Aquatic ecosystems: freshwater, estuarine, mangrove…

Core facts

Freshwater ecosystems:
- Lentic: standing water, such as ponds, lakes and wetlands.
- Lotic: flowing water, such as streams and rivers.
Aquatic organisms by lifestyle:
- Plankton: drift with water. Phytoplankton are producers; zooplankton are consumers.
- Nekton: strong swimmers, such as fish and whales.
- Benthos: live on or in the bottom, such as snails, worms and crabs.
- Neuston: live at the surface film.
Estuary: where a river meets the sea; water is brackish (a mix of fresh and salt water). Estuaries are among the most productive ecosystems and are nurseries for fish and prawns.
Mangroves: salt-tolerant trees and shrubs of tropical and subtropical tidal coasts.
Sundarbans (Ganga–Brahmaputra delta, shared by India and Bangladesh) is the largest mangrove forest in the world.
Mangrove cover in India: about 4,990 km², roughly 0.15% of the country's area (ISFR 2021 and 2023). West Bengal has the most, then Gujarat, then Andaman and Nicobar Islands.
Coral reefs in India: Gulf of Mannar, Gulf of Kachchh, Lakshadweep, Andaman and Nicobar Islands.
Mangroves slow down strong winds and waves, so they protect coastal people from cyclones and tsunamis.

Explained

Zones of a lake

Zone	Where	Life
Littoral	Shallow water near the shore; light reaches the bottom	Rooted plants, many animals
Limnetic	Open, sunlit surface water away from shore	Phytoplankton, zooplankton, fish
Profundal	Deep water below light penetration	Few organisms; decomposers
Benthic	The bottom, at all depths	Bottom-dwellers, decomposers

Lakes by nutrient level

Oligotrophic: low nutrients, clear water, high oxygen, low productivity (deep mountain lakes).
Eutrophic: high nutrients, green water, high productivity, oxygen depletion near the bottom.
Eutrophication is the ageing of a lake by nutrient enrichment. It is natural but slow; fertiliser and sewage run-off (nitrates and phosphates) make it fast ("cultural eutrophication"). It causes algal blooms, low oxygen and fish deaths.

Mangrove adaptations

Pneumatophores: roots that grow upward out of the mud to take in air, because the mud lacks oxygen.
Vivipary: seeds germinate while still on the tree, and the seedling drops ready to root.
Stilt or prop roots give support in soft mud.
Salt management: many species exclude salt at the roots or excrete it through leaf glands.

Marine zones

By light: photic (sunlit, where photosynthesis takes place, roughly the top 200 m) and aphotic (dark).
By position: intertidal (between high and low tide), neritic (over the continental shelf), oceanic (open ocean beyond the shelf).
By habitat: pelagic (open water) and benthic (sea floor).

Deeper

Thermal stratification in deep temperate lakes

In summer, a lake forms three layers:
- epilimnion: warm upper layer
- thermocline (metalimnion): a layer where temperature falls rapidly with depth
- hypolimnion: cold bottom layer
In autumn and spring, surface water cools or warms to about 4 °C, the temperature at which fresh water is densest. It then sinks, and the lake mixes (turnover), bringing oxygen down and nutrients up.
Water is densest at about 4 °C, and ice floats. This keeps lakes from freezing solid, so life survives under the ice.

Why estuaries and mangroves are so productive

Rivers bring nutrients; tides mix them; shallow water lets light reach the bottom.
Mangrove litter supports a rich detritus food chain (see ENV-EC-03).
They trap sediment and stabilise shorelines.
They store large amounts of blue carbon in their soils.

Coral reefs

Corals are colonies of tiny animals (polyps) that build calcium carbonate skeletons.
They live in a mutualism with algae called zooxanthellae, which live in their tissues, photosynthesise and supply food. The algae also give corals much of their colour.
Reefs need warm, clear, shallow, sunlit water.
Coral bleaching: under heat stress, corals expel the zooxanthellae and turn white. Long or repeated bleaching kills them. Ocean acidification also weakens their skeletons.
Reef types: fringing (close to shore), barrier (separated from shore by a lagoon), atoll (a ring around a lagoon, as in Lakshadweep).

