Aquatic Ecosystem

Aquatic ecosystems, where water serves as the primary habitat, are divided based on their salt content. These ecosystems comprise diverse life forms adapted to specific conditions, contributing to global biodiversity and the environment’s health.

Learning Outcomes:

1. Understand the types of aquatic ecosystems and their salt content classification.
2. Identify various aquatic organisms based on their occurrence zones.
3. Comprehend limiting factors affecting aquatic ecosystems.
4. Analyze the dynamics of lake ecology and eutrophication.
5. Recognize conservation strategies for lakes, wetlands, mangroves, and coral reefs.

Classification of Aquatic Ecosystems

Aquatic ecosystems are classified primarily by salt content into three categories:

1. Freshwater Ecosystems: Salt concentration is below 5 ppt. Examples: lakes, ponds, rivers.
2. Marine Ecosystems: Salt concentration is equal to or greater than 35 ppt. Examples: seas, oceans.
3. Brackish Water Ecosystems: Salt content ranges between 5 to 35 ppt. Examples: estuaries, mangrove swamps.

Aquatic Organisms

Aquatic organisms are categorized based on their occurrence zones and their capability to traverse these zones. They are unevenly distributed and classified into five groups:

1. Neuston: Unattached organisms at the air-water interface. Some, like water striders, live on top of the interface, while others, like beetles, live beneath and feed within the water.
2. Periphyton: Organisms that attach to stems, leaves of rooted plants, or substrates above bottom mud, including sessile algae and associated animals.
3. Plankton: Microscopic plants (phytoplankton) and animals (zooplankton) that drift in water. They have limited movement, and currents control their distribution.
4. Nekton: Swimmers like insects and the blue whale. They are relatively large and powerful, swimming against water currents.
5. Benthos: Organisms living at the bottom of water masses. All aquatic ecosystems host a developed benthos community.

Important Note: The depth and quality of light in aquatic ecosystems dictate the distribution of these organisms and plant life.

Factors Limiting Productivity of Aquatic Habitats

Sunlight and dissolved oxygen are critical factors influencing aquatic ecosystems, whereas moisture and temperature are more vital in terrestrial systems.

1. Sunlight: Penetration diminishes with water depth, impacting plant distribution. Aquatic zones based on light:
   1. Photic Zone: The upper, light-penetrating layer where photosynthesis occurs.
   2. Aphotic Zone: The lower layer where light is minimal, restricting plant growth.
2. Dissolved Oxygen: Fluctuates depending on oxygen input/output factors. Oxygen dissolves at the air-water interface and through photosynthesis. Warm water retains less oxygen, hastening depletion and impacting aquatic life when levels drop below 3-5 ppm.
3. Transparency: Suspended particles like clay and silt cause water turbidity, limiting light penetration and photosynthetic activity.
4. Temperature: Water temperatures change more slowly than air due to water’s high specific heat. Aquatic organisms have a narrower tolerance limit for temperature changes than terrestrial ones.

Winterkill: During winter, snow-covered ice prevents sunlight penetration, stopping photosynthesis while respiration continues. In shallow lakes, oxygen depletion can result in fish death.

Lake Ecology

A lake, a standing water body, undergoes ‘ageing,’ gradually filling with organic and mineral matter.

Ageing of Lakes

Lakes age through natural processes like sedimentation and nutrient accumulation, promoting the growth of algae and aquatic fauna. This process is known as natural eutrophication. However, human activities, such as wastewater discharge and agricultural runoff, accelerate this phenomenon, termed cultural eutrophication.

Types of Lakes Based on Nutrient Content

Parameter	Oligotrophic	Eutrophic
Aquatic plant production	Low	High
Oxygen in hypolimnion	Present	Absent
Water quality	Good	Poor
Depth	Deeper	Shallower
Biodiversity	Many species	Fewer species

Important Concept: Oligotrophic lakes have low nutrient content, while eutrophic lakes are nutrient-rich and often turbid, supporting dense vegetation.

Nutrient Removal in Lakes

1. Flushing with nutrient-poor water.
2. Deep water abstraction.
3. Algae removal using filters and adsorbers.
4. Artificial mixing/destratification.
5. Harvesting of fishes and macrophytes.
6. Sludge removal.

