Question

Many temperate lakes exhibit vertical mixing of surface and deeper waters, called overturn, twice annually. During which seasons does overturn occur?

a) Spring and summer
b) Summer and autumn
c) Autumn and winter
d) Winter and spring
e) Spring and autumn

Answer 1

Temperate lakes undergo spring and autumn turnovers due to temperature changes that affect water density. Overturn involves vertical mixing and occurs when surface water reaches 4 °C, causing it to sink and mix with deeper waters, thus redistributing oxygen and nutrients. This process differs in tropical lakes, where constant temperatures prevent such seasonal changes.

Step-by-step explanation:

Many temperate lakes exhibit vertical mixing of surface and deeper waters, called overturn, twice annually. Overturn occurs during the spring and autumn. In springtime, the ice melts and the surface water warms to 4 °C, becoming dense enough to sink and mix with the bottom layers, which is known as spring turnover. Conversely, in autumn, the water cools to 4 °C and the denser surface water again sinks, leading to fall turnover. This process is significant for bringing oxygen and nutrients throughout the water column, supporting aquatic life.

In contrast to temperate regions, tropical lakes generally do not freeze. Therefore, they do not undergo the same type of seasonal overturn as temperate lakes do. Tropical lakes have more constant temperatures and may be stratified year-round, often resulting in lower oxygen levels at the bottom.