Maarg Manthan · Topic 13.13

Dead Zones and Ocean Acidification

Environment & Ecology › Air, Marine, Noise and Other Pollution · Topic 13.13

Dead zones are areas of coastal or open water where dissolved oxygen has fallen so low, below about 2 milligrams per litre (hypoxia), that most marine animals cannot survive. They are mainly caused by nutrient pollution from farms, sewage and industry: nitrogen and phosphorus fuel algal blooms, and when the algae die and decompose, bacteria use up the oxygen, an effect worsened by water stratification. Well-known examples are the seasonal dead zone in the northern Gulf of Mexico fed by the Mississippi river, the Baltic Sea and the Chesapeake Bay; low-oxygen zones also occur in the Arabian Sea and Bay of Bengal. Ocean acidification is the fall in ocean pH as the sea absorbs carbon dioxide from the air, about a quarter of human emissions. Since the Industrial Revolution surface pH has dropped from about 8.2 to about 8.1, roughly a 30 percent rise in acidity, which reduces carbonate ions that corals, shellfish and some plankton need to build shells and skeletons.

5 min read · UPSC Prelims · Environment & Ecology · Air, Marine, Noise and Other Pollution

At a Glance

Dead zone Oxygen below about 2 mg per litre
Main cause Nutrient runoff
Famous zone Gulf of Mexico
pH change 8.2 to 8.1
CO2 absorbed by ocean About a quarter
Exam link UPSC Prelims, Environment & Ecology: Air, Marine, Noise and Other Pollution

Where Does It Fit?

This topic follows the general account of marine pollution and looks at two chemical changes in the sea. It leads to oil spills, ocean trash and microplastics. Previous topic: Marine Pollution: Sources and Effects. Next topic: Oil Spills, Ocean Trash and Debris, Microplastics.

How Do Dead Zones Form and Where Are They?

Aspect	Details
Definition	Hypoxic water with less than about 2 mg per litre of dissolved oxygen, where animals flee or die
Nutrient input	Fertiliser, sewage and animal waste from rivers add nitrogen and phosphorus
Algal bloom	Excess nutrients cause blooms of algae
Decomposition	Dead algae sink and are broken down by bacteria that use up the oxygen
Stratification	Layering of warm fresh water over cold salty water limits mixing and oxygen supply
Gulf of Mexico	One of the largest seasonal dead zones, fed by the Mississippi river basin
Other zones	Baltic Sea, Chesapeake Bay, Black Sea, East China Sea, and coastal areas of India

What Is Ocean Acidification and Its Effects?

Chemistry: CO2 dissolves in sea water and forms carbonic acid, which releases hydrogen ions and lowers pH
pH change: from about 8.2 to about 8.1, a 30 percent increase in acidity
Carbonate loss: fewer carbonate ions for making shells and skeletons
Corals: weaker skeletons and slower reef growth
Shellfish: oysters, clams and mussels have trouble forming shells, hitting aquaculture
Pteropods: tiny sea snails whose shells dissolve in acidified water, a key food for fish
Food chain: knock-on effects on fish and fisheries
Solution: cut CO2 emissions; local steps include restoring seagrass and mangroves

What Else Should You Remember?

Hypoxia: low oxygen
Anoxia: no oxygen
Number of dead zones: hundreds have been recorded worldwide, with numbers rising since the 1960s
Gulf of Mexico size: the summer zone has covered over ten thousand sq km in some years
Fish response: mobile fish leave, while bottom animals die
Natural dead zones: occur in deep basins and oxygen minimum zones
Arabian Sea: has a large natural oxygen minimum zone, which may be growing with warming
Warming effect: warm water holds less oxygen and strengthens stratification
Controls: cut nitrogen and phosphorus, upgrade sewage treatment and use fertiliser more efficiently
Wetland restoration: wetlands filter nutrients before they reach the sea
Ocean CO2 uptake: the ocean takes up about 25 percent of CO2 emitted by humans
Carbonic acid: forms when CO2 dissolves in water
Bicarbonate and carbonate: chemical forms that buffer ocean pH
Aragonite saturation: a measure of how easily corals and shells can form; falling saturation harms calcifiers
Cold water is more vulnerable: colder water absorbs more CO2, so polar seas acidify first
Coral bleaching vs acidification: bleaching is from heat, acidification weakens skeletons
Shellfish hatcheries: on the US Pacific coast faced larval die-offs from acidic upwelling
SDG 14.3: target to reduce and address ocean acidification
Monitoring: Global Ocean Acidification Observing Network
India: research on acidification in the Arabian Sea and Bay of Bengal by NIO and NCPOR
Climate link: acidification and warming are two effects of excess CO2
Link to previous: see Marine Pollution: Sources and Effects
Link to next: see Oil Spills, Ocean Trash and Debris, Microplastics
Link to eutrophication: see Algal Bloom and Eutrophication
Caution: dead zone counts and sizes change yearly; quote them as approximate
Exam hint: separate the cause of dead zones (nutrients) from the cause of acidification (CO2)
Calcifiers: organisms that build calcium carbonate shells or skeletons, including corals, molluscs and some plankton
Coral reefs and acidification: weaker skeletons make reefs more prone to damage by storms and erosion
Seagrass and mangrove refuge: dense plant meadows can raise local pH by taking up CO2, offering local relief for shellfish
Fisheries risk: the loss of shell-forming species hits food security and incomes of coastal people

