The acidification of oceans is increasing. Why is this phenomenon a cause of concern: 1. The growth and survival of calcareous phytoplankton will be adversely affected 2. The growth and survival of coral reefs will be adversely affected 3. The survival of some animals that have phytoplanktonic larvae will be adversely affected 4. The cloud seeding and formation of clouds will be adversely affected Which of the statements given above is/are correct?
Contents16
- A1, 2 and 3 only
- B2 only
- C1 and 3 only
- D1, 2, 3 and 4
Show answer
Answer: (A) 1, 2 and 3 only
Ocean acidification occurs when oceans absorb excess CO₂ from the atmosphere, leading to a fall in ocean pH and a reduction in carbonate ions.
Statement 1 correct — calcareous phytoplankton (such as coccolithophores) depend on calcium carbonate for their shells, and acidic conditions make shell formation difficult.
Statement 2 correct — coral reefs are made of calcium carbonate, so ocean acidification weakens coral growth and survival.
Statement 3 correct — many marine organisms with planktonic larvae are affected because acidification disrupts marine food chains and harms larval development.
Statement 4 is incorrect — although phytoplankton can influence atmospheric processes indirectly, ocean acidification is not considered a direct concern for cloud seeding or cloud formation in the context of this question.
Therefore:
- Statements 1, 2 and 3 are correct.
- Statement 4 is incorrect.
Hence, the correct answer is 1, 2 and 3 only.
Ocean acidification affects marine ecosystems through a cascade starting with calcium carbonate dissolution, impacting shells, coral reefs, and the entire marine food web.
The connection between phytoplankton decline and cloud formation through dimethyl sulphide (DMS) is the advanced concept that separates this question from basic ocean acidification knowledge.
UPSC is testing whether students understand both direct chemical effects (calcium carbonate dissolution) and indirect ecosystem effects (food web disruption and climate feedback loops).
Ocean Acidification: Causes & Chemistry
Environment acidification of oceans
Ocean Acidification: Chemistry & Drivers
Oceans absorb 30% of atmospheric CO₂, forming carbonic acid that lowers pH
Ocean pH has dropped from 8.2 to 8.1 since pre-industrial times (0.1 unit = 26% increase in acidity)
Calcium carbonate dissolution increases in acidic water, threatening shell-forming organisms
What Is Ocean Acidification
Ocean acidification occurs when seawater absorbs excess carbon dioxide (CO₂) from the atmosphere. The CO₂ reacts with seawater to form carbonic acid, which releases hydrogen ions and lowers the ocean's pH.
Chemical Process
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**CO₂ Absorption**
Atmospheric **CO₂** dissolves into seawater`"]
s2["`**Carbonic Acid Formation**
**CO₂ + H₂O → H₂CO₃** (carbonic acid)`"]
s3["`**Ion Release**
**H₂CO₃ → H⁺ + HCO₃⁻** (hydrogen ions + bicarbonate)`"]
s4["`**pH Reduction**
More **H⁺ ions** = lower pH = more acidic ocean`"]
s1 --> s2
s2 --> s3
s3 --> s4Key Drivers
Fossil fuel emissions are the primary source of excess atmospheric CO₂
Deforestation reduces CO₂ absorption by land, forcing oceans to absorb more
Cold water absorbs more CO₂ than warm water - polar regions most affected
Impact on Calcareous Marine Life
Environment calcareous phytoplankton coral reefs
Calcareous Organisms & Ocean Acidification
Calcareous organisms use calcium carbonate (CaCO₃) to build shells and skeletons
Acidic water dissolves calcium carbonate, making shell formation difficult
Coccolithophores (phytoplankton) and coral polyps are most vulnerable
Why Calcium Carbonate Matters
Calcareous organisms are marine species that build their shells, skeletons, or protective plates from calcium carbonate (CaCO₃). This includes phytoplankton like coccolithophores, coral polyps, mollusks, and some crustaceans.
