How does stratification influence dissolved oxygen in a reservoir?

• A. The epilimnion is oxygen-rich and the hypolimnion is oxygen-poor due to limited mixing.
• B. The entire water column becomes equally oxygenated.
• C. Stratification increases oxygen in bottom water.
• D. Stratification has no effect on oxygen distribution.

Answer: (A)

Stratification creates two distinct layers in the water column: a warm, well-mixed upper layer (epilimnion) and a cooler, deeper lower layer (hypolimnion) separated by a thermocline that limits vertical mixing. The surface layer constantly gains oxygen from the air and from photosynthesis, so it tends to be rich in dissolved oxygen. In contrast, the bottom layer receives little oxygen from the surface because the barrier to mixing keeps water from the epilimnion from renewing the hypolimnion. Microbial decomposition of organic matter in the deeper waters consumes whatever oxygen is present, so the hypolimnion often becomes oxygen-poor (and can become hypoxic) over time. When turnover occurs and mixing resumes, oxygen-rich surface water can mix downward and help re-oxygenate the bottom waters. This behavior aligns with the idea that stratification yields an oxygen-rich epilimnion and an oxygen-poor hypolimnion due to limited vertical mixing.