Dissolved Oxygen in Water
Of all the chemical substances in natural waters oxygen is one of the most significant. It is significant both as a regulator of metabolic processes of community and organism and as an indicator of water conditions.
The oxygen available for metabolic relationships in natural waters is the oxygen held in simple solution. The volume of oxygen dissolved in water at any given time is dependent upon:
(1) the temperature of the water,
(2) the partial pressure of the gas in the atmosphere and contact with the water, and
(3) the concentration of dissolved salts in the water.
Temperature not only influences the amount of soluble oxygen, but also is responsible indirectly for the consumption of dissolved oxygen. Increased temperature speeds up biochemical reactions including those of biological systems.
Fish are more active when the water is warm, thus increasing their demand on dissolved oxygen. Microorganisms responsible for degradation of organic material also exhibit accelerated oxygen uptake at elevated temperatures.
The dissolved oxygen content may be reported in terms of the actual concentration or as a percentage of saturation. The ultimate source of oxygen in water exposed to air is the atmosphere, but some oxygen is contributed by as a byproduct of photosynthesis.
Water bodies in which there is much organic productivity often display wide fluctuations of dissolved oxygen in response to biological activities. If organic material and other oxygen consuming substances are present, the dissolved oxygen may be depleted to very low concentrations after the water has moved only a short distance in the saturated zone.
The turbulence of a stream or the mixing of a lake can enhance the rate of dissolving of atmospheric oxygen. In small, turbulent streams, the oxygen content is normally near or above saturation. However, if mixing is inhibited as is sometimes the case in lakes, oxygen in the deep water may be depleted.
The dissolved oxygen content of water is an indicator of the biochemical condition of the water at that time and place. Fish and other desirable clean water biota require relatively high dissolved oxygen levels at all times.
Streams with large loads of organic material, however, may have oxygen consuming organic and inorganic reactions that deplete oxygen to levels unfavorable for clean water species. Therefore, we can consider the dissolved oxygen content as an indication of the status of the water with respect to the balance between oxygen consuming and oxygen producing processes at the moment of sampling.
Dissolved oxygen determinations usually have not been made in geochemical studies of ground water, but they have some value relating to the behavior of iron in ground water. Oxygen may be retained in solution for a long time in ground water if it is circulating far out of contact with the air and if there are no oxidizable materials of any consequence in contact with the water. If organic material and other oxygen consuming substances are present, however, the dissolved oxygen may be depleted to very low concentrations after the water has moved only a short distance in the saturated zone.
Excessive dissolved oxygen arising from algal growths may adversely or unreasonably affect some waters for one or more beneficial uses. However, we are usually concerned with a deficiency or complete absence of dissolved oxygen.
Inadequate dissolved oxygen in surface waters may contribute to unfavorable environments for fish and other aquatic life. Also, the absence of dissolved oxygen may give rise to odiferous products of anaerobic decomposition.
No general statement can be made to give the minimum dissolved oxygen concentration required to support fish life owing to the fact that the oxygen requirements of fish vary with species and age of the fish, with prior acclimatization, with temperature, with concentration of other substances in the water and with several other conglomerating factors.
Investigators have pointed out that temperature affects not only the amount of available dissolved oxygen in the water, but also the rate at which fish utilize oxygen. It is generally agreed that those species which are sluggish in movement, in other words, carp, pike and eel can withstand lower oxygen concentrations than active fish such as trout or salmon. If these fish remain in water of low DO, respiratory distress develops, accompanied by uneasiness and random movement. The lethal effect of low concentrations of dissolved oxygen appears to be increased by the presence of toxic substances such as excessive dissolved carbon dioxide, ammonia, cyanide, zinc, lead, copper or cresols.
With so many factors influencing the effect of oxygen deficiency, it is difficult to estimate the minimum safe concentration at which fish will be unharmed under natural conditions.
The Water Pollution Research Board of England found the minimum oxygen tensions at which various fish would survive for 84 hours at three temperatures to be as follows:
Studies several species of fish that were confined in boxes of wire netting and lowered into varying depths in lakes. The concentration of dissolved oxygen and temperature were noted at each depth and the reactions of the fish were observed. His results are
Summarized in Table
Under average stream conditions, 3.0 mg/L of dissolved oxygen or less should be regarded as hazardous or lethal and that to maintain a varied fish fauna in good condition, the dissolved oxygen concentration should remain at 5 mg/L or higher. For trout in soft water, the lower limit has been set at 6 mg/L. The recommended minimum permissible oxygen concentration for a well rounded warm water fish population is as follows:
The dissolved oxygen content of warm water fish habitat shall not be less than 5 mg/L during 16 hours of any 24 hour period. It may be less than 5 mg/L for a period not to exceed 8 hours within the 24 hour period, but at no time shall the oxygen content be less than 3 mg/L. To sustain a coarse fish population, the dissolved oxygen concentration may be less than 5 mg/L for a period of not more than 8 hours out of any 24 hour period, but at no time shall the concentration be lower than 2 mg/L.
Aside from deoxygenating effects of pollutants that influence the concentration of dissolved oxygen surface waters, there is a diurnal variation owing to the photosynthetic action of algae during daylight hours and the respiration at night.
High fish mortalities have occurred from high oxygen demand caused by decomposition of algae.
There is also a variation of oxygen with the depths of water, especially in lakes and stagnant ponds. The dissolved oxygen concentrations near the bottom muds of lakes and sluggish rivers may approach zero. Under such conditions, the hatching of fish eggs has been delayed, or the fish hatching from such eggs have been deformed or unsuccessful.
On the other hand, supersaturation of dissolved oxygen has also been reported as detrimental to fish. This may be related to gill damage by gas bubbles. Apparently, fish require dissolved oxygen and not air bubbles, the latter proving deleterious at times.
Since dissolved oxygen is a very critical indicator of overall water quality, several methods have been developed to determine the amount of oxygen that will be depleted by organic and inorganic contaminants. The methods discussed below include the biological oxygen demand (BOD), chemical oxygen demand (COD), and total organic carbon (TOC).