7.2 Organisation of an ecosystem

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Question 9
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An investigator set up an experiment to study the gaseous exchange in a closed aquatic ecosystem. A sealed boiling tube containing pond water, a water plant (Elodea\textit{Elodea}Elodea), and a freshwater shrimp was equipped with an oxygen probe connected to a data logger.

For the first 4 days, the tube was kept in a room with constant, bright light. At the end of Day 4, the tube was completely covered with light-proof black foil. On Day 7, the freshwater shrimp died.

A line graph showing the concentration of dissolved oxygen in a sealed boiling tube over a period of 10 days. The vertical axis is labeled 'Oxygen concentration (arbitrary units)' ranging from 0.0 to 10.0. The horizontal axis is labeled 'Time in days' ranging from 0 to 10. From Day 0 to Day 4, the oxygen level is constant at 6.0. At Day 4, there is a vertical line indicating 'Tube covered in black foil'. From Day 4 to Day 7, the line slopes downward from 6.0 to 3.5. At Day 7, there is a label 'Freshwater shrimp died'. From Day 7 to Day 10, the line continues to slope downward, decreasing from 3.5 to 1.5.

a.

Explain why the concentration of oxygen in the tube remained constant between Day 0 and Day 4.

[2]
b.

Suggest why the concentration of oxygen decreased between Day 4 and Day 7.

[2]
c.

Explain why the death of the freshwater shrimp on Day 7 caused the oxygen concentration to continue to decrease after Day 7.

[3]

7.2 Organisation of an ecosystem Questions

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