The so-called ‘dead zones‘ are most often found in shallow waters, such as lakes or shallow sea coasts. However, circulating vortex movement, found in the Atlantic, preventing the exchange of water, so oxygen in them is very quickly consumed.
Dead zones get their name from the fact that they can support very few plants and animals. This is due to the absence of oxygen, known as ‘hypoxia’. Hypoxic conditions can materialise in lakes and other standing bodies of water, but they can also occur in large areas of the ocean.
The main cause of a dead zone is eutrophication – when minerals, usually of agricultural origin, are washed into the water. The heightened nutrient levels stimulate huge blooms of algae on the surface of the water. This then restricts oxygen making its way into the water, which eff ectively smothers everything living beneath it.
Due to the agricultural nature of the nutrient runoff, dead zones often appear close to the shore. There are around 146 known coastal dead zones around the world, with the largest ones located in the Baltic Sea and the Gulf of Mexico.
Several years ago flooding in agricultural areas of the state such as Minnesota and Illinois are washed huge amounts of nitrogen fertilizers in the water of the Gulf of Mexico. These substances have caused an explosion of growth of algae that feed on them and this led to the creation of the largest dead zone ever.
One Danish climate model from 2009 showed that climate change even without fertilizers by the end of the century could ten times increase dead zone areas, and they now make up about two percent of all water. The hot water can absorb smaller amounts of oxygen. At worst dead zone could be extended to one-fifth of sea surface! Read also about Animal Extinction Facts!
How a healthy patch of ocean becomes a hypoxic watery wasteland?
Nutrient runoff – Sun-warmed freshwater runs off the land. Nutrient-loaded water sits on top of seawater, cutting off oxygen to organisms below.
Algal bloom – The nutrients encourage algae growth on the surface. Dead algae also decomposes at depth, using up the limited oxygen levels.
Deep-water hypoxia – Oxygen supplies dwindle and organisms living below can’t survive. Fish can die in shoals and dead matter floats to the surface.
Read also Facts About Dead Sea Salt Lake.