Which statement correctly describes consequences of thermally stratified bodies of water?

• A. Lower oxygen levels at bottom often depleted by midsummer
• B. Increases productivity and bottom oxygen
• C. Algal blooms are prevented by stratification
• D. Stratification reduces organic sedimentation

Answer: (A)

When water is thermally stratified, it splits into distinct layers: a warm, well-lit upper layer that mixes with the air and stays relatively oxygenated, and a cooler bottom layer that is cut off from vigorous mixing. In summer, this stratification becomes strong, so vertical exchange between the layers slows dramatically. Oxygen at the surface can’t easily reach the deeper layer, while the bottom water continues to be consumed by respiration and the decomposition of organic matter. Without replenishment, the dissolved-oxygen levels in the bottom layer drop, often reaching hypoxic or anoxic conditions by midsummer. This is the key consequence of stratification that affects the deeper ecosystem and can influence organisms that live there and the chemistry of the sediments.

The other ideas don’t fit as well because stratification doesn’t inherently boost bottom oxygen or guarantee prevention of blooms; it can actually contribute to surface blooms if nutrients accumulate in the upper layer and light is abundant. It also doesn’t reduce organic sedimentation; organic matter continues to settle and, with low bottom oxygen, may accumulate and deplete further.