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Hypoxia (environmental)
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, oxygen levels are much lower, approximately 7 mg/L in good quality water, and fluctuate locally depending on the presence of photosynthetic organisms and relative distance to the surface (if there is more oxygen in the air, it will diffuse across the partial pressure gradient). In hypoxic bodies of water dissolved oxygen concentrations decrease to 2 or 3 mg/L.

Atmospheric hypoxia
Atmospheric hypoxia occurs naturally at high altitudes. Total atmospheric pressure decreases as altitude increases, causing a lower partial pressure of oxygen which is defined as hypobaric hypoxia. Oxygen remains at 20.9% of the total gas mixture, differing from hypoxic hypoxia, where the percentage of oxygen in the air (or blood) is decreased. This is common in the sealed burrows of some subterranean animals, such as blesmols. Atmospheric hypoxia is also the basis of altitude training which is a standard part of training for elite athletes. Several companies mimic hypoxia using normobaric artificial atmosphere.

Aquatic hypoxia
Oxygen depletion is a phenomenon that occurs in aquatic environments as dissolved oxygen (DO; molecular oxygen dissolved in the water) becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system. Dissolved oxygen is typically expressed as a percentage of the oxygen that would dissolve in the water at the prevailing temperature and salinity (both of which affect the solubility of oxygen in water; see oxygen saturation and underwater). An aquatic system lacking dissolved oxygen (0% saturation) is termed anaerobic, reducing, or anoxic; a system with low concentration&#x2014;in the range between 1 and 30% saturation&#x2014;is called hypoxic or dysoxic. Most fish cannot live below 30% saturation (2 or 3 mg/L). Hypoxia leads to impaired reproduction of remaining fish via endocrine disruption. A "healthy" aquatic environment should seldom experience less than 80%. The exaerobic zone is found at the boundary of anoxic and hypoxic zones.

Hypoxia can occur throughout the water column and also at high altitudes as well as near sediments on the bottom. It usually extends throughout 20-50% of the water column, but this depends on the water depth and location of pycnoclines (rapid changes in water density with depth). It can occur in 10-80% of the water column. For example, in a 10-meter water column, it can reach up to 2 meters below the surface.

Causes of hypoxia
Oxygen depletion can result from a number of natural factors including water stratification due to temperature or salinity gradients within the water column. However, it is most often a concern as a consequence of pollution and eutrophication in which plant nutrients enter a river, lake, or ocean, and phytoplankton blooms are encouraged. While phytoplankton, through photosynthesis, will raise DO saturation during daylight hours, the dense population of a bloom reduces DO saturation during the night by respiration. When phytoplankton cells die, they sink towards the bottom and are decomposed by bacteria, a process that further reduces DO in the water column. If oxygen depletion progresses to hypoxia, fish kills can occur and invertebrates like worms and clams on the bottom may be killed as well. Hypoxia may also occur in the absence of pollutants. Naturally lakes stratify during specific periods throughout the year. Some lakes only stratify once a year and are known as Monomictic lakes, while others may stratify twice a year in the summer and winter months. These lakes are known as Dimictic. During these stratification periods different layers of a lake appear called the epilimnion, metalimnion (or thermocline), and hypolimnion. Without the process of mixing during these stratification months these layers can not transfer oxygen between one another. Therefore the lowest layer, the hypolimnion is devoid of any source of oxygen (e.g., from the atmosphere or photosynthesis) for a certain period of time while the epilimnion may continue to receive oxygen from its contact with the atmosphere and photosynthesis. This naturally occurring process then allows for a depletion of oxygen in the lowest sections of lakes and allows to hypoxic water to form in these areas. The process then limits the livable habitat for oxygen-dependent organisms such as fish and crab during stratification and can even result in massive fish kills.

