Effect on Water Bodies - Dissolved Oxygen (DO)

Most aquatic, organisms respire with oxygen dissolved in water. The quantity of dissolved oxygen in a unit volume of aerated water is only .0084g which is about one thirtieth of that present in the same volume of air at 25°C. The quantity decreases further with the increase of temperature. Hence the availability of oxygen is one of the critical factors for the survival of aquatic species. The amount of oxygen gas dissolved in a given quantity of water at a particular temperature and atmospheric pressure is referred to as DO and generally expressed as parts per million (ppm). DO in natural water is influenced by

• The degree of aeration,
• The photosynthetic activity in the water column,
• The consumption of dissolved oxygen by plants, aquatic organisms and decomposers and
• The ambient temperature.

Discharge of sewage in large quantities results in a drop in DO because decomposer organisms use up a lot of dissolved oxygen in decomposing organic matter. The water containing DO below 8 ppm is considered polluted. The gravely polluted water has DO below 4 ppm to nil. Volume and flow of water also affect biodegradation because consumed oxygen is rapidly renewed in large flowing water, whereas in stagnant water biodegradation is sharply reduced particularly in summer due to decrease in DO.