Anoxic waters

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Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen and are a more severe condition of hypoxia. The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. [1] This condition is generally found in areas that have restricted water exchange.

Groundwater water located beneath the ground surface

Groundwater is the water present beneath Earth's surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. Groundwater is recharged from and eventually flows to the surface naturally; natural discharge often occurs at springs and seeps, and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology.

Hypoxia refers to low oxygen conditions. Normally, 20.9% of the gas in the atmosphere is oxygen. The partial pressure of oxygen in the atmosphere is 20.9% of the total barometric pressure. In water however, oxygen levels are much lower, approximately 1%, and fluctuate locally depending on the presence of photosynthetic organisms and relative distance to the surface.


In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier [2] 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 of organic matter by bacteria is greater than the supply of dissolved oxygen.

Redox chemical reaction

Redox is a chemical reaction in which the oxidation states of atoms are changed. Any such reaction involves both a reduction process and a complementary oxidation process, two key concepts involved with electron transfer processes. Redox reactions include all chemical reactions in which atoms have their oxidation state changed; in general, redox reactions involve the transfer of electrons between chemical species. The chemical species from which the electron is stripped is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. It can be explained in simple terms:

Bacteria A domain of prokaryotes – single celled organisms without a nucleus

Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about half of the bacterial phyla have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

Anoxic waters are a natural phenomenon, [3] and have occurred throughout geological history. In fact, some postulate that the Permian–Triassic extinction event, a mass extinction of species from world's oceans, resulted from widespread anoxic conditions. At present anoxic basins exist, for example, in the Baltic Sea, [4] and elsewhere (see below). Recently, there have been some indications that eutrophication has increased the extent of the anoxic zones in areas including the Baltic Sea, the Gulf of Mexico, [5] and Hood Canal in Washington State. [6]

Permian–Triassic extinction event most severe extinction event of Earths chronology, occurring approx 252 million years ago, ending the Paleozoic era (and the Permian period) and beginning the Mesozoic era (and the Triassic period)

The Permian–Triassicextinction event, colloquially known as the Great Dying, the End-Permian Extinction or the Great Permian Extinction, occurred about 252 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as between the Paleozoic and Mesozoic eras. It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects. Some 57% of all biological families and 83% of all genera became extinct. Because so much biodiversity was lost, the recovery of land-dwelling life took significantly longer than after any other extinction event, possibly up to 10 million years. Studies in Bear Lake County, near Paris, Idaho, showed a relatively quick rebound in a localized marine ecosystem, taking around 2 million years to recover, suggesting that the impact of the extinction may have been felt less severely in some areas than others.

Baltic Sea A sea in Northern Europe bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands

The Baltic Sea is a marginal sea of the Atlantic Ocean, enclosed by Denmark, Estonia, Finland, Latvia, Lithuania, Sweden, northeast Germany, Poland, Russia and the North and Central European Plain.

Eutrophication ecosystem response to the addition of substances

Eutrophication, or hypertrophication, is when a body of water becomes overly enriched with minerals and nutrients which induce excessive growth of plants and algae. This process may result in oxygen depletion of the water body. One example is an "algal bloom" or great increase of phytoplankton in a water body as a response to increased levels of nutrients. Eutrophication is often induced by the discharge of nitrate or phosphate-containing detergents, fertilizers, or sewage into an aquatic system.

Causes and effects

Anoxic conditions result from several factors; for example, stagnation conditions, density stratification, [7] inputs of organic material, and strong thermoclines. Examples of which are fjords (where shallow sills at their entrance prevent circulation) and deep ocean western boundaries where circulation is especially low while production at upper levels is exceptionally high.[ citation needed ] In wastewater treatment, the absence of oxygen alone is indicated anoxic while the term anaerobic is used to indicate the absence of any common electron acceptor such as nitrate, sulfate or oxygen.

Water stagnation occurs when water stops flowing. Stagnant water can be a major environmental hazard.

Thermocline A distinct layer in a large body of fluid in which temperature changes more rapidly with depth than it does in the layers above or below

A thermocline is a thin but distinct layer in a large body of fluid in which temperature changes more rapidly with depth than it does in the layers above or below. In the ocean, the thermocline divides the upper mixed layer from the calm deep water below.

