What is a simple definition of Estuary?........(Estuaries)

 

Estuaries

Definition

• Estuaries are partially enclosed coastal bodies of water where freshwater from rivers meets and mixes with saltwater from the ocean.
• According to D.W. Pritchard (1967), an estuary is defined as:

“An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage.”

Characteristics

• Transitional Zone: Estuaries act as a transition between oceans and continents.
• Connection to Ocean: They have a free connection with the ocean.
• Salinity: The salt content is diluted by freshwater input, creating brackish water.
• Types of Names: Commonly referred to as bays, sounds, inlets, harbors, and sloughs, but not all these bodies of water are true estuaries.

Primary Productivity

• Key Organisms:
• Phytoplankton
• Benthic diatoms
• Seagrasses
• Algal mats
• Detritus: Organic particles, bacteria, algae, and protozoans that form a food source for estuarine organisms.
• Food Web: Traditionally, the food web is based on detritus due to low primary productivity in the water column.

Estuarine Area in India

• Approximately 9.47 lakh hectares of brackish water.
• Distribution:
• West Bengal: 23.28%
• Gujarat: 20.73%
• Kerala: 15.74%

Ecological Characteristics

• Meeting Place: Estuaries are where rivers meet the sea.
• Brackish Water: Characterized by a mix of salt and freshwater.
• Oxygen Levels: Generally well-oxygenated, especially in shallow areas.
• Ecotone: They serve as a transition zone between marine and freshwater habitats.
• Sediment Accumulation: Sediments from rivers accumulate at the river mouth, forming deltas.
• Fluctuating Water Levels: Water levels change with seasons and tides.
• Salinity Variations: Salinity fluctuates based on freshwater inflow and tidal actions.

Types of Estuaries

Based on Geomorphology

1. Coastal Plain Estuaries:
• Formed by rising sea levels filling river valleys.
• Examples: Chesapeake Bay, Charleston Harbor.
2. Tectonic Estuaries:
• Created by the folding or faulting of land surfaces.
• Example: San Francisco Bay area.
3. Bar Built Estuaries:
• Contain sand bars that separate the sea from the estuary.
• Temporary and shallow, often landlocked without river flow.
• Examples: Eastern Seaboard and Gulf Coast of North America.
4. Fjords:
• Deep, U-shaped valleys cut by glaciers, then flooded by the sea.
• Characterized by a shallow lip at the mouth.
• Found in areas with glacial activity, such as Northern Europe and Alaska.

Based on Circulation

• Estuaries can also be classified based on the mixing and circulation of water, but specific details on this classification were not provided.

Estuaries

Overview

• Estuaries are areas where freshwater from rivers meets and mixes with saltwater from the sea.
• They play a crucial role in the ecosystem, affecting organisms and the retention or flushing of toxic substances.

Types of Estuaries

1. Salt Wedge Estuary
• Formed when a large river flows into a tideless sea.
• Freshwater is less dense and flows over saltwater, creating a "salt wedge" at the bottom.
• Features:
• Halocline: A narrow zone between freshwater and saltwater with a sharp salinity change.
• Entrainment: Waves can mix small amounts of saltwater into the freshwater.
• Example: Mississippi River Estuary.
2. Well Mixed Estuary
• Occurs when a river flows slowly into an area with moderate to high tidal range.
• Ocean water mixes almost completely with river outflow.
• Features:
• Minimal vertical salinity change.
• Horizontal residual circulation established.
3. Negative (Evaporate) Estuary
• Found in desert climates with low freshwater input and high evaporation rates.
• Saltwater enters at the surface and becomes hypersaline due to evaporation.
• Features:
• Hypersaline water sinks and moves out as a bottom current.
• Salinity profile is reversed: highest at the bottom, lowest at the top.
4. Partially Mixed (Moderately Stratified) Estuary
• Formed when a river flows into the sea with significant tidal movements.
• Shares properties of both salt wedge and well-mixed estuaries.
• Features:
• Moderate density stratification.
• Good exchange of water with the ocean.
• Example: San Francisco Bay.

Categories of Estuaries

1. Open Estuary
• Located at the mouth of a river, connected to the sea.
• Can be perennial (filled year-round) or seasonal (drying up in summer).
• Example: Chilka Lake in India.
2. Embanked Estuary
• Confined area of brackish water, exposed to the sea only at low tides.
• Surrounded by barrier islands.
• Example: "Bhasabandha fisheries" or "Bheries" in West Bengal, India.

