“What Is Lake Turnover?”Post-Reading Questions

1. What times of year does turnover typically occur?• Turnover usually occurs in the spring and

fall.

2. What is the main cause of turnover?• The main cause of turnover is seasonal

temperature changes of a lake’s surface water.

3. What two conditions dictate where fish are found in a lake?• The two conditions that dictate where fish

are found in a lake are water temperatures and oxygen levels.

4. At what temperature is water densest and heaviest?• Water is densest and heaviest at 4ºC or

approximately 39ºF.

5. When are oxygen levels of water the highest? Explain why?• Oxygen levels in a lake are highest when

water temperatures are colder. This is because colder water holds more oxygen in solution.

6. Snow covered ice has what two effects on a lake?• Ice cover does not allow lake water to

circulate and pick up oxygen from the atmosphere at the surface.

• Snow covered ice prevents sunlight from entering the water, and this slows oxygen production by aquatic plants.

7. What two factors can also deplete oxygen levels in a lake?• Oxygen levels can become depressed due

to respiration by aquatic animals and plants, as well as from the decomposition of organic materials such as dead plants.

8. Where are fish located in the winter? Explain why?• During winter, most fish tend to seek the

warmer water near the bottom of the lake so they do not freeze.

9. Explain what happens in the spring when surface water reaches 4ºC?

• When surface water reaches 4ºC in spring, it becomes heavier than the water immediately below it and therefore sinks.

10. Besides water temperature, what helps water circulate in the spring and fall?

• Spring winds also help promote water circulation.

11. What is the name given to the zone of rapid temperature and oxygen change?

• The zone of rapid temperature and oxygen change is known as the thermocline.

12. What name is given to the phenomenon that occurs in the spring and summer in which distinct temperature layers form?

• The phenomenon in which distinct temperature layers form during the spring and summer is called “thermal stratification.”

13. Explain why a lake may smell like rotten eggs during the fall turnover.

• The rotten-egg odor comes from sulfur dioxide gas produced by decomposition that occurred in the summer. When the deep water (hypolimnion) reaches the surface during fall turnover, the sulfur dioxide is released into the atmosphere.

14. Is water at 0ºC more or less dense than water at 4ºC?• Water at 0ºC is less dense than water at

4ºC.

15. What is the best way to determine lake turnover and stratification patterns throughout the year?

• The best way to determine lake turnover and stratification patterns throughout the year is to measure temperature and oxygen profiles from top to bottom using electronic temperature and oxygen meters.

16. Finding what three conditions will improve your odds of catching fish?

• If the lakes you fish have good water quality; 1. proper water temperature, 2. high levels of dissolved oxygen and 3. plenty of submerged structures, you will increase your chances of catching fish.