Oxygenation (environmental)

Environmental oxygenation can be important to the sustainability of a particular ecosystem. Insufficient oxygen (environmental hypoxia) may occur in bodies of water such as ponds and rivers, tending to suppress the presence of aerobic organisms such as fish. Deoxygenation increases the relative population of anaerobic organisms such as plants and some bacteria, resulting in fish kills and other adverse events. The net effect is to alter the balance of nature by increasing the concentration of anaerobic over aerobic species.[1]

Oxygenation by water aeration can be part of the environmental remediation of a usually stagnant body of water. For example, Bubbly Creek in Chicago, Illinois, was hypoxic (deficient in oxygen) due to its use as an open sewer by Chicago's meat packing industry but has been oxygenated (made more oxic) by introducing compressed air into its waters, increasing the fish population. A similar technique has previously been used in the Thames.

Annual mean sea surface dissolved oxygen for the World Ocean. Data from the World Ocean Atlas 2009.

Pacific Ocean sections of dissolved oxygen and apparent oxygen utilisation. Data from the World Ocean Atlas 2009.[2]

Dissolved oxygen (DO) is measured in standard solution units such as millilitres O₂ per litre (mL/L), millimoles O₂ per litre (mmol/L), milligrams O₂ per litre (mg/L) and moles O₂ per cubic meter (mol/m³). For example, in freshwater under atmospheric pressure at 20 °C, O₂ saturation is 9.1 mg/L.

In aquatic environments, oxygen saturation is a relative measure of the amount of oxygen (O₂) dissolved in the water compared to equilibrium conditions.

Supersaturation of oxygen (i.e. saturation levels above 100%) can occur naturally. The most common cause is oxygen production by photosynthetically active species such as plants and algae. According to Henry's law, the equilibrium oxygen concentration is proportional to the partial pressure of oxygen gas. As air contains about 21% oxygen the equilibrium concentration of pure oxygen gas corresponds to nearly 500% air saturation. The other reason is that the oxygen concentration can be slow to adjust to changes in the environment. A rapid increase in temperature can reduce the equilibrium concentration of oxygen to a value below the actual concentration in the water, giving raise to more than 100% saturation until the system has had time to equilibrate through diffusion.[3] Supersaturation can sometimes be harmful for organisms and cause decompression sickness.

Solubility tables (based upon temperature) and corrections for different salinities and pressures can be found at the USGS web site.[4] Tables such as these of DO in millilitres per litre (mL/L) are based upon empirical equations that have been worked out and tested:[5]

$${\displaystyle \ln(DO)=A_{1}+A_{2}*100/T+A_{3}*\ln(T/100)+A_{4}*T/100+S\left(B_{1}+B_{2}*T/100+B_{3}*(T/100)^{2}\right)}$$

where ln is the symbol for natural logarithm and the coefficients take the following values:

Temperature and salinity constants for DO model

${\displaystyle A_{1}}$	=	−173.4292	${\displaystyle B_{1}}$	=	−0.033096
${\displaystyle A_{2}}$	=	249.6339	${\displaystyle B_{2}}$	=	0.014259
${\displaystyle A_{3}}$	=	143.3483	${\displaystyle B_{3}}$	=	−0.001700
${\displaystyle A_{4}}$	=	−21.8492
${\displaystyle T}$	=	temperature (kelvin)	${\displaystyle S}$	=	salinity (g/kg)

To convert the calculated DO above from mL/L to mg/L, multiply the answer by ${\displaystyle (P/T)*0.55130}$, P=mmHg, T=Kelvin