Anoxic waters are areas of

sea water Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approx ...

fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...

, or

groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit ...

that are depleted of

dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It ca ...

. The

US Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and ...

defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. Anoxic waters can be contrasted with hypoxic waters, which are low (but not lacking) in dissolved oxygen. This condition is generally found in areas that have restricted water exchange. In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier, as well as by a pronounced density stratification, in which, for instance, heavier hypersaline waters rest at the bottom of a basin. Anoxic conditions will occur if the rate of

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...

of organic matter by

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

is greater than the supply of

. Anoxic waters are a natural phenomenon, and have occurred throughout geological history. The

Permian–Triassic extinction event The Permian–Triassic (P–T, P–Tr) extinction event, also known as the Latest Permian extinction event, the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, ...

, a mass extinction of species from the world's oceans, may have resulted from widespread anoxic conditions combined with ocean acidification driven by a massive release of carbon dioxide into Earth's atmosphere. Many lakes have a permanent or temporary anoxic layer created by respiration depleting oxygen at depth and thermal stratification preventing its resupply. Anoxic basins exist in the

Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain. The sea stretches from 53°N to 66°N latitude and from ...

, the

Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Rom ...

, the Cariaco Trench, various fjord valleys, and elsewhere.

Eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phyt ...

has likely increased the extent of anoxic zones in areas including the Baltic Sea, the

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...

, and

Hood Canal Hood Canal is a fjord forming the western lobe, and one of the four main basins,density stratification, inputs of organic material or other reducing agents, and physical barriers to water circulation. In fjords, shallow sills at the entrance may prevent circulation, while at continental boundaries, circulation may be especially low while organic material input from production at upper levels is exceptionally high. In

wastewater treatment Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environm ...

, the absence of oxygen alone is indicated ''anoxic'' while the term

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

is used to indicate the absence of any common electron acceptor such as

nitrate Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insolu ...

sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...

or oxygen. When oxygen is depleted in a basin, bacteria first turn to the second-best electron acceptor, which in sea water, is

Denitrification Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denit ...

occurs, and the nitrate will be consumed rather rapidly. After reducing some other minor elements, the bacteria will turn to reducing

. This results in the byproduct of

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The under ...

(H₂S), a chemical toxic to most biota and responsible for the characteristic "rotten egg" smell and dark black sediment color: :2 CH₂O + → 2 + H₂S + chemical energy These sulfides will mostly be oxidized to either sulfates (~90%) in more oxygen-rich water or precipitated and converted into

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...

(~10%), according to the following chemical equations: Some chemolithotrophs can also facilitate the oxidation of hydrogen sulfide into elemental

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

, according to the following chemical equation: :H₂S + O₂ → S + H₂O₂ Anoxia is quite common in muddy ocean bottoms where there are both high amounts of organic matter and low levels of inflow of oxygenated water through the sediment. Below a few centimeters from the surface the interstitial water (water between sediment) is oxygen free. Anoxia is further influenced by biochemical oxygen demand (BOD), which is the amount of oxygen used by marine organisms in the process of breaking down organic matter. BOD is influenced by the type of organisms present, the pH of the water, temperature, and the type of organic matter present in the area. BOD is directly related to the amount of dissolved oxygen available, especially in smaller bodies of water such as rivers and streams. As BOD increases, available oxygen decreases. This causes stress on larger organisms. BOD comes from natural and anthropogenic sources, including: dead organisms, manure, wastewater, and urban runoff.

Human caused anoxic conditions

, an influx of nutrients (phosphate/nitrate), often a byproduct of agricultural run-off and sewage discharge, can result in large but short-lived algae blooms. Upon a bloom's conclusion, the dead algae sink to the bottom and are broken down until all oxygen is expended. Such a case is the Gulf of Mexico where a seasonal dead zone occurs, which can be disturbed by weather patterns such as hurricanes and tropical convection. Sewage discharge, specifically that of nutrient concentrated "sludge", can be especially damaging to ecosystem diversity. Species sensitive to anoxic conditions are replaced by fewer hardier species, reducing the overall variability of the affected area. Gradual environmental changes through eutrophication or

global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

can cause major oxic-anoxic regime shifts. Based on model studies this can occur abruptly, with a transition between an oxic state dominated by

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

, and an anoxic state with sulfate-reducing bacteria and phototrophic sulfur bacteria.

Daily and seasonal cycles

The temperature of a body of water directly affects the amount of dissolved oxygen it can hold. Following Henry's law, as water becomes warmer, oxygen becomes less soluble in it. This property leads to daily anoxic cycles on small geographic scales and seasonal cycles of anoxia on larger scales. Thus, bodies of water are more vulnerable to anoxic conditions during the warmest period of the day and during summer months. This problem can be further exacerbated in the vicinity of industrial discharge where warm water used to cool machinery is less able to hold oxygen than the basin to which it is released. Daily cycles are also influenced by the activity of photosynthetic organisms. The lack of photosynthesis during nighttime hours in the absence of light can result in anoxic conditions intensifying throughout the night with a maximum shortly after sunrise.

