What is the effect of autumn turnover on the vertical distribution of dissolved oxygen in a temperate lake?

• A. It reduces oxygen in deep waters.
• B. It re-oxygenates deep water by mixing, leading to more uniform DO throughout the water column.
• C. It only affects surface DO and leaves deep water unchanged.
• D. It eliminates DO entirely.

Answer: (B)

Autumn turnover is the seasonal mixing that ends thermal stratification in temperate lakes. As surface water cools, it becomes denser and sinks, driving turbulence that mixes the entire water column. This physical stirring moves oxygen-rich surface water downward and brings deeper water up, replenishing DO in the depths and making the oxygen distribution more uniform throughout the lake. It’s the mixing itself, not surface photosynthesis, that re-oxygenates deep water and equalizes DO across depths.