Fertiliser washed into a lake causes algae to grow rapidly and then die. Bacteria in the water then decompose the large amount of dead algae. How does the a...
Question 1 Report
Fertiliser washed into a lake causes algae to grow rapidly and then die. Bacteria in the water then decompose the large amount of dead algae. How does the action of these bacteria reduce the amount of oxygen in the water?
Answer Details
The dead algae are a huge food supply for decomposing bacteria, so the bacteria multiply rapidly. These bacteria are aerobic, meaning they respire using oxygen:
\[\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}\]
The oxygen they use is the oxygen dissolved in the lake water, so as decay proceeds the bacteria use up the dissolved oxygen as they respire and fish and other aerobic organisms suffocate.
The lake does not evaporate away, and oxygen atoms cannot turn into nitrogen atoms, since that would require a change in the nucleus rather than a chemical reaction. Decaying algae are dead, so they photosynthesise no longer and release no oxygen.
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