Marine Ecosystem and Marine Organisms
Environment & Ecology › Terrestrial and Aquatic Ecosystems · Topic 4.4
The marine ecosystem is the largest ecosystem on Earth, covering about 71 percent of the surface and holding about 97 percent of its water, with an average salinity of about 35 parts per thousand. It is divided horizontally into the intertidal or littoral zone between high and low tides, the neritic zone over the continental shelf, and the oceanic zone beyond. Vertically, the pelagic realm is the open water and the benthic realm is the sea floor, with the sunlit epipelagic zone down to 200 m, the mesopelagic to 1,000 m, the bathypelagic and the abyssal zone and the hadal zone in the deepest trenches. Marine organisms include plankton, nekton such as fish, squid and whales, and benthos such as corals, crabs, sea stars and sponges. Special marine ecosystems are coral reefs, mangroves, seagrass beds, estuaries and hydrothermal vents.
5 min read · UPSC Prelims · Environment & Ecology · Terrestrial and Aquatic Ecosystems
At a Glance
- Surface covered About 71 percent
- Salinity About 35 ppt
- Horizontal zones Intertidal, neritic, oceanic
- Vertical zones Epipelagic to hadal
- Dark-zone energy Chemosynthesis
- Exam link UPSC Prelims, Environment & Ecology: Terrestrial and Aquatic Ecosystems
Where Does It Fit?
This topic follows the account of freshwater ecosystems. It covers the ocean in detail and prepares for the next topics on plankton and phytoplankton. Previous topic: Freshwater Ecosystem. Next topic: Plankton: Phytoplankton and Zooplankton.
What Are the Zones of the Marine Ecosystem?
| Zone | Features |
|---|---|
| Intertidal (littoral) zone | Between high and low tide marks. Faces drying, waves, salinity change. Barnacles, mussels, crabs, seaweeds |
| Neritic zone | Shallow water over the continental shelf, up to 200 m. High light and nutrients. Most fisheries |
| Oceanic zone | Deep open sea beyond the shelf |
| Epipelagic (sunlit) zone | Surface to 200 m. Photosynthesis occurs. Phytoplankton, most fish |
| Mesopelagic (twilight) zone | 200 to 1,000 m. Dim light; many animals migrate to the surface at night. Bioluminescent fish |
| Bathypelagic (midnight) zone | 1,000 to 4,000 m. Dark, cold. Anglerfish, squid |
| Abyssal and hadal zones | Abyssal 4,000 to 6,000 m; hadal below 6,000 m in trenches such as Mariana Trench (about 11,000 m). Very high pressure, near-freezing |
Which Organisms and Special Ecosystems Live in the Sea?
- Plankton: phytoplankton (diatoms, dinoflagellates) and zooplankton (copepods, krill, jellyfish larvae)
- Nekton: fish, squid, sea turtles, dolphins and whales
- Benthos: corals, sponges, sea stars, crabs, sea cucumbers, worms and sea grasses
- Coral reefs: built by coral polyps with symbiotic zooxanthellae in warm, clear water; high biodiversity; threatened by bleaching
- Mangroves and seagrass: coastal nurseries and carbon sinks
- Hydrothermal vents: deep sea vents with chemosynthetic bacteria feeding tube worms and clams
- Polar seas: cold, productive waters supporting krill, whales, seals and penguins
- Sargasso Sea: a sea without land boundaries in the North Atlantic, bounded by currents and with floating Sargassum weed
What Else Should You Remember?
- Marine biodiversity: oceans hold most animal phyla
- Census of Marine Life: a global programme of 2000 to 2010
- Bioluminescence: light made by deep sea animals
- Marine mammals: whales, dolphins, dugong and seals; the dugong is India’s marine herbivore mammal
- Marine turtles: Olive ridley mass nesting at Gahirmatha in Odisha
- Deep sea mining: polymetallic nodules in the Indian Ocean are explored by India
- Deep Ocean Mission: India’s 2021 mission for ocean resources and technology
- Ocean currents: affect distribution of life and climate
- Thermohaline circulation: deep global current system
- Marine pollution: plastic, oil and chemicals harm marine life
- Overfishing: depletes stocks and food webs
- Ocean warming: drives coral bleaching and shifts in species
- Acidification: harms corals and shellfish
- Marine protected areas: areas restricted for fishing and use
- Law of the Sea: UNCLOS 1982 sets rights over ocean zones
- Exclusive Economic Zone: 200 nautical miles from the coast
- High seas treaty: the BBNJ Agreement was adopted in 2023 to protect biodiversity beyond national jurisdiction
- Link: see Plankton: Phytoplankton and Zooplankton
- Link to coasts: see Coastal and Wetland Ecosystems macro
- Link to productivity: see Aquatic Ecosystems and Factors Affecting Their Productivity
- Caution: depth limits of zones differ slightly by source
- Rocky shore zonation: barnacles live high on the shore, mussels in the middle and seaweeds low down, forming bands according to exposure to air
- Sandy and muddy shores: burrowing worms, clams and crabs live in sediments
- Tidal range: varies with the moon and coast, with high tides in the Bay of Fundy
- Marine food chain length: longer than on land because producers are tiny
- Deep sea adaptations: slow metabolism, bioluminescence and large eyes or none
- Coral triangle: Southeast Asian seas have the highest coral diversity
- Indian reefs: Gulf of Mannar, Gulf of Kachchh, Lakshadweep and Andaman and Nicobar have coral reefs
- Indian marine biodiversity: India has a coastline of about 11,098 km including islands, and an EEZ of about 2 million square km
- Marine ecosystem services: food, oxygen, climate regulation, tourism and coastal protection
- Ocean heat: the oceans have absorbed most of the extra heat from global warming
Why Does It Matter for the Exam?