Importance of Estuaries

• Habitat: Vital for thousands of marine species, serving as nurseries for fish and shellfish.
• Biodiversity: Supports a wide range of marine life, including birds and marine mammals.
• Water Quality: Filters sediment and pollutants, leading to cleaner ocean water.
• Flood Protection: Acts as a buffer, reducing flooding and storm surge impacts.
• Cultural Significance: Historically important for human settlements and activities.

Estuaries are essential for maintaining healthy marine ecosystems and providing numerous benefits to both wildlife and humans.

Estuaries and Organisms

Types of Animals

• Sea Cow and Manatee
• Terrestrial Mammals
• Examples: Rodents, rabbits, tigers
• Reptiles
• Examples: Crocodiles, sea snakes, tree snakes
• Birds
• Examples: Cormorants, pelicans

Principal Estuaries of India

Estuaries are located along the coastal areas where rivers meet the sea. Below are the principal estuaries categorized by the East and West Coast River Systems.

East Coast River System

1. Adyar Estuary
• Location: Tamil Nadu
• Source of River Water: Adyar River
• Source of Marine Water: Bay of Bengal
• Area: 6.4 km from river mouth
2. Chilka Estuary
• Location: Puri and Ganjam, Orissa
• Source of River Water: Mahanadi River
• Source of Marine Water: Bay of Bengal
• Area: 906 sq.km (summer) to 1165 sq.km (flood)
3. Cauvery Estuary
• Location: Near Tiruchirappalli, Tamil Nadu
• Source of River Water: Two branches of Cauvery River
• Source of Marine Water: Bay of Bengal
• Area: Embanked
4. Godavari Estuary
• Location: Andhra Pradesh
• Source of River Water: Branches of Godavari
• Source of Marine Water: Kakinada Bay
• Area: 45 km from river mouth
5. Hooghly Matlah Estuary
• Location: Sunderban, West Bengal
• Source of River Water: Many rivers in the area
• Source of Marine Water: Bay of Bengal
• Area: 2340 sq.km
6. Krishna Estuary
• Location: Machillipatnam, Andhra Pradesh
• Source of River Water: Krishna and its tributaries
• Source of Marine Water: Nizampatam, Bay of Bengal
• Area: 6-28 km from river mouth
7. Mahanadi Estuary
• Location: Cuttak and Puri, Orissa
• Source of River Water: Mahanadi River
• Source of Marine Water: Bay of Bengal
• Area: 42 km from river mouth
8. Pulicat Lake
• Location: Nellore, Andhra Pradesh and Chengalpat, Tamil Nadu
• Source of River Water: Rivulets and River Kalangi
• Source of Marine Water: Bay of Bengal
• Area: 4400 sq.km
9. Vellar Estuary
• Location: Tamil Nadu
• Source of River Water: Vellar River
• Source of Marine Water: Bay of Bengal
• Area: 262 hectares
10. Vaigai and Others
• Location: Ramnad district, Tamil Nadu
• Source of River Water: Vellar system
• Source of Marine Water: Palk Bay and Gulf of Mannar
• Area: 93-77 hectares

West Coast River System

1. Choodapur Estuary
• Location: South Karnataka
• Source of River Water: River and South West Monsoon
• Source of Marine Water: Arabian Sea
• Area: 2 km from river mouth
2. Kalinadi Estuary
• Location: North Karnataka
• Source of River Water: South West Monsoon
• Source of Marine Water: Arabian Sea
3. Nethravathi Gurupur Estuary
• Location: South Karnataka
• Source of River Water: River and South West Monsoon
• Source of Marine Water: Arabian Sea
• Area: 2 km from river mouth
4. Narmada Estuary
• Location: Gulf of Cambay, Gujarat
• Source of River Water: Narmada River
• Source of Marine Water: Arabian Sea
5. Tapti Estuary
• Location: Gulf of Cambay, near Surat, Gujarat
• Source of River Water: Tapti River
• Source of Marine Water: Arabian Sea

Adaptations of Estuarine Organisms

Organisms in estuaries face challenges like changing salinity and staying in place. They adapt in various ways:

1. Behavioral Adaptations

• Blue Mussel: Closes its valves when salinity is low.
• Marine Worms and Amphipods: Bury themselves in sediments.
• Fish: Move to find suitable salinity levels.
• Oyster Larvae: Drop to the bottom during low tide and rise with the incoming tide.

