Millions of dead fish wash up amid heat wave in Australia

Subject : Environment

Section: Ecosystem

Concept :

Millions of fish have washed up dead in southeastern Australia in a die-off that authorities and scientists say is caused by depleted oxygen levels in the river after recent floods and hot weather.
People north of Menindee say there’s cod and perch floating down the river everywhere.
The Department of Primary Industries said the fish deaths were likely caused by low oxygen levels as floods recede,a situation made worse by fish needing more oxygen because of the warmer weather.
State agencies also started to release higher-quality water where possible to boost dissolved oxygen levels in the area.

Dissolved Oxygen (DO)

Dissolved oxygen (DO) is defined as the amount of oxygen that is present in water.
The average dissolved oxygen content in freshwater is 0.0010 percent by weight (10 parts per million or 10 ppm), which is 150 times lower than the oxygen concentration in an equivalent volume of air.
Adequate dissolved oxygen is crucial for all forms of life and is required for excellent water quality.
Aquatic life is stressed when dissolved oxygen levels fall below 5.0 mg/L. Lower concentrations result in more stress.
Large fish kills can occur when oxygen levels fall below 1-2 mg/L even for a few hours.
The decrease in the level of dissolved oxygen is termed as Biological Oxygen Demand (BOD).
Because bacteria that feed on sewage multiply in numbers and respire, any release of raw sewage into a river results in a fall in dissolved oxygen level. This respiration depletes oxygen in the water quickly, resulting in fish deaths.

Factors affecting DO

Presence of organic and inorganic wastes in water decreases the dissolved oxygen content of the water.
A number of factors like surface turbulence, photosynthetic activity, O2 consumption by organisms and decomposition of organic matter are the factors which determine the amount of DO present in water.
The quality of water increases with an increase in DO levels.

Biochemical Oxygen Demand

Water pollution by organic wastes is measured in terms of Biochemical Oxygen Demand (BOD).
BOD is the amount of dissolved oxygen needed by bacteria in decomposing the organic wastes present in water. It is expressed in milligrams of oxygen per litre of water.
The higher value of BOD indicates low DO content of water.
Since BOD is limited to biodegradable materials, it is not a reliable method of measuring water pollution.

Chemical Oxygen Demand (COD)

COD measures the amount of oxygen in parts per million required to oxidise organic (biodegradable and non-biodegradable) and oxidizable inorganic compounds in the water sample.