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Global Governance of the Oceans

Sources and consequences of ocean pollution

  • 3 things to know
  • Component 2

Overview

Terrestrial run-off, gulf of Mexico dead zone and garbage patch.

What you need to know

Answer each one before you reveal it.

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Key term

What share of marine pollution comes from land?

Key term

What is terrestrial run-off?

Key term

How does nutrient run-off cause a dead zone?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What share of marine pollution comes from land?

    Model answer

    Around 80% of marine pollution originates on land. Rivers, run-off, sewage and airborne deposition carry it to the sea.

  2. 2

    What is terrestrial run-off?

    Model answer

    Terrestrial run-off is water flowing off the land carrying soil, fertiliser, pesticide and waste. Rain falling on bare ploughed fields or on a city produces the largest loads.

  3. 3

    How does nutrient run-off cause a dead zone?

    Model answer

    Nitrate and phosphate feed a bloom of algae in coastal water. Bacteria consume the dead algae and strip oxygen from the water below.

  4. 4

    Define eutrophication.

    Model answer

    Eutrophication is the enrichment of water by nutrients that triggers excessive plant and algal growth. Oxygen loss and a change of species follow.

  5. 5

    Where is the largest dead zone in the world?

    Model answer

    The Arabian Sea holds the largest dead zone, while the Baltic Sea has the largest caused by human activity, covering tens of thousands of square kilometres. Slow exchange with the North Sea makes the Baltic's recovery very difficult.

  6. 6

    Describe the Gulf of Mexico dead zone.

    Model answer

    Nitrate carried by the Mississippi creates a summer dead zone off the coast of Louisiana. The area measured about 17,000 km² in 2024.

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