Fjord A long, narrow inlet with steep sides or cliffs, created by glacial activity

Geologically, a fjord or fiord is a long, narrow inlet with steep sides or cliffs, created by a glacier. There are many fjords on the coasts of Alaska, Antarctica, British Columbia, Chile, Greenland, the Faroe Islands, Iceland, Kamchatka, the Kerguelen Islands, New Zealand, Norway, Novaya Zemlya, Labrador, Nunavut, Newfoundland, Quebec, Scotland, South Georgia Island, and Washington state. Norway's coastline is estimated at 29,000 kilometres (18,000 mi) with nearly 1,200 fjords, but only 2,500 kilometres (1,600 mi) when fjords are excluded.

When oxygen is depleted in a basin, bacteria first turn to the second-best electron acceptor, which in sea water, is nitrate. Denitrification occurs, and the nitrate will be consumed rather rapidly. After reducing some other minor elements, the bacteria will turn to reducing sulfate. This results in the byproduct of hydrogen sulfide (H2S), a chemical toxic to most biota and responsible for the characteristic "rotten egg" smell and dark black sediment color. [8]

Nitrate anion

Nitrate is a polyatomic ion with the molecular formula NO
and a molecular mass of 62.0049 u. Organic compounds that contain the nitrate ester as a functional group (RONO2) are also called nitrates.


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 denitrification as a type of respiration that reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. The preferred nitrogen electron acceptors in order of most to least thermodynamically favorable include nitrate (NO3), nitrite (NO2), nitric oxide (NO), nitrous oxide (N2O) finally resulting in the production of dinitrogen (N2) completing the nitrogen cycle. Denitrifying microbes require a very low oxygen concentration of less than 10%, as well as organic C for energy. Since denitrification can remove NO3, reducing its leaching to groundwater, it can be strategically used to treat sewage or animal residues of high nitrogen content. Denitrification can leak N2O, which is an ozone-depleting substance and a greenhouse gas that can have a considerable influence on global warming.

Sulfate anion

The sulfate or sulphate ion is a polyatomic anion with the empirical formula SO2−
. Sulfate is the spelling recommended by IUPAC, but sulphate is used in British English. 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 are prepared from that acid.

SO4−2 + H+1 → H2S +H2O + chemical energy

If anoxic sea water becomes reoxygenized, sulfides will be oxidized to sulfate according to the chemical equation:[ citation needed ]

Sulfide salt or other derivative of hydrogen sulfide or organic compound having the structure RSR (R ≠ H)

Sulfide (British English sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. Sulfide also refers to chemical compounds large families of inorganic and organic compounds, e.g. lead sulfide and dimethyl sulfide. Hydrogen sulfide (H2S) and bisulfide (SH-) are the conjugate acids of sulfide.

HS + 2 O2 → HSO4

or, more precisely:

(CH2O)106(NH3)16H3PO4 + 53 SO42− → 53 CO2 + 53 HCO3 + 53 HS +16 NH3 + 53 H2O + H3PO4

Anoxia is quite common in muddy ocean bottoms where there are both high amounts of organic matter and the low levels 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 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. [9]

In the Baltic Sea, the slowed rate of decomposition under anoxic conditions has left remarkably preserved fossils retaining impressions of soft body parts, in Lagerstätten.[ citation needed ] [10]

Human caused anoxic conditions

Eutrophication, 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. [8]

Gradual environmental changes through eutrophication or global warming 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, and an anoxic state with sulfate-reducing bacteria and phototrophic sulfur bacteria. [11]

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 the larger scale. Thus, bodies of water are more vulnerable to anoxic conditions during warmest period of the day and during the 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. [12]

Biological adaptation

Organisms have adapted a variety of mechanisms to live within anoxic sediment. 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 toxic H2S results in low levels of biologic activity and a lower level of species diversity if the area is not normally anoxic. [8]

Anoxic basins

See also

Related Research Articles

The purple sulfur bacteria (PSB) are part of a group of Proteobacteria capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in stratified water environments including hot springs, stagnant water bodies, as well as microbial mats in intertidal zones. Unlike plants, algae, and cyanobacteria, purple sulfur bacteria do not use water as their reducing agent, and therefore do not produce oxygen. Instead, they can use sulfur in the form of sulfide, or thiosulfate (as well, some species can use H2, Fe2+, or NO2) as the electron donor in their photosynthetic pathways. The sulfur is oxidized to produce granules of elemental sulfur. This, in turn, may be oxidized to form sulfuric acid.