These naturally occurring processes aren't restrained to lakes alone. In estuaries, for example, because freshwater flowing from a river into the sea is less dense than salt water, stratification in the water column can result. Vertical mixing between the water bodies is therefore reduced, restricting the supply of oxygen from the surface waters to the more saline bottom waters. The oxygen concentration in the bottom layer may then become low enough for hypoxia to occur. Areas particularly prone to this include shallow waters of semi-enclosed water bodies such as the Waddenzee or the Gulf of Mexico, where land run-off is substantial. In these areas a so-called "dead zone" can be created. Low dissolved oxygen conditions are often seasonal, as is the case in Hood Canal and areas of Puget Sound, in Washington State. The World Resources Institute has identified 375 hypoxic coastal zones around the world, concentrated in coastal areas in Western Europe, the Eastern and Southern coasts of the US, and East Asia, particularly in Japan. Hypoxia may also be the explanation for periodic phenomena such as the Mobile Bay jubilee, where aquatic life suddenly rushes to the shallows, perhaps trying to escape oxygen-depleted water. Recent widespread shellfish kills near the coasts of Oregon and Washington are also blamed on cyclic dead zone ecology.

Phytoplankton breakdown
Scientists have determined that high concentrations of minerals dumped into bodies of water causes significant growth of phytoplankton blooms. As these blooms are broken down by bacteria, oxygen is depleted by these bacteria and increases the chances of forming hypoxic habitats.

Environmental factors
The limiting factor of a majority of phytoplankton populations is usually either phosphorus or nitrogen. That is why we see an increase in algal blooms when runoff of these nutrients find their way into bodies of water such as the Lake Erie Algal bloom in 2014. Algal blooms cause hypoxia because not all of the phytoplankton within these blooms can be eaten by their predators such as zooplankton and certain fish species. Therefore these remaining phytoplankton organisms die and sink to lower layers of lakes or oceans and are decomposed by bacteria. This process of decomposition then depletes the small amounts of DO in these deep layers causing hypoxic waters to form. As the breakdown of phytoplankton takes place, organic phosphorus is mineralized into phosphates, and organic nitrogen into ammonium. This can fuel additional algal growth and may ultimately deplete the oxygen even more so in the environment, further creating hypoxic zones in higher quantities. As more minerals such as phosphorus and nitrogen are displaced into these aquatic systems, the growth of phytoplankton greatly increases, and after their death when bacteria begin to decompose them in the lower layers of these bodies of water DO is depleted even more as hypoxic zones are formed.

Solutions
To combat hypoxia, it is essential to reduce the amount of land-derived nutrients reaching rivers in runoff. Farmers can help reduce runoff through a variety of different ways:


 * 1) Farmer's creation and adaptation of nutrient management plans. By figuring out when each of their crops need fertilizer, the right amount of that fertilizer, where it is needed, and with the correct methods farmers can drastically reduce their runoff.
 * 2) Fixing Drainage Methods. Water collected in drainage has the potential to carry excess nutrients such as phosphorus and nitrogen to watersheds, rivers, lakes and other bodies of water. A subsurface tile drainage can help manage these water movements.
 * 3) Cover Crops. Farmers can keep their fields occupied by having year-round plant coverage to prevent erosion into waterways of the soil and the subsequent nutrients that it contains.
 * 4) Field Buffers. The planting of shrubs, bushes, trees, and tall grasses around crop fields can help prevent runoff through the absorption and filtering out of nutrients by said plants before they can get to bodies of water.
 * 5) Reducing Tillage. By implementing a conservative approach to the tillage of their fields, where fields are tilled less than a conventional agriculture, farmers can help increase the health of their soil while also reducing the chances of erosion and runoff of nutrients.

Alternately, this can be done by restoring natural environments along a river; marshes are particularly effective in reducing the amount of nutrients such as phosphorus and nitrogen in water. Other natural habitat-based solutions include restoration of shellfish populations, such as oysters. Oyster reefs remove nitrogen from the water column and filter out suspended solids, subsequently reducing the likelihood or extent of harmful algal blooms or anoxic conditions. Foundational work toward the idea of improving marine water quality through shellfish cultivation was conducted by Odd Lindahl et al., using mussels in Sweden. More involved than single-species shellfish cultivation, integrated multi-trophic aquaculture mimics natural marine ecosystems, relying on polyculture to improve marine water quality.

Technological solutions are also possible, such as that used in the redeveloped Salford Docks area of the Manchester Ship Canal in England, where years of runoff from sewers and roads had accumulated in the slow running waters. In 2001 a compressed air injection system was introduced, which raised the oxygen levels in the water by up to 300%. The resulting improvement in water quality led to an increase in the number of invertebrate species, such as freshwater shrimp, to more than 30. Spawning and growth rates of fish species such as roach and perch also increased to such an extent that they are now amongst the highest in England.