The presence of a large population of water lice in a water source indicates that organic pollution is moderate but declining. Oxygen levels are just beginning to increase (option A).
What are indicator species?
Indicator species are living organisms that tells us about the pollution status of our environment (water, air, land) by being absent or present.
Examples of indicator species are as follows;
Water louse
Lichen
According to this question, there is an abundance of large population of water lice in a water body.
This could indicate that organic pollution is moderate but declining and oxygen levels are just beginning to increase because water lice are known to thrive in water bodies with moderate pollution.
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