General Factors

• Turbulence increases the rate at which oxygen dissolves in water.Jupiterimages/Photos.com/Getty Images
  
  The amount of oxygen dissolved in a body of water is measured in milligrams per liter (mg/L). This measure is affected by physical and biological factors.
  
  The main physical determinants of dissolved oxygen content are temperature, salinity, atmospheric oxygen pressure, and turbulence. Cooler water is capable of containing more oxygen because the gas is less volatile. Consequently, dissolved oxygen concentration is higher during the cooler months. Conversely, salty water is less able to hold dissolved gases, so seawater has a lower oxygen concentration than fresh water.
  
  When water is still, there is relatively little interaction between the atmosphere and the body of water, whereas flowing or churning water gets much more exposure to the air. Naturally, the amount of oxygen that's actually present in the air is also important. This is why dissolved oxygen concentration is lower at high altitudes where the air is thinner.
  
  In addition to these physical factors, organisms and chemical processes in the water consume and release oxygen. The consumption and production of oxygen by organisms has a dramatic impact on the dissolved oxygen concentration. Water oxygen levels become stable when the rate of consumption equals the rate of release.
  
  

Hypoxic and Anoxic Water

• Algae blooms often result in the death of aquatic animals due to hypoxia or infection.David McNew/Getty Images News/Getty Images
  
  Some bodies of water have areas of low (hypoxic) or no (anoxic) dissolved oxygen, making it difficult for animals to live. These low-oxygen conditions are most frequent where the oxygenated water on the surface does not mix with lower layers of water due to a pycnocline (temperature or salinity difference). If an event such as an oceanic upwelling causes the layers to mix, the surface water becomes hypoxic.
  
  Hypoxic conditions are most dangerous when water becomes eutrophic (enriched) when pollution adds nitrates and phosphates to the water column. This causes an algal bloom that initially releases oxygen. When the algae dies, it decomposes, which makes the oxygen concentration plummet. This results in massive die-offs of aquatic life. Algal blooms such as the "red tide" can create expansive dead zones in oceanic and estuarine waters.
  
  

Oxygenated Water

• Tropical fish require a bit less oxygen, having adapted to the relatively low oxygen content of warm water.Jupiterimages/Photos.com/Getty Images
  
  Some areas of the water have high oxygen content. This is generally less of a problem for organisms, though "supersaturation" of oxygen can be dangerous since it results in small gas bubbles in the blood. Areas with naturally high algae concentrations often have high oxygen levels. Oxygen levels can be artificially raised (as is done in aquariums) by passing small bubbles or foam through the water, which increases the interaction between the water and the atmosphere.
  
  

Degassing Water

• Small bubbles dissolved in water can interfere with industrial and research processes such as high-performance liquid chromatography (HPLC).Jupiterimages/Comstock/Getty Images
  
  To remove gases from solutions, research and industrial processes take advantage of the physical mechanisms that lead to oxygen dissolution in water. This is called degasification. The two main methods are vacuum degassing and inert gas bubbling. Additionally, simpy heating liquids can degass them to some extent.
  
  Vacuum degassing involves putting a solution into an airtight container and pumping out all the air, forcing the dissolved oxygen to come out of solution. For inert gas bubbling, an unreactive gas (usually helium) is bubbled through the solution. The helium temporarily dissolves into the solution, replacing the dissolved oxygen and nitrogen. Helium is a volatile gas and quickly leaves the solution, resulting in a degassed liquid.