Sulfate-reducing microorganisms microorganisms which "breathe" sulfates

Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO42–) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S). Therefore, these sulfidogenic microorganisms "breathe" sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration.

Anoxic event Intervals in the Earths past where parts of oceans were depleted of oxygen at depth over a large geographic area

Oceanic anoxic events or anoxic events (anoxia conditions) were intervals in the Earth's past where portions of oceans become depleted in oxygen (O2) at depths over a large geographic area. During some of these events, euxinia, waters that contained H
hydrogen sulfide, developed. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events coincided with several mass extinctions and may have contributed to them. These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating. Many geologists believe oceanic anoxic events are strongly linked to slowing of ocean circulation, climatic warming, and elevated levels of greenhouse gases. Researchers have proposed enhanced volcanism (the release of CO2) as the "central external trigger for euxinia".

In biogeochemistry, remineralization refers to the breakdown or transformation of organic matter into its simplest inorganic forms. These transformations form a crucial link within ecosystems as they are responsible for liberating the energy stored in organic molecules and recycling matter within the system to be reused as nutrients by other organisms.

f-ratio In oceanic biogeochemistry, the fraction of total primary production fuelled by nitrate

In oceanic biogeochemistry, the f-ratio is the fraction of total primary production fuelled by nitrate. The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the first papers estimating global oceanic production. This fraction was originally believed significant because it appeared to directly relate to the sinking (export) flux of organic marine snow from the surface ocean by the biological pump. However, this interpretation relied on the assumption of a strong depth-partitioning of a parallel process, nitrification, that more recent measurements has questioned.

Cariaco Basin

The Cariaco Basin lies off the north central coast of Venezuela and forms the Gulf of Cariaco. It is bounded on the east by Margarita Island, Cubagua Island, and the Araya Peninsula; on the north by Tortuga Island and the Tortuga Banks; on the west by Cape Codera and the rocks known as Farallón Centinela; and on the south by the coast of Venezuela.

Microbial metabolism is the means by which a microbe obtains the energy and nutrients it needs to live and reproduce. Microbes use many different types of metabolic strategies and species can often be differentiated from each other based on metabolic characteristics. The specific metabolic properties of a microbe are the major factors in determining that microbe's ecological niche, and often allow for that microbe to be useful in industrial processes or responsible for biogeochemical cycles.

Secondary treatment A treatment process for wastewater or sewage

Secondary treatment is a treatment process for wastewater to achieve a certain degree of effluent quality by using a sewage treatment plant with physical phase separation to remove settleable solids and a biological process to remove dissolved and suspended organic compounds. After this kind of treatment, the wastewater may be called as secondary-treated wastewater.

Freshwater environmental quality parameters are the natural and man-made chemical, biological and microbiological characteristics of rivers, lakes and ground-waters, the ways they are measured and the ways that they change. The values or concentrations attributed to such parameters can be used to describe the pollution status of an environment, its biotic status or to predict the likelihood or otherwise of a particular organisms being present. Monitoring of environmental quality parameters is a key activity in managing the environment, restoring polluted environments and anticipating the effects of man-made changes on the environment.

Baltic Sea hypoxia refers to low levels of oxygen in bottom waters, also known as hypoxia, occurring regularly in the Baltic Sea. As of 2009 the total area of bottom covered with hypoxic waters with oxygen concentrations less than 2 mg/l in the Baltic Sea has averaged 49,000 km2 over the last 40 years. The ultimate cause of hypoxia is excess nutrient loading from human activities causing algal blooms. The blooms sink to the bottom and use oxygen to decompose at a rate faster than it can be added back into the system through the physical processes of mixing. The lack of oxygen (anoxia) kills bottom-living organisms and creates dead zones.

Isorenieratene chemical compound

Isorenieratene is a carotenoid light harvesting pigment with the chemical formula C40H48. Isorenieratene and its derivatives are useful to marine chemists studying the carbon cycle as biomarkers that indicate photic zone anoxia.

<i>Chondrites</i> (genus) Ichnogenus.

