what is a lagoon?.....(Lagoons)

 

Lagoons

Definition and Origin

• The term "lagoon" comes from the Italian word "laguna," which means a shallow body of water.
• It is derived from the Latin word "lacuna," meaning pool, hollow, or gap.
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Types of Lagoons

Lagoons are commonly classified into two main types:

1. Coastal Lagoons
• Shallow bodies of water separated from the ocean by barriers.
• Barriers can be coral reefs, barrier islands, sand bars, or spits.
2. Oceanic/Atoll Lagoons
• Circular coral reefs or strings of coral islands surrounding a lagoon.
• Generally deeper than coastal lagoons, sometimes reaching depths of about 20  m

Characteristics of Lagoons

• Lagoons are usually parallel to the shoreline and longer than they are wide.
• They typically do not have large rivers flowing into them, although some large lagoons may.
• Depth rarely exceeds a few meters, making them sensitive to changes in precipitation, evaporation, and wind.
• Salinity and temperature can vary significantly due to tidal exchange and freshwater inflows.

Salinity Levels

• Freshwater: Low salinity.
• Brackish Water: Saltier than freshwater but less salty than seawater.
• Salt Water: Found in the sea.
• Hypersaline/Brine: Highly concentrated salt water.

Seasonal Variations

• In temperate and Mediterranean climates, salinity is low during cold wet seasons and high during warm dry periods.
• In tropical climates, salinity increases during dry seasons and decreases during monsoons.

Classification of Lagoons by Water Exchange

1. Choked Lagoons
• Narrow channel to the sea, high wave energy.
• Limited tidal inflow and mixing of water.
• Example: Mundel Lagoon, Sri Lanka.
2. Restricted Lagoons
• More than one channel to the sea, allowing for good water exchange.
• Wind influences surface currents and mixing.
• Example: Uppar Lagoon, Sri Lanka.
3. Leaky Lagoons
• Wide channels to the sea with fast water currents.
• Unhindered interchange of water.
• Example: Mississippi Sound, USA.

 

Distribution of Lagoons

• Found along low-lying coasts globally, except Antarctica.
• Occupy about 13% of the world's coastline.
• Extent by continent:
• Africa: 18%
• North America: 17.6%
• South America: 12.22%
• Asia: 13.8%
• Australia: 11.4%
• Europe: 5.3%
• The longest coastal lagoons stretch about 2,800 , km along the Atlantic and Gulf coasts of the USA.

Biodiversity of Lagoons

• Lagoons create ecotones, which are areas of transition between land and sea.
• They host diverse habitats, including barrier islands, beaches, salt marshes, and mangroves.
• Ecotones often have higher species diversity and population densities, known as the "edge effect."

Examples of Ecotones

1. From Inland Terrestrial Vegetation
• Includes mangroves, salt marshes, and submerged vegetation like seagrasses.
2. Salt Marsh
• A specific type of ecotone found in lagoon environments.

Lagoons and Their Ecosystems

Overview of Lagoons

• Definition: Lagoons are shallow bodies of water separated from the ocean by barriers like sandbars or coral reefs.
• Ecotone: They exist within a salinity gradient from freshwater inland to saltwater at the ocean's mouth.

Characteristics of Lagoons

• Benthic Zone:
• Lagoons have a flourishing benthic zone where light penetrates to the bottom.
• Supports benthic plants like algae and seagrasses.
• Safe Habitats:
• Protected by barriers, lagoons serve as nurseries and feeding grounds for many species, including commercially important shellfish and finfish.

Adaptations of Plants and Animals in Lagoons

1. Coping with Salinity Changes

• Animals:
• Oysters close their shells when salinity is low and open them when it increases.
• Some worms and mollusks cover themselves with slime to protect against salinity changes.
• Plants:
• Xeromorphic Characteristics:
• Leathery, waxy leaves and sunken stomata to conserve water.
• Halophytic Characteristics:
• Salt glands that excrete salt, found in species like Grey Mangroves  (Avicennia ).

2. Coping with Tides and Currents

• Animals:
• Mussels and barnacles anchor themselves to prevent being washed away.
• Plants:
• Mangroves   (Rhizophora ) have stilt roots for support.
• Seagrasses use underground stems (rhizomes) to anchor themselves.

3. Avoiding Desiccation

• Animals:
• Barnacles and bivalves shut their shells to retain moisture.
• Plants:
• Exhibit xeromorphic characteristics to prevent water loss.

4. Coping with Low Oxygen Content

• Mangroves:
• Have aerial roots (pneumatophores) to absorb oxygen from the air.
• Animals:
• Fish may move to areas with higher oxygen or reduce activity to lower oxygen needs.

5. Coping with Reproductive Challenges

• Mangroves:
• Exhibit vivipary, where seeds develop while still attached to the parent plant.

Importance of Lagoons

Ecosystem Services

• Provisioning Services:
• Support coastal fisheries and provide safe anchorage for fishermen.
• Supporting Services:
• High primary productivity comparable to tropical rainforests.
• Nutrient cycling that supports coastal biodiversity.

Threats to Lagoons

1. Overexploitation:
• Fish harvested above sustainable levels.
2. Coastal Development:
• Construction disrupts tidal flow and circulation patterns.
3. Pollution:
• Point and non-point source pollution can lead to eutrophication and harmful algal blooms.
4. Sedimentation Changes:
• Increased sedimentation affects benthic organisms and can lead to habitat loss.
5. Dredging:
• Can destroy benthic habitats and degrade water quality.

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Aquaculture and Coastal Ecosystems

Impact of Aquaculture

• Clearing of Habitats: Mangroves and salt marshes are often removed to create aquaculture ponds.
• Water Extraction: The process of extracting water for aquaculture alters the natural water quantity.
• Water Flow Changes: Dams and diversions can significantly disrupt natural water flows, posing serious threats to ecosystems.

Regulating Services of Lagoons

• Storm Protection: Lagoons act as buffer zones, protecting coastal communities from:
• Storm surges
• Floods
• Cyclones
• Flood Absorption: During flooding, lagoons absorb excess water and gradually release it back to the sea, functioning like large sponges.
• Erosion Protection: They help protect rivers from erosion caused by water and wind.

Water Filtration and Sediment Trapping

• Habitat Diversity: Lagoons contain various habitats (mangroves, salt marshes, seagrass meadows) that:
• Filter pollutants
• Remove excess nutrients and sediments from wastewater and stormwater runoff
• Nutrient Cycling: High rates of nutrient cycling in lagoons help quickly absorb and eliminate pollutants.

Cultural Services

• Human Habitation: People have historically settled along riverbanks and lagoons due to nutrient-rich soils.
• Recreational Importance: Lagoons are vital for various recreational activities, including:
1. Sailing and boating
2. Fishing
3. Swimming
4. Water sports (jet skiing, kite surfing)
5. Bird watching
• Traditional Fisheries: Traditional fishing methods, such as fencing and amassing fish, have been practiced in lagoons since the 1st century, particularly in South and Southeast Asia. A common method is brush pile fishery.

·         Conclusion

·         Lagoons are vital ecosystems that provide numerous services and support diverse life forms. However, they face significant threats that require careful management and conservation efforts