Anoxic water refers to aquatic environments where dissolved oxygen is extremely low or completely absent. This condition occurs in various bodies of water, including oceans, freshwater lakes, and groundwater.
Understanding Anoxic Water
Anoxic water is characterized by dissolved oxygen concentrations typically less than 0.5 milligrams per liter (mg/L). This contrasts with hypoxic conditions, which represent low but not entirely absent oxygen levels, often defined as less than 2 mg/L.
When oxygen is depleted, bacteria turn to other electron acceptors, leading to the production of compounds like hydrogen sulfide (H2S). Hydrogen sulfide is a poisonous gas with a characteristic rotten egg odor, often associated with black mud and sediments. The presence of hydrogen sulfide and other reduced substances, such as iron and manganese, can also impact water quality.
How Anoxic Conditions Form
Anoxic conditions frequently arise from natural processes and human-influenced factors. A primary contributor is eutrophication, where excess nutrient runoff, often from agricultural activities and wastewater, enters water bodies. These nutrients fuel rapid algal growth, known as blooms, which eventually die and sink. Their decomposition by bacteria consumes dissolved oxygen from the water, leading to anoxia.
Thermal stratification also plays a role, particularly in lakes and deep basins. This occurs when layers of water with different temperatures form, preventing proper mixing and oxygen replenishment to deeper layers. Warmer, less dense water stays at the surface, while colder, denser water at the bottom becomes isolated, allowing oxygen to be depleted without being resupplied.
Physical barriers, such as sills at the entrance of fjords or specific geological formations, can restrict water circulation. This limitation prevents oxygenated water from reaching deeper areas, exacerbating oxygen depletion. Sediment oxygen demand, the consumption of oxygen by microorganisms degrading organic matter on the seabed, also contributes to anoxic conditions.
The Impact on Aquatic Life and Ecosystems
Anoxic water affects aquatic life and ecosystem health. The lack of dissolved oxygen leads to the mortality of most aerobic organisms, including fish, shellfish, and benthic invertebrates. Mobile organisms, like many fish species, may attempt to escape these low-oxygen zones, but less mobile species are often unable to flee and perish.
These oxygen-depleted areas are often referred to as “dead zones” because they become largely devoid of life. Dead zones disrupt aquatic food webs, as species reliant on affected organisms for food are impacted. Only certain anaerobic bacteria or highly tolerant species can survive, leading to a reduction in biodiversity and shifts in species composition.
Anoxic conditions can trigger the release of harmful substances from sediments into the water column. Hydrogen sulfide, a byproduct of anaerobic decomposition, is toxic to most aquatic life. The release of other compounds like phosphates, ammonia, iron, and manganese can degrade water quality and exacerbate environmental problems.
Global Occurrence and Significance
Anoxic zones, commonly known as dead zones, are found globally in marine and freshwater environments. Examples include the recurring dead zone in the Gulf of Mexico, influenced by nutrient runoff from the Mississippi River. Other anoxic areas are found in the Baltic Sea and the Black Sea, where restricted circulation contributes to oxygen depletion.
These zones are not isolated occurrences but represent a widespread environmental concern. The number and size of dead zones have increased over the past several decades, with over 400 systems reported worldwide. The largest known dead zone is found in the Arabian Sea, specifically the Gulf of Oman.