink to the bottom of the water bed, they provide a rich food source for bacteria, which when decomposing consume dissolved oxygen from surrounding waters, depleting the supply of marine life. With too many nutrients, however, cyanobacteria grow out of control, which can be harmful. At normal levels, an organism called cyanobacteria – or blue-green algae – feeds on these nutrients. What are dead zones, how many are there in the world and how can they be prevented?Ī dead zone occurs as a result of eutrophication, which happens when a body of water is inundated with too many nutrients, such as phosphorus and nitrogen.

Hypoxic zones can occur naturally, but human activities can also lead to the creation of new dead zones or the enhancement of existing ones. The term “dead zone” or “hypoxia” refers to low-oxygen areas in the world’s lakes and oceans and is so called because very few organisms can survive in hypoxic conditions.

The dead zone