Anoxic waters arebodies or areas ofsea water,fresh water orgroundwater that are depleted ofdissolved oxygen. Anoxic waters can be contrasted withhypoxic waters, which are low but not completely lacking in dissolved oxygen, often defined as having oxygenconcentration less than 2 milligrams per litre.[1] This condition is generally found in areas that havewater stagnation and/ordeadstratification. TheUnited States Geological Survey defines anoxic groundwater as that with a dissolved oxygen concentration of less than 0.5 milligrams per litre.[2]
In most cases, oxygendiffusion is prevented from the shallowerphotic zone to deeper levels by a physical barrier,[3] as well as by pronounced stratifications due totemperature orsalinity, in which, for instance, denser, colder orhypersaline waters rest at the bottom of a basin. Anoxic conditions will occur if the rate ofoxidativedecomposition (aerobic digestion) oforganicdetritus byaerobicmicroorganisms (mainlyfungi and aerobicbacteria) is greater than fresh supply of dissolved oxygen.
Anoxic waters are anatural phenomenon[4] and have occurred throughoutEarth's geological history. ThePermian–Triassic extinction event, amass extinction that wiped out most species from the world's oceans, may have resulted from widespread anoxic conditions combined withocean acidification driven by a massive release ofvolcaniccarbon dioxide intoEarth's atmosphere by thesupereruption ofSiberian Traps,[5] which concurrently also devastated theterrestrialvegetations responsible forphotosyntheticallyremoving carbon from the atmosphere. Many lakes have a permanent or temporary anoxic layer created by biotic aerobic respiration depleting oxygen at depth and thermal stratification preventing its replenishment to the depth.[6]
Anoxic basins exist in theBaltic Sea,[7] theBlack Sea, theCariaco Trench, variousfjord valleys, and elsewhere.[8]Eutrophication has likely increased the extent of anoxic zones in areas including the Baltic Sea, theGulf of Mexico,[9] andHood Canal inWashington State.[10]
Anoxic conditions result from a combination of environmental conditions, includingdensity stratification,[11] inputs of organic material or otherreducing agents, and physical barriers to water circulation. In fjords, shallowsills at the entrance may prevent circulation. In contrast, at continental boundaries, the circulation may be especially low while the organic material input from production at upper levels is exceptionally high.[12] Inwastewater treatment, the absence of oxygen alone is indicatedanoxic while the termanaerobic is used to indicate the absence of any common electron acceptor such asnitrate,sulfate or oxygen.
When oxygen is depleted in a basin, bacteria first turn to the second-best electron acceptor, which in seawater isnitrate.Denitrification occurs, and the nitrate will be consumed relatively rapidly. After reducing some other minor elements, the bacteria will turn toreducingsulfate. This results in the byproduct ofhydrogen sulfide (H2S), a chemical toxic to most biota and responsible for the characteristic "rotten egg" smell and dark black sediment color:[13][14]
These sulfides will mostly be oxidized to eithersulfates (~90%) in more oxygen-rich water or precipitated and converted intopyrite (~10%), according to the following chemical equations:[14]
Somechemolithotrophs can also facilitate the oxidation of hydrogen sulfide into elementalsulfur, according to the following chemical equation:[15]
Anoxia is quite common in muddy ocean bottoms where there are both high amounts oforganic matter and low levels of inflow of oxygenated water through the sediment. Below a few centimetres from the surface, the interstitial water (pore water between sediment grains) is oxygen-free.
Anoxia is further influenced bybiochemical oxygen demand (BOD), which represents the amount of oxygen utilised by marine organisms during 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.[16]
Eutrophication, a type ofnutrient pollution (typicallyphosphates andnitrates) often as byproduct ofagricultural runoff andsewage discharge, can result in large-scale but short-livedalgal blooms. Althoughalgae arephotosyntheticautotrophs, themetabolic consumption of oxygen in rapidly proliferating algal populations within a relatively small area of water will quickly overwhelm anyoxygen production, anddecomposition of dead algal biomass that sink to the bottom further expends oxygen, creating expanding areas ofhypoxia. When the localoxygen saturation drops too low to sustain the metabolic demands of other aquatic organisms,mass die-offs often occur, especially among more metabolically activenektonicaquatic animals such asfish. A notable example of suchharmful algal blooms is theGulf of Mexico, where a seasonaldead zone forms, which can be disrupted by weather patterns such as hurricanes and tropical convection. Sewage discharge, specifically that of nutrient-concentrated "sludge", can be especially damaging to ecosystem diversity. Largefish kills often happen to species sensitive to hypoxic conditions, who are replaced by fewer variety of hardier species, and these changes inecological niches will spread up and down thefood web, reducing the overallbiodiversity of the affectedaquatic ecosystem.[13][page needed]
Gradual environmental changes through eutrophication orglobal warming can cause major oxic-anoxic regime shifts. Based on model studies, this can occur abruptly, with a transition between an oxic state dominated bychlorophyll-bearingcyanobacteria, and an anoxic state withanoxygenic,sulfate-reducingpurple bacteria andarchaea.[17]
The temperature of a body of water directly affects the amount ofdissolved oxygen it can hold. FollowingHenry's law, as water becomes warmer, oxygensolubility decreases. 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.
The activity ofphotosynthetic organisms also influences daily cycles. The lack ofphotosynthesis during nighttime hours in the absence of light can result in anoxic conditions intensifying throughout the night, with a maximum shortly after sunrise.[18]
The reactions of individual species toeutrophication can vary widely. For example, some organisms, such as primary producers, can adapt quickly and even thrive under anoxic conditions. However, most organisms are highly susceptible to slight changes in aquatic oxygen levels. When arespiring organism is presented with little to no oxygen, the chances of survival decrease. Therefore, eutrophication and anoxic conditions in water lead to a reduction inbiodiversity.
For example, the soft coralXenia umbellata can resist some anoxic conditions for short periods. Still, after about three weeks, mean survival decreases to about 81%, and about 40% of surviving species experience size reductions, a lessening in coloration, and compromised pinnate structures.[19] Another example of a susceptible organism is theSydney cockle,Anadara trapezia. Enriched sediments have lethal and sublethal effects on this cockle, and, as stated in Vadillo Gonzalez et al. (2021), "movement of cockles was reduced in enriched sediments compared to natural treatments."[20]
A study collecting over 850 published experiments "reporting oxygen thresholds and/or lethal times for a total of 206 species spanning the fulltaxonomic range ofbenthicmetazoans."[21]
Individual species will exhibit different adaptive responses to anoxic conditions, depending on their biological makeup and the condition of their habitat. While some can pump oxygen from higher water levels down into the sediment, other adaptations include specifichaemoglobins for low-oxygen environments, slow movement to reduce the rate ofmetabolism, andsymbiotic relationships withanaerobic bacteria. In all cases, the prevalence of excess nutrients results in low levels of biological activity and a lower level of species diversity, unless the area is ordinarily anoxic.[13]
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