Biological adaptation

Individual species’ reactions to eutrophication can vary widely. For example, some organisms, such as primary producers, can adapt very quickly and even thrive under anoxic conditions. However, most organisms are extremely susceptible to slight changes in aquatic oxygen levels. Simply put - if a respirating organism is presented with little to no oxygen, its chances of survival will decrease. Therefore, eutrophication and anoxic conditions in water leads to a decrease in biodiversity. For example, the soft coral ''Xenia umbellata'' can resist some anoxic conditions for short periods of time, but after about 3 weeks, mean survival decreases to about 81% and about 40% of surviving species experience size reductions, lessening in coloration, and compromised pinnate structures (Simancas-Giraldo et al., 2021). Another example of a susceptible organism is observed with The Sydney Cockle, ''Anadara trapezia''. Enriched sediments have lethal and sublethal effects on this Cockle and, as stated in adillo Gonzalez et al., 2021 “movement of cockles was reduced in enriched sediments compared to natural treatments.” These are just a few examples of the hundreds of thousands of aquatic species that exist, but these and other examples show important results. A study collecting over 850 published experiments "reporting oxygen thresholds and/or lethal times for a total of 206 species spanning the full taxonomic range of benthic metazoans." Individual species will have different adaptive responses to anoxic conditions based on their biological makeup and the condition of their habitat. While some are able to pump oxygen from higher water levels down into the sediment, other adaptations include specific hemoglobins for low oxygen environments, slow movement to reduce rate of metabolism, and symbiotic relationships with anaerobic bacteria. In all cases, the prevalence of excess nutrients results in low levels of biologic activity and a lower level of species diversity if the area is not normally anoxic.

Anoxic basins

* Bannock Basin in

Levantine Sea The Levantine Sea (Arabic: بحر الشام, tr, Levanten Denizi, el, Θάλασσα του Λεβάντε) is the easternmost part of the Mediterranean Sea. Geography The Levantine Sea is bordered by Turkey in the north and north-east ...

, eastern Mediterranean Sea; *

Basin, off eastern Europe, below 50 metres (150 feet); *

Caspian Sea The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad steppe of Central A ...

Basin, below 100 metres (300 feet); * Cariaco Basin, off north central

Venezuela Venezuela (; ), officially the Bolivarian Republic of Venezuela ( es, link=no, República Bolivariana de Venezuela), is a country on the northern coast of South America, consisting of a continental landmass and many islands and islets in th ...

; * Gotland Deep, in the Baltic off

Sweden Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic countries, Nordic c ...

; * L'Atalante basin, eastern Mediterranean Sea *

Mariager Fjord With a length of approximately 35 km (22 statute miles), Mariager Fjord is the longest fjord in Jutland, and the 2nd longest fjord (Tied with Ise Fjord on Zealand), behind Roskilde Fjord in Denmark (Excluding the self-governing territory of Gree ...

, off

Denmark ) , song = ( en, "King Christian stood by the lofty mast") , song_type = National and royal anthem , image_map = EU-Denmark.svg , map_caption = , subdivision_type = Sovereign state , subdivision_name = Kingdom of Denmark , establish ...

; * Orca Basin, northeast

; *

Saanich Inlet , image = Saanich Inlet from Gowlland Tod Provincial Park, Vancouver Island, Canada 13.jpg , image_size = 260px , caption = Saanich Inlet from Gowlland Tod Provincial Park , image_bathymetry = , caption_bathymetry = , location = Northwest o ...

, off

Vancouver Island Vancouver Island is an island in the northeastern Pacific Ocean and part of the Canadian province of British Columbia. The island is in length, in width at its widest point, and in total area, while are of land. The island is the largest by ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...

;

References

* * Hallberg, R.O. (1974) "Paleoredox conditions in the Eastern Gotland Basin during the recent centuries". ''Merentutkimuslait''. Julk./Havsforskningsinstitutets Skrift, 238: 3-16. * * Fenchel, Tom & Finlay, Bland J. (1995) ''Ecology and Evolution in Anoxic Worlds'' (Oxford Series in Ecology and Evolution) Oxford University Press. * Richards, F.A. (1965) "Anoxic basins and fjords", in Riley, J.P., and Skirrow, G. (eds) ''Chemical Oceanography'', London, Academic Press, 611-643. * * Sarmiento, J.A. et al. (1988-B) "Ocean Carbon-Cycle Dynamics and Atmospheric pCO2". ''Philosophical Transactions of the Royal Society of London, Series A, Mathematical and Physical Sciences'', Vol. 325, No. 1583, ''Tracers in the Ocean'' (May 25, 1988), pp. 3–21. * . * * * * {{aquatic ecosystem topics Chemical oceanography Aquatic ecology

Human caused anoxic conditions

Daily and seasonal cycles

Biological adaptation

Anoxic basins

See also

References