What happens if the number of plankton in the ocean decreases?

In one line

The rule

Plankton are tiny living things in the ocean that form the base of the marine food chain and help produce oxygen for the Earth.

Let's understand it simply

Imagine you are watching a cricket match. The opening batsmen are like plankton—they start the game and set the stage for everyone else. If the openers get out quickly, the whole team struggles. In the ocean, plankton are the 'openers' for food. They are eaten by small fish, which are then eaten by bigger fish, and so on. If plankton numbers drop, the whole food chain is in trouble.

Plankton also act like tiny plants. They use sunlight to make food and release oxygen, just like the tulsi plant in your home. Less plankton means less oxygen for fish—and even for us.

How it works

1. Plankton (tiny plants and animals) float near the ocean surface.
2. They use sunlight to make food (photosynthesis) and release oxygen.
3. Small fish and animals eat plankton.
4. Bigger fish, crabs, and whales eat those small fish.
5. If plankton decrease, small fish have less food and their numbers drop.
6. This causes bigger fish and other animals to also suffer or die.
7. Less plankton also means less oxygen is made for the ocean and air.

The story behind it

Scientists noticed that when plankton numbers went down in some places, fish catches also dropped. This happened in the 20th century, especially near coasts where pollution or warm water killed plankton. One famous study in the 1970s showed that plankton are not just food—they are also the world's biggest oxygen makers, even more than forests!

Where you see it in real life

• Fish markets in Kerala or West Bengal: When plankton are low, fishermen catch fewer fish because small fish have less to eat.
• Coral reefs: Corals eat plankton. If plankton are missing, corals get weak and reefs die.
• Whale watching in Gujarat: Whales eat tons of plankton (krill). Fewer plankton means fewer whales.
• Oxygen in the air: Plankton make about half the oxygen we breathe. Less plankton means less fresh air.
• Clear blue water: Sometimes, water looks very clear because there is little plankton. But this is not always good—it means less food for fish.
• Algal blooms: Sometimes, pollution causes too many plankton (algae), which is also bad. But if they suddenly die off, it can crash the food chain.

Why it matters

• Food security: Millions of people in India and around the world depend on fish for food and jobs.
• Oxygen supply: Plankton help make the oxygen we need to live.
• Climate control: Plankton absorb carbon dioxide (CO₂), helping slow down global warming.
• Healthy oceans: Without plankton, the whole ocean ecosystem can collapse.
• Exams and future studies: This idea is important for biology, environmental science, and geography.

Worked example

Given: Plankton population drops by 50% in a region.
Step 1: Small fish eat less, so their numbers drop by 40%.
Step 2: Bigger fish and whales now have less food, so their numbers drop by 30%.
Step 3: Less plankton means less photosynthesis, so oxygen levels in water drop.
Step 4: Some fish die due to low oxygen.
Step 5: Fishermen catch fewer fish, affecting their income.

Common mistakes

• Thinking only big animals matter: Tiny plankton are just as important as big fish or whales.
• Believing oxygen comes only from trees: Plankton make about half of Earth's oxygen.
• Ignoring the chain reaction: A drop in plankton affects every level of the food chain, not just one type of fish.
• Assuming clear water is always healthy: Sometimes, clear water means no plankton, which is bad for marine life.

Remember it

Plankton are the ocean’s base—if they fall, everything above them falls too.