Year 8 Science · Unit 1 · Lesson 23

Matter and Energy Cycling, Carbon, Water and Nutrients

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Learning Goals

Trace a cause-and-effect chain linking agricultural runoff to ecosystem collapse.
Compare the carbon, nitrogen and phosphorus cycles across key features.
Apply nutrient cycling concepts to a real Australian environmental problem.

Warm Up

Because… chain

The boxes below show a chain of cause-and-effect events related to nutrient pollution in the Murray–Darling Basin. Fill in the missing effects in the empty boxes. Each cause leads to the next step.

Excess nitrogen fertiliser is applied to farms in the Murray–Darling catchment.

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Nitrogen-rich water washes into the Murray–Darling River system.

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A massive algal bloom forms across large sections of the river.

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When the algae die, decomposers break down the huge mass of organic material.

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Dissolved oxygen in the river is almost completely used up.

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Overall outcome for the Murray–Darling ecosystem:

Your Turn

Compare three cycles

Complete the table to compare the carbon cycle, nitrogen cycle and phosphorus cycle across five key features.

Feature	Carbon cycle	Nitrogen cycle	Phosphorus cycle
Does it have an atmospheric reservoir?
Key process that moves it INTO plants
Key process that returns it to the environment from dead organisms
Main human impact on this cycle
One Australian environmental problem linked to this cycle

Show What You Know

1. Explain why decomposers are essential to all nutrient cycles. What would happen to an ecosystem if all decomposers disappeared?

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2. In 2019, blue-green algal blooms killed hundreds of thousands of fish in the Darling River near Menindee, NSW. Using the Because… chain you completed above, explain the role of the nitrogen cycle in causing this disaster.

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Wrap Up

In one sentence, what was the main idea of this lesson?