Chondrites is a trace fossil ichnogenus, preserved as small branching burrows of the same diameter that superficially resemble the roots of a plant. The origin of these structures is currently unknown. Chondrites is found in marine sediments from the Cambrian period of the Paleozoic onwards. It is especially common in sediments that were deposited in reduced-oxygen environments.

Western Interior Seaway anoxia

Three Western Interior Seaway anoxic events occurred during the Cretaceous in the shallow inland seaway that divided North America in two island continents, Appalachia and Laramidia. During these anoxic events much of the water column was depleted in dissolved oxygen. While anoxic events impact the world's oceans, Western Interior Seaway anoxic events exhibit a unique paleoenvironment compared to other basins. The notable Cretaceous anoxic events in the Western Interior Seaway mark the boundaries at the Aptian-Albian, Cenomanian-Turonian, and Coniacian-Santonian stages, and are identified as Oceanic Anoxic Events I, II, and III respectively. The episodes of anoxia came about at times when very high sea levels coincided with the nearby Sevier orogeny that affected Laramidia to the west and Caribbean large igneous province to the south, which delivered nutrients and oxygen-adsorbing compounds into the water column.

Dissimilatory nitrate reduction to ammonium (DNRA), also known as nitrate/nitrite ammonification, is the result of anaerobic respiration by chemoorganoheterotrophic microbes using nitrate (NO3) as an electron acceptor for respiration. In anaerobic conditions microbes which undertake DNRA oxidise organic matter and use nitrate (rather than oxygen) as an electron acceptor, reducing it to nitrite, then ammonium (NO3→NO2→NH4+).

Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly stratified, have an oxic, highly productive, thin surface layer, and have anoxic, sulfidic bottom water. The word euxinia is derived from the Greek name for the Black Sea- Εὔξεινος Πόντος (Euxeinos Pontos)- which translates to "hospitable sea". Euxinic deep water is a key component of the Canfield ocean, a model of oceans during the Proterozoic known as the Boring Billion proposed by Donald Canfield, an American geologist, in 1998. There is still debate within the scientific community on both the duration and frequency of euxinic conditions in the ancient oceans. Euxinia is relatively rare in modern bodies of water, but does still happen in places like the Black Sea and certain fjords.

Microbial oxidation of sulfur

Microbial oxidation of sulfur is the oxidation of sulfur by microorganisms to produce energy. The oxidation of inorganic compounds is the strategy primarily used by chemolithotrophic microorganisms to obtain energy in order to build their structural components, survive, grow and reproduce. Some inorganic forms of reduced sulfur, mainly sulfide (H2S/HS) and elemental sulfur (S0), can be oxidized by chemolithotrophic sulfur-oxidizing prokaryotes, usually coupled to the reduction of oxygen (O2) or nitrate (NO3).

The sulfate-methane transition zone (SMTZ) is a zone in oceans, lakes, and rivers found below the sediment surface in which sulfate and methane coexist. The formation of a SMTZ is driven by the diffusion of sulfate down the sediment column and the diffusion of methane up the sediments. At the SMTZ, their diffusion profiles meet and sulfate and methane react with one another, which allows the SMTZ to harbor a unique microbial community whose main form of metabolism is anaerobic oxidation of methane (AOM). The presence of AOM marks the transition from dissimilatory sulfate reduction to methanogenesis as the main metabolism utilized by organisms.

An oxygen minimum zone (OMZ) is characterized as an oxygen-deficient layer in the world oceans. Typically found between 200m to 1500m deep below regions of high productivity, such as the western coasts of continents. OMZs can be seasonal following the spring-summer upwelling season. Upwelling of nutrient-rich water leads to high productivity and labile organic matter, that is respired by heterotrophs as it sinks down the water column. High respiration rates deplete the oxygen in the water column to concentrations of 2mg/l or less forming the OMZ. Under these oxygen-starved conditions, energy is diverted from higher trophic levels to microbial communities that have evolved to use other biogeochemical species instead of oxygen, these species include Nitrate, Nitrite, Sulphate etc. Several Bacteria and Archea have adapted to live in these environments by using these alternate chemical species and thrive. The most abundant phyla in OMZs are Proteobacteria, Bacteroidetes, Actinobacteria, and Planctomycetes.


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