1. Aquatic Ecosystems

Aquatic ecosystems are ecosystems where plants and animals live in water.

Classification of Aquatic Ecosystems

1. Freshwater ecosystems
   • Very low salt content (< 5 ppt)
   • Examples:
     • Lentic (still water): lakes, ponds, swamps
     • Lotic (running water): rivers, streams

2. Marine ecosystems
   • High salt content (≈ 35 ppt or more)
   • Examples: oceans, seas
3. Brackish water ecosystems
   • Salt content between 5–35 ppt
   • Examples: estuaries, mangroves, salt marshes

Estuaries and mangroves belong to brackish water ecosystems.

2. Estuarine Ecosystem

What is an Estuary?

An estuary is a partially enclosed coastal area where:

• Freshwater from rivers
• mixes with
• Saline seawater

Examples:

• River mouths
• Coastal bays
• Lagoons
• Deltas

Key Features of Estuaries

• Salinity keeps changing (0–35 ppt)
• Strongly influenced by tides
• Acts as a transition zone (ecotone) between land and sea
• Organisms show osmoregulation (adaptation to changing salinity)
• Very nutrient-rich

Formation of Estuaries

Estuaries are formed due to:

• Rise in sea level
• Tectonic activity
• Glacial processes
• Sand and sandbar movement

Importance of Estuaries

Estuaries are among the most productive ecosystems in the world. Why?

• Mixing of freshwater and seawater brings nutrients
• Supports species from both marine and freshwater ecosystems

Major Importance

• Highly productive fishing grounds
• Act as natural pollution filters
• Trap sediments → help in delta formation
• Mangroves near estuaries protect coasts from cyclones
• Buffer between land and sea
• Ideal locations for ports and harbours
• Support livelihoods of coastal populations

About 60% of the world’s population lives near coasts and estuaries.

3. Lagoon vs Estuary

4. Estuarine Vegetation & Life

• Vegetation depends on salinity & flooding
• Includes:
  • Mangroves
  • Salt marshes
  • Seagrasses
  • Mudflats
• Highly dynamic ecosystem

Common organisms:

• Phytoplankton (diatoms, algae)
• Fish, prawns, crabs
• Birds like pelicans
• Sea turtles, sea catfish

5. Estuarine Ecosystem in India

• India has 14 major, 44 medium, and 162 minor rivers
• Most major estuaries are on the East Coast
• West coast estuaries are smaller but deeper

Examples:

• East coast: Hooghly, Godavari, Krishna
• West coast: Mandovi, Zuari

Issues in Indian Estuaries

• Industrial & sewage pollution
• Overfishing
• Aquaculture expansion
• Dredging & navigation
• Blocking fish migration routes
• Habitat destruction (Chilika, Pulicat)

6. Mangroves

What are Mangroves?

Mangroves are:

• Salt-tolerant (halophyte) trees and shrubs
• Found in coastal brackish water areas
• Grow in intertidal zones (between land and sea)
• Best example of an ecotone ecosystem

Key Characteristics of Mangroves

• Evergreen forests
• Grow in muddy, oxygen-poor soils
• Highly productive ecosystems
• Protect coastlines from:
  • Cyclones
  • Tsunamis
  • Coastal erosion

Adaptations of Mangroves (Very Important)

1. Salt tolerance
   • Salt-secreting glands
   • Salt exclusion at roots
2. Special roots
   • Prop roots (Rhizophora)
   • Pneumatophores (Avicennia) – for breathing
   • Stilt roots
3. Vivipary
   • Seeds germinate on the tree itself
   • Helps survival in saline water
4. Low oxygen adaptation
   • Roots adapted to anaerobic muddy soil

7. Mangroves in India

Distribution

• India has ~3% of the world’s mangroves
• Mostly along:
  • East coast
  • Andaman & Nicobar Islands
  • Gujarat coast

Major Mangrove Regions in India

1. Sundarbans (West Bengal)

• The largest mangrove forest in the world
• Home to the Royal Bengal Tiger
• Formed by Ganga–Brahmaputra delta

2. Bhitarkanika (Odisha)

• Second largest mangrove forest in India
• High biodiversity

3. Godavari–Krishna Delta (Andhra Pradesh)

• Dense mangrove swamps

4. Pichavaram & Vedaranyam (Tamil Nadu)

• Degraded due to aquaculture & salt pans

5. Gujarat (Gulf of Kutch)

• Dominant species: Avicennia
• Scrubby and dwarf mangroves
• Improving condition (Kori Creek)

6. Andaman & Nicobar Islands

• Dense, diverse, undisturbed mangroves

8. Importance of Mangroves

Ecological Importance

• Prevent coastal erosion
• Reduce the impact of cyclones & tsunamis
• Act as carbon sinks
• Support marine biodiversity

Economic & Social Importance

• Fish breeding & nursery grounds
• Livelihoods for coastal communities
• Source of timber, fuelwood, and medicines

9. Threats to Mangroves

• Conversion to agriculture & aquaculture
• Urbanisation & industrialisation
• Oil spills
• Pollution
• Climate change & sea-level rise