User:Brownem5/Anoxic waters

Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen. While hypoxia and anoxia are used interchangeably, hypoxia refers to areas with low oxygen concentrations and anoxia refers to area absent of oxygen. This condition is generally found in areas that have restricted water exchange.

Anoxic marine zones
Anoxic marine zones (AMZs) are large oxygen-depleted regions in oceans. AMZs can provide habitats for anaerobic microbes and can also provide temporary environments for aerobic microbes. While there are a few metazoan taxa that can tolerate anoxic conditions for a short period of time, there is limited evidence that some metazoans can survive permanent anoxic conditions for their life cycle. However, in the Mediterranean Sea and Black Sea, there has been presence of living fauna, loriciferans, in the oxygen-depleted anoxic zones. Metazoans that live under these anoxic conditions have similar anaerobic metabolisms to unicelled eukaryotes.

Geologic History
Oceans were anoxic throughout the Precambrian area. During this time, oxygen-producing organisms were beginning to evolve. The Great Oxidation Event allowed for increasing atmospheric oxygen which in turn reduced ocean anoxia. The Proterozoic era was the last time where long duration deep ocean anoxia existed. While smaller intervals of deep ocean anoxia persisted during the Phanerozoic area, a widespread deep-water anoxia existed around the time of the Permian-Triassic extinction event. Some speculate that this mass extinction was a result of widespread anoxic conditions. As time went on, anoxic conditions were not prevalent in deep-water, but instead were mostly limited to shallow marine environments or short-term deep-water events. The last known deep-water anoxia occurred during the Pliocene-Pleistocene epoch in the Eastern Mediterranean.

Anoxic Process
When oxygen is depleted in a basin, bacteria first turn to the second-best electron acceptor, which in sea and freshwater, 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.

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]

HS− + 2 O2 → HSO4−

or, more precisely:

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

Iron reduction can also impact anoxia. Anoxic waters and sediments create conditions in which iron-phosphorus compounds can be reduced, organic matter can be decomposed, and dissolved inorganic phosphorus can increase. In anoxic conditions, where oxygen is absent, iron in sediments exists in a reduced state, Fe2+. This state of iron is soluble in water making the phosphate ion also soluble. The phosphorus is then released into the water column where it can be used by phytoplankton and promote its growth. This can create a positive feedback loop where anoxia waters cannot trap the phosphorus in the sediments and the released phosphorus is taken up by phytoplankton, which make the water more eutrophic.

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.

Causes of Anoxia
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. Anoxia is also driven by high primary production and increased organic matter on the seafloor. Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen.

Anoxic conditions result from several factors; for example, stagnation conditions, density stratification, 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] Stratification can also impact the anoxia in lakes. In eutrophic lakes, the hypolimnion is often anoxic, especially in the summer season when mixing does not occur. 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.

Anoxic waters and oxygen depletion are usually the result of eutrophication. Excess nutrients enter the water causing an increase in algae that feed on the nutrients. The influx of nutrients is often a byproduct of agricultural run-off and sewage discharge. This also leads to an increase in phytoplankton that feed on the algae and a reduced amount that light that can reach heterotrophic organisms. Both of these combined lead to lower oxygen levels which may cause wildlife like fish to die, resulting in anoxic waters, where dissolved oxygen is absent. In some cases, this can also result from 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.

Anoxia is further influenced by biochemical oxygen demand (BOD), which is the amount of oxygen used by aquatic 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.

Areas of Anoxia
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.

Coastal regions of the eastern tropical Pacific Ocean experience anoxic waters. Here, nutrient-rich waters result in increased primary production and sinking of organic matter.

The deeper waters in lakes are more prone to anoxia. If lakes are deep enough, they can thermally stratify. This prevents the mixing of oxygen throughout the lake, depleting the deep waters of oxygen and making them vulnerable to anoxia.

The interior of the Black Sea and hypersaline basins of the Mediterranean Sea are known to have anoxic conditions.

Climate change
Climate change is contributing to the depletion of oxygen. Climate change is causing a increase in water temperatures and warmer water holds less oxygen. Warm water is less dense than cool water, so it remains at the surface and does not mix; warm water also increases metabolic rates in some species, causing them to consume more oxygen. Warmer waters can also make algal blooms more severe. Warming temperatures promote stratification, which allow the algae to grow at a quicker rate. In many lakes, the bottom waters of the hypolimnion are under threat to becoming anoxic during summer stratification. Depletion of oxygen during stratification and reduction of phosphorus loading have affected hypolimnetic lakes which impact fish and other aquatic species.

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.

Human effects
Anoxic water can also affect human life by decreasing quality of drinking water. Anoxic conditions housing algal blooms can produce toxins that are unsafe for human consumption. According to the World Health Organization, water is considered unsafe if there is greater than 1 part per billion in water consumed. Consumption of this water can cause headaches, nausea, vomiting, and damage to the digestive system and liver. Toledo, Ohio ran into this problem in 2014 when residents were told that the water unsafe to use. Half a million people were cut off from their water supply in included tap drinking water, cooking water, water to brush teeth, and shower water. A harmful algal bloom releasing toxins caused this abrupt change that lasted almost a week and still impacts the community today. Due to increasing temperatures, algal blooms are becoming more frequent which is depleting oxygen and leaving areas vulnerable to anoxia, all of which can threaten surrounding ecosystems.

Anoxic Wastewater Treatment
Anoxic waters can also be a source for wastewater treatment as a part of denitrification.

Wastewater can be a result of both domestic and industrial activities and can pose a threat to aquatic environments. Through anthropogenic activities, nutrients such as ammonia, nitrates, nitrites, and phosphorous have discharged into bodies of water like rivers, streams, and lakes which may lead to eutrophication. Wastewater treatment consists of four steps: preliminary treatment, primary treatment, secondary treatment, and tertiary treatment. Preliminary treatment removes large solid objects such as sticks, plastics, and debris through separators. Primary treatment solids and organics that settle, secondary treatment removes suspended and attached organisms, and tertiary treatment removed trace organics and phosphorus. Anoxic treatment can be used in the primary and secondary treatments to remove nitrogen and phosphorus, by using bioreactors packed with wood and iron, from wastewater before it gets discharged into bodies of water, thus preventing eutrophication of rivers, streams, and lakes. There are multiple available technologies that use anoxic conditions in wastewater treatment. For example, the Wuhrmann System is a process used post-denitrification by adding a anoxic tank to aerobic nitrification. The Ludzack-Ettinger System (LES) uses an anoxic primary reactor as a pre-denitrification process, the Modified Ludzack-Ettinger takes nitrate from the aerobic reactor directly into the anoxic reactor, and the Bardenpho 4-Stage Process uses a combination of pre-anoxic and post-anoxic systems.

Anoxic Basins

 * Bannock Basin in Levantine Sea, eastern Mediterranean Sea;
 * Black Sea Basin, off eastern Europe, below 50 metres (150 feet);
 * Caspian Sea Basin, below 100 metres (300 feet);
 * Cariaco Basin, off north central Venezuela;
 * Gotland Deep, in the Baltic off Sweden;
 * L'Atalante basin, eastern Mediterranean Sea
 * Mariager Fjord, off Denmark;
 * Orca Basin, northeast Gulf of Mexico;
 * Saanich Inlet, off Vancouver Island, Canada;

Also See

 * Anoxic event
 * Dead zone (ecology)
 * Hypoxia (environmental)
 * Meromictic
 * Mortichnia
 * Ocean deoxygenation
 * Oxygen minimum zones