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Oceans and Life: 16 MCQs with answers

All 16 multiple-choice questions from this kit, free and with no sign-up.

Full kit · everything Flashcards · 20 Quick Revision · 15

Warm-up · 5 questions

Q1. The ocean covers approximately what fraction of Earth's surface?

Beginner
A About 71% B About 90% C About 30% D About 50%
Show answer & explanation

About 71%

The ocean covers about 71% of Earth's surface and holds about 97% of its water.

Q2. Which zone is defined by receiving enough sunlight for photosynthesis?

Beginner
A Dark zone B Sunlit (euphotic) zone C Abyssal zone D Twilight zone
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Sunlit (euphotic) zone

The sunlit or euphotic zone extends to about 200 m and is the only zone where photosynthesis can occur.

Q3. Which organisms form the base of the marine food web?

Beginner
A Sharks B Sea cucumbers C Phytoplankton D Whales
Show answer & explanation

Phytoplankton

Phytoplankton are microscopic photosynthetic organisms at the base of the marine food web.

Q4. A deep-sea fish produces its own light to attract prey. What is this ability called?

Beginner
A Photosynthesis B Bioluminescence C Chemosynthesis D Thermohaline circulation
Show answer & explanation

Bioluminescence

Bioluminescence is the production of light by living organisms, used for hunting and defence.

Q5. The ocean covers about 71% of Earth's surface and contains about 97% of its water.

Beginner
A True B False
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True

The ocean covers about 71% of Earth's surface and contains about 97% of its water.

Building up · 6 questions

Q6. Which pair of factors most directly affects water density and drives ocean currents?

Intermediate
A Oxygen and carbon dioxide B Depth and latitude C Light and pressure D Salinity and temperature
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Salinity and temperature

Salinity and temperature affect water density; density differences drive ocean currents including thermohaline circulation.

Q7. At a hydrothermal vent, bacteria make food using hydrogen sulphide instead of sunlight. This process is:

Intermediate
A Evaporation B Respiration C Chemosynthesis D Photosynthesis
Show answer & explanation

Chemosynthesis

Chemosynthesis uses chemical energy (e.g., from hydrogen sulphide) to make food, supporting vent communities without sunlight.

Q8. Why can't photosynthesis occur in the twilight zone?

Intermediate
A The pressure is too low B Light is too faint C There is no carbon dioxide D The water is too salty
Show answer & explanation

Light is too faint

The twilight zone (200–1000 m) receives only faint light, insufficient for photosynthesis.

Q9. A student says the ocean is one uniform body of water. Which observation best refutes this?

Intermediate
A The ocean contains salt B Warm surface water floats above cold deep water C The ocean is very large D The ocean is blue in colour
Show answer & explanation

Warm surface water floats above cold deep water

The ocean is layered and dynamic: warm, less salty water stays near the surface while cold, salty water sinks.

Q10. Which statement best explains why the ocean is called a climate buffer?

Intermediate
A It reflects all sunlight back to space B It absorbs carbon dioxide and heat C It prevents all evaporation D It produces all of Earth's oxygen
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It absorbs carbon dioxide and heat

The ocean absorbs carbon dioxide and heat, buffering climate change — though this capacity has limits.

Q11. Nutrient runoff from farms enters coastal waters. What is the most likely consequence?

Intermediate
A Faster thermohaline circulation B Increased water salinity C Algal blooms that deplete oxygen D Colder surface water
Show answer & explanation

Algal blooms that deplete oxygen

Nutrient runoff causes algal blooms, which deplete oxygen and create dead zones.

Topper level · 5 questions

Q12. Which adaptation is most useful for a fish living under crushing deep-sea pressure?

Advanced
A Flexible, watery tissues without gas spaces B A gas-filled swim bladder C Brightly coloured scales D Rapid growth to large size
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Flexible, watery tissues without gas spaces

Deep-sea organisms avoid gas-filled spaces; flexible, watery tissues tolerate high pressure.

Q13. Assertion (A): Phytoplankton are more important to the marine food web than whales. Reason (R): Phytoplankton form the base of the food web and produce a large share of Earth's oxygen.

Advanced
A Both A and R are true, but R does not explain A. B A is false but R is true. C A is true but R is false. D Both A and R are true, and R correctly explains A.
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Both A and R are true, and R correctly explains A.

Phytoplankton's role as the base of the food web and oxygen producers is exactly why they matter more than top predators like whales.

Q14. Consider these statements: I. The dark zone lies between 1000 and 4000 m. II. Photosynthesis occurs throughout the dark zone. III. The dark zone is cold and under high pressure. Which combination is correct?

Advanced
A II and III only B I and II only C I, II and III D I and III only
Show answer & explanation

I and III only

The dark zone (1000–4000 m) is cold and high-pressure, but no sunlight reaches it, so photosynthesis cannot occur there.

Q15. Two students debate why deep-sea vent communities survive without sunlight. Which explanation is correct?

Advanced
A They absorb nutrients directly from seawater B They migrate to the sunlit zone to feed C They rely on chemosynthesis by bacteria using chemicals like hydrogen sulphide D They use bioluminescence to power photosynthesis
Show answer & explanation

They rely on chemosynthesis by bacteria using chemicals like hydrogen sulphide

Vent communities depend on chemosynthesis: bacteria use chemical energy from hydrogen sulphide to make food.

Q16. Which change would most directly reduce the oxygen produced by the ocean?

Advanced
A A decline in phytoplankton populations B A rise in ocean salinity C An increase in deep-sea pressure D Faster thermohaline circulation
Show answer & explanation

A decline in phytoplankton populations

Phytoplankton produce a significant share of Earth's oxygen; their decline would reduce ocean oxygen production.

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