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A million fish have died in the Murray Darling basin, as oxygen levels plummet due to major algal blooms. Experts have warned we could see more mass deaths this week.
Fingers have been pointed at poor water management after a long period of drought. However, mass fish deaths can also be caused by floods, and even raw sewage.
Oxygen - Water
So what's going on when oxygen gets "sucked out of the water"?
The phenomenon is very well known to water quality engineers; we call it "biochemical oxygen demand". To understand it, we need to talk about a little bit of biology and a little bit of chemistry.
Oxygen - Molecules - Water - Way - Sugar
Oxygen molecules are soluble in water in the same way that sugar is soluble in water. Once its dissolved, you can't see it (and, unlike sugar, oxygen is tasteless).
The maximum amount of oxygen that you can dissolve in water depends on a number of factors, including the water temperature, ambient air pressure, and salinity. But roughly speaking, the maximum amount of dissolvable oxygen, known as the "saturation concentration" is typically around 7-10 milligrams of oxygen per litre of water (7-10 mg/L).
Oxygen - Use - Water - Mouths - Force
This dissolved oxygen is what fish use to breathe. Fish take water in through their mouths and force it through their gill passages. Gills, like our lungs, are full of blood vessels. As water passes over the thin walls of the gills, dissolved oxygen is transferred into the blood and then transported to the fish's cells. The higher the oxygen concentration in the water, the easier it is for this transfer to occur.
Once in the cells, the oxygen molecules play a key role in the process of "aerobic respiration". The oxygen reacts with energy-rich organic substances, such as sugars, carbohydrates and fats to break them down and release energy for the cells. The main waste product from this process...
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