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DISSOLVED OXYGEN

 


The solubility of oxygen in water


Like all gas, oxygen dissolves in water, but this solubility is limited by a threshold value, called the 'standard concentration' or the 'saturation content' which varies according to temperature, atmospheric pressure and salinity.
 


The influence of temperature


The higher the temperature, the lower the solubility of oxygen in water. At 0°C for example, the saturation content is 14.16 Mg/l while at 20°C, it is 8.84 mg/l (at sea level). A complex equation gives the standard concentration in relation to temperature, but the following simplified formula (to an accuracy of 1% ) can also be used:



Cs = 468,41 / (31,64 + T°C)


Cs: standard concentration in mg/l
T°C: water temperature



Table 1 can also be used. It gives the satu-ration levels of oxygen according to tempera-ture at an atmospheric pressure of 1013 (i.e. for a temperature of 22.3 "C, read the value at the intersection of line 22 and column 0.3)










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The influence of salinity


The solubility of oxygen in water decreases with an increase in salinity. Table 2 gives the satu-ration levels of oxygen in relation to tempera-ture and salinity at an
atmospheric pressure of 1013 hPa.
 


The influence of atmospheric pressure


The standard concentration of oxygen decreases in proportion to atmospheric pressure. The equation above is valid for an atmospheric pressure of 1013 hPa (or 769 mmHg) but at the pressure p (in hPa), standard concentration equal to:



Cs (p) = Cs * p / 1013


Cs (p) : standard concentration at pressure p (in hPa)
Cs: standard concentration at pressure 1013 hPa


Atmospheric pressure varies with climatic conditions, but also with altitude and therefore you can use this as a correction factor according to the following equation :



Cs (h) = Cs * Ca


Cs (h): Standard concentration at h altitude
CA:Cs: correction coefficient given by table 3.


Table 3 : correction coefficient of the level of oxygen saturation according to altitude.




Expression of the dissolved oxygen content


The oxygen content in water can be expressed either in concentration (mg/l or ppm) or in percentage saturation. The latter relates the content measured to the standard concentration according to the relationship.



% S = 100 * C/Cs



  % S : percentage of oxygen saturation
C : measured oxygen concentration (mg/l)
Cs : standard concentration (mg/l)
 


The percentage saturation therefore gives an indication of the degree of equilibrium (for oxygen) between the air and water. When % S < 100 % the water is undersaturated in oxygen, and when % S > 100%. The water is oversaturated with oxygen.












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 Processes which have an influence on the dissolved oxygen


 


 


The dissolved oxygen content of an aquatic medium changes continuously under the influence of chemical, biological and physical processes. The atmosphere, on the other hand, has a relatively constant oxygen concentration which varies only slightly
with atmospheric pressure.
Chemical reactions occur in the aquatic medium which take up or release oxygen depending on whether it is a question of oxidation or reduction. The consequence of these chemical reactions is expressed, in general, by what is called the C.O.D (Chemical Oxygen Demand). The main physical process influencing the oxygene content is the diffusion of oxygen between the water and the atmosphere, which occurs from water to air if the water is undersaturated in oxygen
(%S < 100 %), this exchange takes place until an equilibrium is reached between the air and the water (% S = 100 %).
The rate of diffusion depends to a large degree on the extent of the contact surface between the air and water. Hence, disturbed water mixed either naturally (current, turbulence, waterfalls) or artificially (aerators) will rapidly reach a state of equilibrium, even more so if the droplets of water projected in the air are very small.
On the other hand, in calm water which is hardly disturbed (pool, lake) is a very slow process.
Biological processes which have an effect on the dissolved oxygen in the water include the photosynthesis of vegetation and respiration of all autotrophic organisms which allows them to, in the light, to synthesise their own matter from mineral carbon dioxide according to the following equation:
Oxygen is a waste product of this metabolic process. A waste product which is then discarded into the medium. The extent of this process depends on the abundance of vegetation, temperature, light, quality of the nutrients and water turbulence.
Photosynthesis plays an important role, above all in stagnant water which is rich in phytoplankton. In this type of medium, it is the most influential process on the development of the dissolved oxygen content (table 4)


6CO2 + 6H2O = C6H12O6 + 6O2


Respiration is the metabolic process which provides energy to organisms by oxidation of organic carbon by the reaction; It is therefore an oxygen requiring process, which occurs in animals as well as plants.
Finally, the aerobic degradation of organic material is, in general, also an important operation in the uptake of oxygene, most of the organic matter is in the sediment.
The relative importance of each of the processes mentioned ( C.O.D, photosynthesis, diffusion, respiration) in the development


C6H12O6 + 6O2 = 6CO2+ 6H2O + heat


of the dissolved oxygen content depends largely on the conditions of the milieu. In a turbulent milieu, cold and not rich in organisms (especially phytoplankton), as would be the case in a mountain river, diffusion plays a very important role. In a stagnant medium, subject to warming and rich in organisms, it is photo-synthesis and respiration which play key roles, this is shown in table 4.
       
Table 4 :In decreasing order, the gains and losses due to the main processes involved in the development of oxygen in a pond.












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 Development of oxygen content in the aquatic milieu


The seasonal and daily development of dissolved oxygen is the result of the main processes outlined above.
In a turbulent medium, diffusion provides a constant supply of oxygen, and the oxygen content, is relatively stable over the course of the nycthemere (space of time composed of one day and one night 24 h) and varies only
slightly according to the season as a result of the influence of temperature on the solubility of oxygen in the water.
In a stationary milieu such as a pond, over the course of the nycthemere, the oxygen content is maximum when the rate of photosynthesis is significant (high temperature, high light intensity) and is minimum in darkness.
It therefore follows that, in a daily cycle where the range varies mainly according to the abundance of phytoplankton, there is a maximum at the end of the day and a minimum at the end of the night, a minimum which can reach lethal levels for fish and can promote major mortalities. The knowledge of this daily cycle is therefore important as it allows for risk periods to be determined and for methods to deal with deoxygenation to be thought out.
The seasonal component is indirectly due to the temperature, which influences the solubility of oxygen and determines the development of organisms.
Moreover, in a barely disturbed medium, dissolved oxygen decreases with depth, in general, as a result of diminished light penetration and therefore of a reduction in photosynthesis and the decrease is also due to significant uptake by the sediment in the degradation process of organic matter.






The needs of fish


The oxygen uptake of fish for respiration depends mainly on the species, the size, activity, temperature, feed and the quality of the water.
Pond species consume between 200 and 500 mg of oxygen per kg of live weight per hour at rest at 17 - 20°C and between 300 and 900 mg of oxygen/kg/hour when active. The hourly uptake of oxygen (per kg) decreases with the size of the fish. It increases with the oxygen content of the medium.
As regards temperature (for most fish), the uptake doubles with an increase in temperature of 10°C and with an increase in activity and feeding, e.g. a catfish (lctalurus punctatus) takes up 300-500 mg/kg/hr on an empty stomach and 680 mg/kg/hr an hour after feeding.
The minimum content of oxygen also depends mainly on the species, the temperature, the water quality and the length of exposure. Generally speaking, pond fish die after a few hours at under 0.3 mg/l. They can survive several hours (at rest) if the oxygen content reaches 1 mg/l and many days if it reaches 1.5 mg/l. It is generally considered that the required minimum in the pool is 5 mg/l.
Although they are just indicative values, table 5 gives the minimum values at which some of the pond species can survive as a fonction of temperature. It is evident that the level of oxygen must be much higher to permit normal life and growth.

Tab - 5 : Critical values of dissolved oxygen for certain species of pond fish.




To contact AQUALOG


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