Question

What happens if the dissolved oxygen reduces in lake water​

Answer 1

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If the dissolved oxygen reduces in lake, water, then the water turned more alkaline in nature. Organisms die due to lack of sufficient amounts of oxygen.

Answer 2

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When dissolved oxygen becomes toolow, fish and other aquatic organisms cannot survive. The colder water is, the more oxygen it can hold. As the water becomes warmer, less oxygen can be dissolved in the water.

The dissolved oxygen is used by the fishes and other aquatic organisms to breathe. So if the oxygen level reduces they would die.