Maarg Manthan · Topic 4.7

Sea-Grass and Seaweeds

Environment & Ecology › Terrestrial and Aquatic Ecosystems · Topic 4.7

Seagrasses and seaweeds are two different groups of marine plant-like organisms. Seagrasses are true flowering plants that grow in shallow, sandy or muddy coastal waters, with roots, stems, leaves, flowers and seeds, and they form underwater meadows. Seaweeds are marine algae, simple organisms without true roots, stems or flowers, attached to rocks by a holdfast, and are classed as green, brown and red algae. Seagrass meadows are among the most productive ecosystems and are called the lungs of the sea and blue carbon ecosystems; they stabilise sediments, nurse fish, feed dugongs and sea turtles, and store large amounts of carbon. Seaweeds supply food, agar, algin and carrageenan and are used as fertiliser and biofuel. India has seagrass in the Gulf of Mannar and Palk Bay and has launched a seaweed mission.

5 min read · UPSC Prelims · Environment & Ecology · Terrestrial and Aquatic Ecosystems

At a Glance

Seagrass Flowering plant
Seaweed Algae
Seagrass role Blue carbon, dugong food
Seaweed products Agar, algin
Indian site Gulf of Mannar
Exam link UPSC Prelims, Environment & Ecology: Terrestrial and Aquatic Ecosystems

Where Does It Fit?

This topic follows the account of phytoplankton factors. It covers two kinds of marine vegetation of shallow waters, which are important for coastal biodiversity, carbon and livelihoods. Previous topic: Factors Affecting Phytoplankton Biodiversity and Their Importance. Next topic: Human-Modified Ecosystems.

How Do Seagrasses and Seaweeds Differ?

Feature	Seagrass and seaweed
Type of organism	Seagrass: flowering plants (angiosperms). Seaweed: macroscopic algae
Structure	Seagrass has true roots, rhizomes, stems and leaves. Seaweed has a holdfast, stipe and blades (thallus), without true roots, stems or leaves
Reproduction	Seagrass produces flowers, fruits and seeds, including underwater pollination. Seaweed reproduces by spores and fragmentation
Habitat	Seagrass grows in soft sand or mud in sheltered shallow water. Seaweed attaches to rocks and hard surfaces
Groups	Seagrass: Zostera, Posidonia, Halophila, Thalassia, Cymodocea. Seaweeds: green (Ulva), brown (Sargassum, Laminaria) and red (Gracilaria, Gelidium)
Depth	Both need light, so they grow in the upper tens of metres
Distribution	Seagrass in tropical and temperate coasts worldwide; seaweeds in all oceans, with kelp forests in cool waters

What Are the Values, Uses and Threats?

Blue carbon: seagrass meadows store carbon in sediments for centuries, up to 35 times faster than tropical rainforests per area
Habitat: nurseries for fish and prawns, and feeding grounds for dugongs and green turtles
Coastal protection: trap sediments and reduce wave energy
Water quality: filter nutrients and add oxygen
Seaweed products: agar (from red algae, used in labs and desserts), algin (from brown algae, used as a thickener) and carrageenan (food and cosmetic)
Other uses: food in Japan, Korea and China, fertiliser, animal feed, biofuel and bioplastic
Threats: coastal development, dredging, pollution and nutrient run-off, trawling, warming and eutrophication
Loss: about 7 percent of seagrass area has been lost each year in recent decades according to global studies

What Else Should You Remember?

Dugong: the sea cow, India’s marine herbivore mammal, a vulnerable species protected under Schedule I of the Wildlife Protection Act
Dugong Conservation Reserve: India’s first was declared in Palk Bay, Tamil Nadu, in 2022
Seagrass in India: Gulf of Mannar, Palk Bay, Lakshadweep, Andaman and Nicobar, Chilika and Kutch
Seaweed in India: Tamil Nadu coast, Gujarat coast and Lakshadweep
Seaweed mission: Pradhan Mantri Matsya Sampada Yojana supports seaweed farming, with a multipurpose seaweed park planned in Tamil Nadu
Kelp: giant brown algae forming forests in cool seas
Sargassum: floating brown seaweed in the Sargasso Sea
Seaweed farming: uses ropes or nets in shallow water and needs no land or fresh water
Carbon market: blue carbon projects may earn credits
Eelgrass: Zostera, widespread in temperate seas
Epiphytes: algae growing on seagrass blades
Seagrass wasting disease: has hit eelgrass in the past
Gulf of Mannar Biosphere Reserve: covers seagrass, corals and mangroves
Restoration: replanting seagrass is being tried
Climate: seagrass and mangrove protection is part of climate mitigation
Ocean acidification: may favour seagrass
Link: see Marine Ecosystem and Marine Organisms
Link to wetlands: see Coastal and Wetland Ecosystems macro
Link to dugong: see the protected species lists later
Caution: seaweed is not a plant in the strict sense, it is algae
Seagrass pollination: pollen drifts underwater in strands, an unusual adaptation
Kelp forests: support rich communities and are cooling the water around them
Carrageenan source: Kappaphycus seaweed is farmed along the Tamil Nadu coast
Livelihoods: women self-help groups in coastal Tamil Nadu farm seaweed
Seaweed nutrition: rich in iodine, minerals and fibre
Seagrass loss: dredging and trawling in the Palk Bay have damaged some beds, so fishing rules matter
Exam hint: seagrass is a plant, seaweed is an alga, and both are not corals

