Discussion
In an aquatic ecosystem level of D.O. from top to bottom shows :
Answer: D
Dissolved oxygen shows a decrease from top to bottom.
Oxygen enters water at the surface, by diffusion from the air and from photosynthesis by algae in the sunlit upper layer. Deeper down, light is too weak for photosynthesis while bacteria decomposing settled organic matter keep using oxygen up. In a thermally stratified lake the bottom layer (hypolimnion) is also cut off from the surface, so it can become nearly anoxic in summer.
'Less on the top and more at bottom' and 'an increase from top to bottom' describe the same reversed profile, which is why neither can be right. A uniform level is found only briefly, when a lake mixes completely at turnover.
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