- Zones: epipelagic to hadal
- Life forms: plankton, nekton, benthos
- Special: reefs, vents
- Current affairs link: BBNJ treaty and Deep Ocean Mission
- Common errors: mixing neritic with oceanic or saying vents depend on sunlight
As an analytical point, the ocean regulates climate, supplies food and absorbs carbon, yet is stressed by warming, acidification, pollution and overfishing. Marine protected areas and agreements such as the high seas treaty are key policy tools.
Why Is This Topic Important?
- Zones Horizontal, vertical
- Life Three forms
- Threats Warming, pollution
What Came Next?
The next topic covers plankton, phytoplankton and zooplankton. Next topic: Plankton: Phytoplankton and Zooplankton.
Key People and Terms
- Pelagic Open water
- Benthic Sea floor
- Neritic Over continental shelf
- Abyssal Deep ocean plain
- Hadal Trench zone
- Zooxanthellae Algae in corals
Exam Corner
Points to Remember
- Ocean covers 71 percent of Earth.
- Sunlit zone to 200 m.
- Mariana Trench about 11,000 m.
- EEZ: 200 nautical miles.
- BBNJ treaty adopted 2023.
Do Not Confuse With
- Neritic and oceanic zone Shelf waters versus open sea
- Pelagic and benthic Open water versus bottom
- Epipelagic and mesopelagic Sunlit versus twilight
Memory Hook
Sun, twilight, midnight, abyss, hadal: five steps down; neritic hugs the shelf.
Mains Angle
Questions ask for zones and organisms. These points help.
- Zones Structure
- Life Organisms
- Threats Stress
A Question You May Face
An original practice question, not a past paper question.
“Describe the zonation of the marine ecosystem and the organisms found in each zone, and discuss major threats.”
How to Answer
- Zones Horizontal, vertical.
- Organisms Groups.
- Special Reefs, vents.
- Threats Warming.
GS Relevance
Frequently Asked Questions
What are the main zones of the ocean?
The ocean is divided horizontally into the intertidal zone between tides, the neritic zone over the continental shelf and the oceanic zone beyond. Vertically, the sunlit epipelagic zone extends to 200 m, followed by the mesopelagic, bathypelagic, abyssal and hadal zones, which get darker, colder and higher in pressure with depth.
What is the difference between pelagic and benthic zones?
The pelagic zone is the open water column of the sea, where plankton and nekton live, while the benthic zone is the bottom of the sea, where benthos such as corals, crabs and worms live. Both exist at all depths from the shore to the deepest trenches.
What are hydrothermal vents?
Hydrothermal vents are openings on the sea floor, usually along mid-ocean ridges, from which hot water rich in minerals and gases such as hydrogen sulphide flows. Chemosynthetic bacteria use these chemicals to make food and support communities of tube worms, clams and shrimps, without sunlight.
What is the neritic zone?
The neritic zone is the shallow part of the ocean above the continental shelf, with depth up to about 200 metres. It receives plenty of sunlight and nutrients from land, so it is highly productive and supports most of the world's marine fisheries.
What is the BBNJ Agreement?
The Agreement on Biodiversity Beyond National Jurisdiction, also called the High Seas Treaty, was adopted by the United Nations in June 2023 to conserve and sustainably use marine biodiversity in areas beyond national jurisdiction. It allows marine protected areas on the high seas and rules for sharing benefits from marine genetic resources.
PYQ Practice — Statement Analysis
1 The neritic zone lies above the continental shelf.
Up to about 200 m.
2 Hydrothermal vent communities depend on sunlight.
They depend on chemosynthesis.
3 The epipelagic zone is the sunlit zone.
Down to about 200 m.
4 The BBNJ treaty was adopted in 2023.
For the high seas.
Past Mains Questions on This Topic
This topic comes under Terrestrial and Aquatic Ecosystems. UPSC Mains questions asked on the matching syllabus topic, year-wise with marks:
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