Oxygen sustains life for the creatures of this earth, whether they live on the land or in the ocean. For our oceans, events both natural and man made can occur which cause the levels of oxygen to drop to such an extent that life for many creatures may end swiftly.
These areas, where the levels of oxygen are depleted so badly that most marine species cannot survive, are called "dead zones." They are often seasonal, and are often caused by eutrophication - the pollution of water by nitrogen phosphorous-rich run-off from fertilizers used in agriculture. The nitrogen causes massive algal blooms, which eventually die and are broken down by bacteria. The bacteria consume large amounts of oxygen, which leads to hypoxia - dissolved oxygen depletion - where the area becomes so depleted of dissolved oxygen that much of the marine life simply cannot survive, and quickly dies. The image below shows an algal bloom in the Baltic Sea.
The effects on marine life can be swift and widespread. The low levels of oxygen kill not only mature fish, but also their eggs and larvae. Other marine life such as crustaceans are also killed.
Hypoxia can also occur without pollutants. Freshwater flowing from a river into the sea can reduce the oxygen flow from the surface waters to the lower layer of salt water. The oxygen levels may be reduced to an extent that hypoxia occurs. This can happen in the case of excessive freshwater flows, such as following a flood. Areas that are susceptible to this type of hypoxia include Waddenzee (Netherlands) and the Gulf of Mexico. The largest natural dead zone is the Black Sea (see image above) below a depth of 150 metres. The water flowing between the Black Sea and the Mediterranean Sea is limited through the narrow Bosphorous (sometimes known as the Istanbul Strait), causing the mixing of freshwater and saltwater to take place close to the surface. From 150 metres to 2,000 metres the water is entirely anoxic.
The World Resources Institute (WRI) and the Virginia Institute of Marine Science (VIMS) have identified more than 530 hypoxic and anoxic dead zones, as well as 228 sites worldwide showing signs of marine eutrophication (see the image below for three such areas). They have produced an interactive map that shows the areas they have identified as experiencing hypoxia, eutrophication or showing signs of eutrophication.
The map can be seen here.