Anoxic waters

Anoxic waters are areas in sea water, fresh water, or groundwater where there is very little or no dissolved oxygen. This lack of oxygen occurs in places where water does not move much and layers of water stay separated. Without oxygen, only special tiny living things can survive there.

Anoxic waters happen naturally and have been around for a very long time. They have even helped cause big changes in Earth's history. For example, a huge loss of ocean life long ago might have been linked to widespread anoxic conditions in the oceans.

Many places in the world have areas with anoxic water, like parts of the Baltic Sea, the Black Sea, and some deep valleys called fjords. In some places, extra nutrients entering the water can make these oxygen-poor areas even larger.

Causes and effects

Anoxic waters happen when there is very little oxygen in the water. This can occur because of natural conditions like layers of water that don’t mix well, or because a lot of plant and animal matter ends up in the water. When oxygen runs out, special kinds of tiny living things change the nitrogen and sulfur in the water. This creates a smell like rotten eggs and can turn the water and mud a dark color.

The amount of oxygen in water can also change based on what lives there, the temperature, and how much plant and animal matter is around. When there isn’t enough oxygen, bigger animals and plants can get stressed because they need oxygen to live.

Human caused anoxic conditions

When too many nutrients, like phosphates and nitrates, enter water from farms and sewage, they can cause lots of algae to grow quickly. This is called eutrophication. Algae make oxygen during the day, but at night they use up oxygen. When the algae die, their decay also uses up oxygen. This leaves less oxygen for fish and other water animals. This can create areas where there isn’t enough oxygen for many sea creatures to survive, called a dead zone. A famous example is in the Gulf of Mexico.

Changes in the environment, such as global warming, can also cause water to lose its oxygen. These changes can happen fast, going from water full of oxygen to water with very little oxygen. This affects the tiny plants and animals that live there.

Daily and seasonal cycles

The temperature of water changes how much oxygen it can hold. When water gets warmer, it can hold less oxygen. This means that during the hottest parts of the day and in the summer, water can become low in oxygen.

Plants and algae that make their own food also affect oxygen levels. At night, when there is no sunlight, these plants stop making oxygen, which can make the water even lower in oxygen. This usually gets worst a little after sunrise.

Biological adaptation

When water has very little oxygen, most sea creatures have a hard time living there. Some small plants and other simple organisms can survive, but many animals cannot. For example, a type of soft coral named Xenia umbellata can live with low oxygen for a short time, but after three weeks, many of them start to shrink and change color.

Scientists have studied many sea animals and found that each one reacts differently to low-oxygen water. Some animals can get oxygen from the surface to where they live, while others move more slowly or work with special tiny living things to survive. However, when there are too many nutrients in the water, most animals leave or cannot live there anymore, leading to fewer types of creatures in that area.

Anoxic basins

Anoxic basins are special areas in the ocean and other water bodies where there is very little or no oxygen. These places usually happen where water doesn’t mix well and layers stay separated.

Some well-known anoxic basins include the Bannock Basin in the Levantine Sea, Eastern Mediterranean; the Black Sea Basin below 50 metres; the Caspian Sea Basin below 100 metres; the Cariaco Basin off Venezuela; the Gotland Deep in the Baltic near Sweden; the L'Atalante basin in the eastern Mediterranean Sea; Mariager Fjord near Denmark; the Orca Basin in the northeast Gulf of Mexico; and Saanich Inlet near Vancouver Island, Canada.