Deep-sea hydrothermal vents. Bacteria there make food by chemosynthesis, using hydrogen sulphide. They support tube worms, clams and crabs without sunlight.

AP angle

Coringa Wildlife Sanctuary (Kakinada district) protects mangroves in the Godavari estuary, one of the largest mangrove stretches on India's east coast.
Krishna Wildlife Sanctuary protects the mangroves of the Krishna estuary.
Pulicat Lake, shared by AP and Tamil Nadu, is India's second-largest brackish-water lagoon after Chilika. Sriharikota island lies between the lagoon and the sea.
Kolleru Lake, between the Godavari and Krishna deltas, is one of India's largest natural freshwater lakes. It was declared a Ramsar site in 2002. Large areas were turned into fish tanks, and the government demolished many of them in 2006 to restore the lake.

Common traps

✗ "Lentic means flowing water." ✓ Lentic = standing (ponds, lakes); lotic = flowing (rivers).
✗ "Nekton drift with the water." ✓ Plankton drift; nekton swim actively.
✗ "Pulicat is India's largest brackish lagoon." ✓ Chilika (Odisha) is the largest; Pulicat is second.
✗ "Kolleru is a brackish-water lagoon." ✓ Kolleru is a freshwater lake; Ramsar site since 2002.
✗ "Gujarat has the largest mangrove cover in India." ✓ West Bengal has the most; Gujarat is second.
✗ "Corals make their own food by photosynthesis." ✓ Their partner algae, zooxanthellae, do; corals are animals.

Quick revision

Lentic = standing; lotic = flowing.
Plankton drift, nekton swim, benthos live at the bottom.
Lake zones: littoral, limnetic, profundal, benthic.
Thermocline separates the warm epilimnion from the cold hypolimnion.
Estuary: brackish, highly productive, fish nursery.
Sundarbans: largest mangrove forest in the world.
India's mangroves: about 4,990 km² (ISFR 2021/2023); West Bengal > Gujarat > Andaman and Nicobar.
Mangrove features: pneumatophores, vivipary, salt control.
Corals + zooxanthellae = mutualism; heat causes bleaching.
AP: Coringa (Godavari), Krishna WLS, Pulicat (2nd brackish lagoon), Kolleru (Ramsar 2002).

Go deeper: Mains writing (beyond what APPSC Group 1 usually asks)

Mains writing Mains writing

Frame: coastal and freshwater ecosystems give flood control, fisheries, clean water and storm protection, yet they are the most converted ecosystems, for aquaculture, ports, reclamation and sewage disposal.
Analysis points:
1. Mangroves as a bio-shield: after the 2004 tsunami, villages behind mangroves were less affected, for example a mangrove village in Cuddalore district, Tamil Nadu.
2. Aquaculture conversion of mangroves and lakes (Kolleru, Godavari delta) gives quick income but loses fisheries and protection.
3. Eutrophication from untreated sewage and fertiliser run-off is the main threat to urban lakes.
4. Coral reefs face bleaching from marine heatwaves; repeated events leave too little time for recovery.
Way forward:
- protect mangroves through the Coastal Regulation Zone rules and the MISHTI scheme for mangrove planting (2023)
- treat sewage before it reaches lakes; control fertiliser run-off
- community-managed fisheries
- monitor and protect reefs; cut emissions that drive warming
Example: the Sundarbans reduce the force of cyclones such as Amphan (2020) before they reach Kolkata, a service worth far more than the land would yield if converted.

Medha AI writes its own notes and checks them against the sources listed. Medha AI is an independent preparation service. It is not affiliated with, endorsed by or connected to APPSC, TSPSC, MPSC, UPSC or any other commission or government body.

2.5.1.8 Aquatic ecosystems: freshwater, estuarine, mangrove and marine

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