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The River Basin  
 Introduction
Geography
Climate and Weather
Hydrology
Water Quality
 Principles of Water Quality
 Water Temperature
Dissolved Oxygen
 pH
 Total Dissolved Solids and Conductivity
 Suspended Sediment
 Salinity
 Hardness
 Nutrients
 Metals
 Biological Water Quality Parameters
 Spiritual Meaning of Water
 Human Impacts to Water Quality
 Acidity, Heavy Metals and Radionuclides
 Groundwater Quality
 Water Quality Fitness for Use
Ecology and Biodiversity
 References


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Water Quality: Principles of Water Quality:

Dissolved Oxygen

   

The amount of oxygen that dissolves in water can vary daily and seasonally, and decreases with higher temperature, salinity, and elevation. The maximum solubility of oxygen in water at one atmosphere ranges from about 15 mg/L at 0ºC to 8 mg/L at 30ºC. That is, ice-cold water can hold twice as much dissolved oxygen as warm water. Dissolved oxygen comes from the atmosphere and from photosynthesis by aquatic plants, and is depleted through both chemical oxidation and respiration by aquatic animals and microorganisms, especially during the decomposition of plant biomass and other organic material.

Surface water, near the water-atmosphere interface and with sufficient light for photosynthesis, is generally saturated or even supersaturated with oxygen. Deeper water receives oxygen through mixing by wind, currents, and inflows. Mixing and aeration also occur at waterfalls and rapids. Dissolved oxygen can be greatly depleted during summer months, in stagnant pools, or where algal blooms occur.

Dissolved oxygen is essential for a healthy aquatic ecosystem. Fish and aquatic organisms need oxygen dissolved in the water to survive. The need depends on the species and life stage; some are adapted to lower oxygen conditions, others require higher concentrations. Dissolved oxygen can affect the solubility and availability of nutrients, which can be released from sediments under conditions of low dissolved oxygen.

Water is mixed with oxygen in waterfalls.
Source:Source:Lesotho Water Commission 2002
( click to enlarge )

 

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