Affected Organisms
Organism Type | Examples | CaCO₃ Structure | Impact of Acidification |
|---|---|---|---|
Calcareous Phytoplankton | Coccolithophores, Foraminifera | Protective plates/shells | Thinner shells, reduced growth |
Coral Reefs | Hard corals, Coral polyps | Calcium carbonate skeleton | Bleaching, skeleton dissolution |
Mollusks | Oysters, Mussels, Clams | External shells | Shell thinning, vulnerability |
Crustaceans | Some crabs, Sea urchins | Exoskeleton plates | Weakened protection |
Mechanism of Harm
Shell dissolution: Existing calcium carbonate structures dissolve in acidic water
Reduced calcification: Organisms struggle to extract carbonate ions needed for shell-building
Energy diversion: More metabolic energy spent on shell maintenance, less on growth and reproduction
All four statements in the 2012 question are correct - don't eliminate statement 4 about cloud formation
Calcareous ≠ just coral - includes phytoplankton, mollusks, and other shell-forming organisms
Indirect effects through food web disruption are as important as direct shell damage
Food Web & Larvae Impact
Environment phytoplanktonic larvae survival of some animals
Marine Food Web Disruption by Ocean Acidification
Phytoplankton form the base of marine food webs - their decline affects all levels
Many marine animals have planktonic larvae that depend on healthy phytoplankton populations
Cascading effects move up the food chain from primary producers to top predators
Phytoplankton as Foundation
Phytoplankton are microscopic marine plants that form the foundation of ocean food webs. When acidification reduces calcareous phytoplankton like coccolithophores, it disrupts the entire ecosystem's primary productivity.
Food Web Connections
# Phytoplankton Decline
## Direct Impact
- Reduced primary productivity
- Fewer calcareous species
- Changed species composition
## Larvae Affected
- Fish larvae
- Crustacean larvae
- Mollusk larvae
- Echinoderm larvae
## Cascading Effects
- Small fish decline
- Commercial fish impact
- Marine mammal food shortage
- Seabird population changesLarval Stage Vulnerability
Planktonic larvae of many fish, crabs, and mollusks depend on phytoplankton for food during critical early development
Shell-forming larvae (like sea urchin and mollusk larvae) face double impact - food shortage plus difficulty building their own calcium carbonate structures
Timing mismatches occur when phytoplankton blooms shift, leaving larvae without adequate food during crucial windows
Ocean-Atmosphere Climate Feedback
Environment cloud seeding formation of clouds
Phytoplankton, Cloud Formation & Climate Feedback
Phytoplankton produce dimethyl sulphide (DMS) which helps form cloud condensation nuclei
Fewer phytoplankton = less DMS = reduced cloud formation over oceans
This creates a positive feedback loop - less cloud cover = more warming = more acidification
The Ocean-Cloud Connection
Many marine phytoplankton, especially coccolithophores, produce a sulfur compound called dimethyl sulphide (DMS). When DMS reaches the atmosphere, it forms particles that act as cloud condensation nuclei - the seeds around which water droplets form clouds.
DMS-Cloud Process
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Phytoplankton Activity**
Healthy **coccolithophores** and other phytoplankton produce **DMS**`"]
s2["`**DMS Release**
**DMS** escapes from ocean surface into atmosphere`"]
s3["`**Particle Formation**
DMS oxidizes to form **sulfate aerosols** (cloud condensation nuclei)`"]
s4["`**Cloud Seeding**
Water droplets condense around nuclei to form **marine clouds**`"]
s5["`**Climate Regulation**
Clouds reflect sunlight, helping cool Earth's surface`"]
s1 --> s2
s2 --> s3
s3 --> s4
s4 --> s5Climate Feedback Loop
Reduced phytoplankton → less DMS → fewer cloud condensation nuclei → less marine cloud cover
Less cloud cover → more solar radiation reaches ocean → more warming → more CO₂ absorption → more acidification
This positive feedback accelerates both ocean acidification and global warming
Statement 4 is correct - many students eliminate it thinking cloud formation is unrelated to ocean acidification
DMS from phytoplankton is the key link - not direct chemical cloud seeding by acidified water
Remember: healthy phytoplankton = more clouds = cooler planet - acidification disrupts this natural climate regulation