Why Does It Matter for the Exam?

Dead zone cause: Nutrients
Acidification cause: CO2
pH change: 8.2 to 8.1
Current affairs link: Ocean deoxygenation
Common errors: confusing acidification with eutrophication

As an analytical point, dead zones are a local to regional nutrient problem that can be solved by cutting farm and sewage runoff, whereas acidification is global and can be solved only by cutting carbon dioxide. Both stress the same marine life.

Why Is This Topic Important?

Dead zone Nutrients
Acidity CO2
Result Life loss

What Came Next?

The next topic covers oil spills, ocean trash and debris and microplastics. Next topic: Oil Spills, Ocean Trash and Debris, Microplastics.

Key People and Terms

Hypoxia Low oxygen
Dead zone Oxygen-poor water
Acidification Fall in ocean pH
Pteropod Sea snail
Aragonite Shell mineral
Calcifier Shell builder

Exam Corner

Points to Remember

Dead zone: oxygen below 2 mg per litre.
Cause: nutrient runoff.
Ocean absorbs 25 percent of CO2.
pH 8.2 to 8.1.
Famous zone: Gulf of Mexico.

Do Not Confuse With

Dead zone and acidification Low oxygen versus low pH
Hypoxia and anoxia Low versus no oxygen
Bleaching and acidification Heat stress versus chemistry

Memory Hook

Nutrients bloom, bacteria breathe, zone goes dead; CO2 dissolves, pH drops, shells thin.

Mains Angle

Questions ask for cause and effect. These points help.

Dead zones Nutrients
Acidification CO2
Fix Cut inputs

A Question You May Face

An original practice question, not a past paper question.

“Explain the formation of ocean dead zones and the process and impacts of ocean acidification.”

How to Answer

Zones Nutrient chain.
Acid CO2 chemistry.
Impacts Shells, reefs.
Way Cut inputs.

GS Relevance

Prelims: definitions, causes, examples. GS Paper 3: ocean health and climate.

Frequently Asked Questions

What is a dead zone in the ocean?

A dead zone is an area of coastal or open water where dissolved oxygen is so low, usually below about 2 milligrams per litre, that most marine life cannot survive. It usually results from nutrient pollution that causes algal blooms, whose decay uses up oxygen. Hundreds of dead zones have been recorded worldwide.

What causes dead zones?

Dead zones are caused mainly by nutrients such as nitrogen and phosphorus from fertilisers, sewage and animal waste that reach the sea through rivers. They fuel algal blooms. When the algae die and decompose, bacteria use up the oxygen, and layering of the water prevents fresh oxygen from reaching the bottom. Warming makes the problem worse.

What is ocean acidification?

Ocean acidification is the decrease in the pH of the ocean caused by absorption of carbon dioxide from the atmosphere. The ocean takes up about a quarter of human CO2 emissions, forming carbonic acid. Since pre-industrial times surface pH has fallen from about 8.2 to 8.1, a rise in acidity of roughly 30 percent.

How does ocean acidification affect marine life?

Acidification reduces the carbonate ions that corals, oysters, clams, sea urchins and tiny plankton such as pteropods need to build their shells and skeletons. Shells become thinner or dissolve, coral reefs grow more slowly, and the food web that depends on these organisms is disturbed, which affects fisheries and aquaculture.

How can dead zones and acidification be reduced?

Dead zones can be reduced by cutting nutrient runoff through better fertiliser use, sewage treatment, restoring wetlands and buffer strips. Ocean acidification can only be slowed by cutting carbon dioxide emissions, although protecting seagrass, mangroves and kelp can offer local relief, and monitoring helps in early warning.

PYQ Practice — Statement Analysis

1 Ocean acidification is caused by absorption of carbon dioxide.

True

Forming carbonic acid.

2 Dead zones have high dissolved oxygen.

False

They have very low oxygen.

3 The Gulf of Mexico has a large seasonal dead zone.