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, one of the biggest mass extinctions in history, 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 when water cannot mix well, such as in deep parts of the ocean or lakes. 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 and can harm other living things.

The amount of oxygen in water can also drop because many living things are using it up 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 algae die and sink, their decomposition uses up oxygen, creating areas with very little oxygen, called hypoxia. This can lead to mass die-offs of fish and other aquatic 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 suddenly, affecting the types of tiny plants and bacteria that live in the water.

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 anoxic, which means it has very little oxygen.

Plants and algae can also affect oxygen levels. They make oxygen during the day when there is sunlight, but at night, without sunlight, they stop making oxygen. This can cause oxygen levels to drop even more during the night.

Biological adaptation

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

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