Eutrophication

Eutrophication is the ecosystem’s response to nutrient enrichment, particularly nitrates and phosphates, leading to excessive plant and algae growth.

Process of Eutrophication

1. Nutrient accumulation in water.
2. Algal overgrowth (bloom), limiting light penetration.
3. Oxygen depletion due to increased decomposition.
4. Death of aquatic organisms due to low dissolved oxygen.
5. Complete ecosystem degradation.

Types of Eutrophication

1. Natural: Occurs over centuries due to deposition of nutrients in aquatic systems.
2. Manmade: Occurs rapidly due to agricultural runoff, untreated sewage, and industrial effluents.

Flowchart: Nutrient accumulation → Algal Bloom → Oxygen Depletion → Death of Aquatic Life → Ecosystem Degradation.

Wetland Ecosystems

Wetlands are transitional zones between terrestrial and aquatic ecosystems, experiencing periodic flooding.

Wetland Characteristics

1. Hydrology: Covered or waterlogged soil for at least seven days.
2. Flora: Hydrophytes adapted to waterlogged conditions.
3. Soils: Hydric soils, low in oxygen.

Functions of Wetlands

1. Habitat: Supports diverse flora, fauna, and migratory species.
2. Water Filtration: Removes sediments and nutrients.
3. Flood Mitigation: Controls stream flow and groundwater recharge.
4. Economic Resource: Provides drinking water, fish, and raw materials.
5. Tourism: Promotes recreation and cultural heritage.

India’s Wetlands

India has 27,403 wetlands, with significant biodiversity. Major types include Himalayan wetlands, floodplains, estuaries, mangroves, and coral reefs.

Mangroves

Mangroves are coastal vegetation with high salt tolerance, found in tropical and subtropical shores.

Mangrove Characteristics

1. Pneumatophores: Air roots for respiration in anaerobic soil.
2. Viviparity: Seeds germinate while attached to the parent tree.
3. Adaptation: High solar radiation absorption and salt excretion mechanisms.

Role of Mangroves

1. Stabilizes Shorelines: Controls coastal erosion.
2. Breeding Grounds: Supports fish and other marine life.
3. Natural Barriers: Protects against storm surges, tsunamis, and hurricanes.

Important Note: India’s largest mangrove forest is the Sundarbans, hosting the Royal Bengal Tiger.

Coral Reefs

Coral reefs, built by tiny animals called polyps, thrive in nutrient-poor tropical waters due to their symbiosis with zooxanthellae algae.

Coral Reef Features

1. Symbiotic Relationship: Zooxanthellae provide nutrients to corals through photosynthesis.
2. Classification: Includes fringing reefs, patch reefs, barrier reefs, and atolls.
3. Functions: Act as natural protective barriers, support high biodiversity, and stabilize coastal regions.

Coral Bleaching

Occurs when corals lose their zooxanthellae, often due to temperature anomalies, leading to coral death and habitat loss.

Causes of Bleaching: Elevated sea temperatures, solar irradiance, sedimentation, freshwater dilution, and nutrient loading.

Government Measures to Protect Coastal Ecosystems

1. Coastal Regulation Zone (CRZ) Notification: Protects sensitive coastal areas.
2. National Coastal Zone Management Authority: Monitors and enforces coastal conservation.
3. National Wetland Conservation Programme (NWCP): Identifies and funds critical wetland conservation.
4. Clean Ganga Fund (CGF): Engages communities in river conservation initiatives.

Key Initiatives: COMAPS for coastal monitoring, ICMAM for integrated coastal management, and Namami Gange for river rejuvenation.

Lake and Wetland Comparison

Parameter	Lake	Wetland
Water Turnover	Permanent	Permanent or Temporary
Thermal Stratification	Yes	No
Dominant Producer	Phytoplankton	Macrophytes
Flood Control	Less Significant	Significant
Management Objective	Eutrophication Control	Biodiversity Conservation

Multiple-Choice Question

Which of the following is a characteristic of oligotrophic lakes?

1. High aquatic plant production
2. Absent oxygen in the hypolimnion
3. Good water quality for domestic use
4. Increased turbidity

Correct Answer: 3. Good water quality for domestic use