Dissolved Oxygen in Water

Aquatic beings require oxygen to live, although their amounts may differ. When organic matter decomposes, the dissolved oxygen is used. Biological (or biochemical) demand (BOD) is a measure to determine the amount of oxygen in the water (in ppm) required to decompose organic matter.  Pure water has a BOD of 1 ppm. When BOD > 5 ppm the river is polluted.

Importance of oxygen in water: At a pressure of 1 atm and 20C the solubility of oxygen in water is 0.009gdm-3 Oxygen is
 crucial for aquatic plants and animals that require it for aerobic respiration.(fish require at least 3ppm)

Aerobic / Anaerobic Decomposition:If there’s enough oxygen, organic matter decomposes aerobically producing oxides. Anaerobic decay are for organisms that don’t need oxygen 
Eutrophication:  Due to the nitrates of intensive farming and the fertilizers used the sizes of the plants and algae, present in the water, increase. When they die, there won’t be enough oxygen and an anaerobic decomposition will happen. Hydrides are formed, which poison the water. The species die, increase the anaerobic decay. More species will die, until there’s no life (eutrophication). 

Thermal Pollution: In order for the oxygen to become soluble in water, it depends on the temperature. Water is used as a cooling agent, the heated water is thrown in rivers causing a thermal pollution.