Anoxic waters

Anoxic waters are areas in sea water, fresh water, or groundwater that have very little or no dissolved oxygen. Unlike hypoxic waters, which still have small amounts of oxygen, anoxic waters have almost none at all. This usually happens in places where water doesn’t move much and layers of water stay separated, often because of differences in temperature or saltiness.

Oxygen can’t easily reach deeper layers from the sunlit parts of the water because of physical barriers or strong stratifications. Anoxic conditions happen when tiny organisms break down organic material and use up the oxygen faster than it can be replaced. This is a natural process that has happened many times throughout Earth’s history.

Anoxic waters have played a role in major events, like the Permian–Triassic extinction event, which may have been worsened by widespread anoxic conditions in the oceans. Today, anoxic areas can be found in places like the Baltic Sea, the Black Sea, and the Gulf of Mexico, sometimes growing larger because of extra nutrients causing eutrophication.

Causes and effects

Anoxic conditions happen when there is very little or no oxygen in water. This can occur in deep parts of the ocean or lakes where water does not mix well. When oxygen runs out, tiny organisms called bacteria change the way they get energy. They use other substances instead, like nitrate, which can turn into a gas called nitrogen. Later, they may use sulfate, creating hydrogen sulfide, a chemical that smells like rotten eggs.

The amount of oxygen in water can drop when many living things use it to break down organic matter, like dead plants and animals. This is called biochemical oxygen demand. When there is a lot of this demand, oxygen levels fall, making it hard for fish and other larger creatures to survive. This often happens in places where lots of organic material ends up, such as muddy ocean bottoms or polluted rivers.

Human caused anoxic conditions

Eutrophication, a type of nutrient pollution from agricultural runoff and sewage discharge, can cause large algal blooms. When the algae die and sink, their decomposition uses up oxygen. This creates areas with very little oxygen, called hypoxia. This can harm fish and other water animals.

Changes in the environment, like eutrophication or global warming, can cause shifts from areas with normal oxygen levels to anoxic conditions. These changes can happen quickly and affect tiny plants and bacteria in the water.

Daily and seasonal cycles

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

Plants and algae make oxygen during the day with sunlight. But at night, without sunlight, they stop making oxygen. This can cause oxygen levels to drop even more.

Biological adaptation

Different animals and plants react in different ways to eutrophication. Some tiny water plants can grow even when there is no oxygen. But most animals and plants need oxygen to live, so when there is very little oxygen, many of them cannot survive there. This means fewer kinds of plants and animals can be found in such places.

Some creatures, like a type of soft coral called Xenia umbellata, can live for a short time without much oxygen. But after a few weeks, most of them get weaker. Another example is the Sydney cockle, Anadara trapezia, which has trouble moving and can die when there are too many nutrients in the water. Scientists study many animals to learn how they deal with low oxygen. Some can bring oxygen from above into the mud, while others move very slowly or have special ways to live without much oxygen. But when there is too much extra food in the water, most animals and plants cannot thrive, and there are fewer kinds of life.

Anoxic basins

Anoxic basins are special areas in the ocean and other water bodies where there is almost no oxygen. These places usually happen where water doesn't mix well and stays still for long periods.

Some famous anoxic basins include the Bannock Basin in the Levantine Sea, 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 off Denmark, the Orca Basin in the northeast Gulf of Mexico, and Saanich Inlet near Vancouver Island, Canada.