UPSC Prelims 2012 Q99: Environment & Ecology – Ecosystems & Ecology Concepts
What would happen if phytoplankton of an ocean is completely destroyed for some reason?
1. The ocean as a carbon sink would be adversely affected.
2. The food chains in the ocean would be adversely affected.
3. The density of ocean water would drastically decrease.
Select the correct answer using the codes given below :
- (a) 1 and 2 only
- (b) 2 only
- (c) 3 only
- (d) 1, 2 and 3
Answer: (a) 1 and 2 only
Explanation
If ocean phytoplankton were destroyed
Phytoplankton are tiny floating plants.
They do about half of the world's photosynthesis.
| Statement | Verdict | Reason |
|---|---|---|
| 1. The ocean as a carbon sink would be adversely affected | Correct | Phytoplankton absorb carbon dioxide; when they die, the carbon sinks to the deep sea (the biological pump) |
| 2. The food chains in the ocean would be adversely affected | Correct | They are the producers at the base of nearly all marine food chains |
| 3. The density of ocean water would drastically decrease | Incorrect | Density depends on temperature and salinity, not on plankton |
Role of phytoplankton:
| Role | Detail |
|---|---|
| Oxygen | Produce about half of the oxygen in the air |
| Food | Eaten by zooplankton, which feed fish and whales |
| Climate | Draw carbon dioxide out of the air |
| Too many | Cause algal blooms and dead zones |
Answer: (a) 1 and 2 only.
Why each option is right or wrong
- (a) CORRECT. The carbon sink and the food chains would both suffer.
- (b) Incorrect. Statement 1 is also true; the ocean's role as a carbon sink would weaken.
- (c) Incorrect. The density of ocean water does not depend on phytoplankton.
- (d) Incorrect. Statement 3 is false; density is set by temperature and salinity.
Key point
Phytoplankton: base of marine food chains, about half of global photosynthesis and oxygen, drive the biological carbon pump. Sea water density depends on temperature and salinity.
Remember
Phytoplankton: food base and carbon sink of the sea.
Common mistakes
- Accepting the density statement without asking what controls density.
- Forgetting that phytoplankton, not forests alone, supply much of our oxygen.
- Missing the ocean's role as a carbon sink.
Exam tip
Living things affect food and gases, not the physics of water. A claim about density does not fit.
Sources
- NCERT Class XII — Biology, Ch 14 (Ecosystem)
- NASA Earth Observatory — What are Phytoplankton?