Why Does It Matter for the Exam?

Difference: flowering plant versus algae
Seagrass role: blue carbon, dugong
Seaweed products: agar, algin
Current affairs link: Dugong Conservation Reserve and seaweed park
Common errors: calling seagrass an alga or seaweed a flowering plant

As an analytical point, seagrasses and seaweeds are cheap, natural solutions for carbon storage, coastal protection and livelihoods. Protecting meadows and growing seaweed sustainably can support both climate goals and coastal incomes.

Why Is This Topic Important?

Difference Plant versus alga
Value Carbon, nursery
Uses Agar, food

What Came Next?

The next topic covers human-modified ecosystems. Next topic: Human-Modified Ecosystems.

Key People and Terms

Seagrass Marine flowering plant
Seaweed Macroalga
Holdfast Attachment organ of alga
Blue carbon Carbon stored in coastal ecosystems
Agar Gel from red algae
Algin Thickener from brown algae

Exam Corner

Points to Remember

Seagrass is an angiosperm; seaweed is an alga.
Dugong feeds on seagrass.
Agar from red algae; algin from brown algae.
Dugong reserve in Palk Bay (2022).
Seagrass stores blue carbon.

Do Not Confuse With

Seagrass and seaweed Flowering plant versus algae
Agar and algin Red algae gel versus brown algae thickener
Blue carbon and green carbon Coastal sea ecosystems versus forests

Memory Hook

Seagrass flowers and feeds dugongs; seaweed holds on and gives agar and algin.

Mains Angle

Questions ask for differences, uses and Indian sites. These points help.

Value Services
Uses Seaweed economy
Threats Loss

A Question You May Face

An original practice question, not a past paper question.

“Distinguish between seagrass and seaweeds and discuss their ecological and economic importance.”

How to Answer

Difference Plant versus alga.
Services Carbon, nursery.
Uses Products.
Threats Loss.

GS Relevance

Prelims: difference between seagrass and seaweed, uses, locations in India. GS Paper 3: coastal ecology and blue economy.

Frequently Asked Questions

What is the difference between seagrass and seaweed?

Seagrasses are true flowering plants with roots, stems, leaves, flowers and seeds that grow in soft sediments in shallow coastal waters. Seaweeds are marine algae without true roots, stems or flowers, which attach to rocks by a holdfast. Seagrasses are not grasses or algae, and seaweeds are not plants in the strict sense.

Why are seagrass meadows important?

Seagrass meadows are highly productive nursery grounds for fish and prawns and feeding areas for dugongs and sea turtles. They stabilise sediments, protect coasts, improve water quality and store large amounts of carbon in sediments, which is why they are known as blue carbon ecosystems.

What are the uses of seaweeds?

Seaweeds are used as food, especially in East Asia, as fertiliser and animal feed, and as a source of useful chemicals. Red algae give agar, used in microbiology and food, brown algae give algin, used as a thickener, and carrageenan from red algae is used in foods and cosmetics. They are also being studied for biofuel.

Where are seagrass beds found in India?

Seagrass beds occur in the Gulf of Mannar and Palk Bay, around Lakshadweep, the Andaman and Nicobar Islands, Chilika Lake, and the Gulf of Kutch. The Gulf of Mannar and Palk Bay region has the largest meadows and supports the dugong, for which India declared its first Dugong Conservation Reserve in Palk Bay in 2022.

What is blue carbon?

Blue carbon is the carbon stored in coastal and marine ecosystems such as mangroves, seagrass meadows and salt marshes. These ecosystems capture carbon dioxide and bury it in waterlogged sediments for hundreds or thousands of years, so protecting them helps to reduce greenhouse gases in the atmosphere.

PYQ Practice — Statement Analysis

1 Seagrasses are flowering plants.

True

Unlike seaweeds.

2 Algin is obtained from red algae.

False

From brown algae; agar from red algae.

3 Dugongs feed on seagrass.