2. Physiological Adaptations

• Crustaceans: Excrete salt quickly.
• Birds: Have salt glands to excrete excess salt.
• Marine Worms: Contract bodies to reduce salt absorption.
• Oysters: Close shells when salinity is unacceptable.
• Shipworm: Seals its burrow with a pallet if freshwater enters.

These adaptations help organisms survive in the fluctuating conditions of estuaries.

Estuaries: Environmental Characteristics and Organisms

Environmental Characteristics

Estuaries are unique ecosystems where freshwater from rivers meets saltwater from the sea. They have specific environmental characteristics that influence the organisms living there.

Abiotic Factors

• Salinity: Varies with depth, location, and tides.
• Temperature: Shallow waters experience significant temperature changes.
• Wave Action: Minimal in estuaries, leading to sediment deposition.
• Sediment: Influenced by particle size and chemistry.
• Turbidity: High levels due to fine sediments, affecting light penetration.
• Oxygen Levels: Can be low, especially in stratified estuaries.

Biotic Factors

• Phytoplankton: Primary producers in the food web.
• Zooplankton: Consumers that feed on phytoplankton.
• Trophic Levels: Various organisms occupy different levels in the food chain.

Salinity

• Euryhaline Organisms: Can tolerate a wide range of salinities.
• Stenohaline Organisms: Survive only in a narrow salinity range.
• Coping Mechanisms:
• Moving to favorable areas.
• Shutting shells or digging burrows.
• Excreting excess salts.

Temperature

• Eurythermal Organisms: Can withstand temperature variations.
• Temperature Changes:
• Daytime heating from the sun.
• Nighttime cooling from rivers and sea.
• Population Density: High temperature differences can lead to lower population densities in temperate regions.

Oxygen

• Low Oxygen Levels: Common in some estuaries, especially where biological activity is high.
• Mixing: Low tidal effects can lead to minimal oxygen replenishment.

Sediment

• Particle Size and Chemistry: Affect the types of organisms that can survive.
• Nutrient Infiltration: Can influence estuarine productivity.

Wave Action

• Minimal Effect: Leads to sediment settling and rooted plant development.

Turbidity

• High Levels: Due to fine sediments, especially during freshwater discharge.
• Light Penetration: Reduced turbidity is crucial for phytoplankton photosynthesis.

Classification of Estuarine Organisms

Organisms in estuaries are classified based on their salinity tolerance:

1. Oligohaline Organisms:
• Tolerate low salinity (up to 5 ppt).
• Mostly freshwater forms.
2. True Estuarine Organisms:
• Euryhaline forms adapted to estuaries.
• Live in upper and middle reaches.
3. Euryhaline Marine Organisms:
• Extend from the sea to upper estuary.
• Tolerate salinities as low as 5 ppt.
4. Stenohaline Marine Organisms:
• Live in open sea and estuary mouths.
• Do not enter below 25 ppt salinity.
5. Migrants:
• Euryhaline marine organisms that spend part of their lives in estuaries.

Biota of Estuaries

Estuaries host a diverse range of organisms adapted to brackish waters.

Flora

• Diatoms: Navicula sp, Skeletonema costatum, etc.
• Algae: Spirulina, Spirogyra, etc.
• Seagrasses: Cymodocea serrulata, Halophila ovalis, etc.
• Salt Marsh Vegetation: Salicornia, Spartina, etc.
• Mangroves: Avicennia, Rhizophora, etc.

Fauna

• Protozoa: Euglena, Vovox, etc.
• Coelenterata: Sea anemones, Hydrozoan medusa.
• Rotifera: Brachionus sp.
• Annelida: Tubifex sp, Nereis sp, etc.
• Arthropoda: Cyclops, Uca, Scylla, etc.
• Chordata: Finfishes like mullets and mudskippers.
• Aquatic Mammals: Dolphins.

These notes summarize the key aspects of estuaries, their environmental characteristics, and the organisms that inhabit them.