AQUATIC HABITAT

TOPIC: AQUATIC HABITATS

An aquatic habitat is a natural environment where organisms live in or around water. It includes all areas covered by water either permanently or temporarily. In aquatic habitats, water serves as the medium for supporting life, providing food, oxygen, and shelter to plants and animals.
 Aquatic habitats vary in salinity, depth, temperature, light penetration, and movement of water, which determine the kinds of organisms found there.

Examples of organisms found in aquatic habitats include fish, crabs, water snails, frogs, algae, and water lilies.

Types of Aquatic Habitat

Aquatic habitats are broadly divided into three. They are: Marine or salt water habitat, estuarine or brackish water habitat and fresh water habitat.

Marine Habitats
This type of habitat contains water with high salt concentration, such as oceans, shores, lakes and open seas. Marine habitats make up the largest portion of the Earth’s surface and support a wide range of organisms adapted to salty conditions.

Characteristics of Marine Habitats

1. High Salinity: Salinity is defined as the degree of saltiness or concentration of salt solution in oceans. Marine water contains a high concentration of salts, especially sodium chloride (NaCl). The average salinity is about 35 parts per thousand (ppt) or 3.5%.

2. Large Size and Depth: Oceans are very large and deep, extending to great depths that can exceed several thousand meters. Because of this, light, temperature, and pressure vary greatly with depth. Examples of oceans are the Atlantic ocean, Indian ocean, Pacific Ocean (which is the largest), etc.

3. Tidal Influence: The level of the sea rises and falls periodically due to the gravitational pull of the moon and the sun. These tides affect the organisms living near the shore, which must adapt to both wet and dry conditions.

4. Temperature Variation: Temperature decreases with depth. Surface waters are warmer due to sunlight, while deep waters remain cold.

5. Light Penetration
 Sunlight decreases with depth, dividing the ocean into:

• Euphotic zone – enough light for photosynthesis. Producers, consumers and decomposers are found here.
•   Disphotic zone – dim light, no photosynthesis. Consumers and decomposers are found here.
• Aphotic zone – It represents the bottom or bed of seas and oceans. It is completely dark and very few living organisms are found here.

6. Pressure: Water pressure increases with depth, so deep-sea organisms are specially adapted to survive high pressure.

7. Nutrient Content: Nutrients are more concentrated near the shore and in the upper layers due to wave action and upwelling. Deep ocean waters are nutrient-poor.

8. Oxygen Availability: Oxygen is highest near the surface where water mixes with air and photosynthesis occurs. Deeper waters have less oxygen.

9. Diverse Organisms: Marine habitats support a wide variety of life forms, including phytoplankton, zooplankton, fish, mollusks, crustaceans, corals, seaweeds, and marine mammals.

10. Density: The density of marine water is high, hence many organisms can float in it. While the density of ocean water is 1.028, that of Fresh water is 1.00.

11. Zonation
 Marine habitats are divided into zones based on depth and distance from the shore:

• Supratidal or Splash Zone - This is the exposed zone of the marine habitat. It has occasional moisture since it is the area where water splashes when the waves break at the shore. 
• Intertidal(or Neritic) zone – It is also called the planktonic or euphotic zone. It is exposed at low tide or covered by water at high tide. It has high photosynthetic activities due to abundant sunlight. There is also fluctuation of water temperature. 
• Littoral (or Subtidal) zone – This zone is about 200m deep. It is constantly under water. It has abundant sunlight and therefore abundant nutrients.
• Benthic zone –It is under water and is about 500m deep. It has low light penetration and low nutrients.
• Pelagic (or Abyssal zone): This zone is about 7000m deep. It has low temperature, low light penetration, high pressure, low photosynthetic activities and the primary production of food is by chemosynthesis.
• Hadal or Aphotic Zone:  This is the deepest zone of the marine habitat. It is over 7000m deep. There is no light penetration and no photosynthetic activity.
  
   

General Adaptations of Plants in Salt Water (Marine Plants)

Plants that live in salt water are called halophytes. They have special features that help them survive in salty, submerged, and oxygen-poor environments like seas, oceans, and mangrove swamps. Below are their general adaptations:

1. Salt Tolerance: These plants can tolerate or get rid of excess salt in their tissues. Some have salt glands on their leaves that excrete salt, while others store salt in old leaves that later fall off.

2. Succulent Leaves and Stems: Many saltwater plants have thick, fleshy leaves or stems that store water, helping them dilute the salt concentration in their cells.

3. Aerial Roots (Pneumatophores): Plants in swampy saltwater areas like mangroves have special roots that grow upward above the water surface to absorb oxygen directly from the air since the soil is often waterlogged and lacks air.

4. Prop or Stilt Roots: Mangroves and similar plants develop stilt or prop roots that anchor them firmly in soft, muddy soils and provide stability against waves and tides.

5. Vivipary (Seed Germination on Parent Plant): Some marine plants like mangroves show vivipary, meaning their seeds germinate while still attached to the parent plant, ensuring young plants can survive when they drop into the muddy water.

6. Waxy or Thick Cuticle: Their leaves often have a waxy surface to reduce water loss and protect against the salt in seawater.

7. Small or Reduced Leaves: Some saltwater plants have small, narrow, or needle-like leaves to reduce water loss through transpiration.

8. Specialized Osmoregulation: These plants maintain the balance of water and salts inside their cells through osmotic control mechanisms, preventing dehydration caused by high salt concentration outside.

9. Floating Structures: Some marine plants like seaweeds and seagrasses have air-filled spaces or bladders that help them float near the water surface to receive enough sunlight for photosynthesis.

10. Flexible Stems and Leaves: Their stems and leaves are soft and flexible, allowing them to bend easily with water currents and waves without breaking.

Plants in Marine Habitats

1. Phytoplankton: These are microscopic, free-floating plants (mainly algae) found near the surface of the ocean where sunlight is available. They form the base of the marine food chain, serving as food for many small animals like zooplankton and fish. Examples: Diatoms, dinoflagellates, and cyanobacteria.

2. Seaweeds (Marine Algae): These are large, multicellular algae that grow attached to rocks or the ocean floor in shallow waters. They contain chlorophyll and other pigments that enable them to photosynthesize even at different depths. Seaweeds are classified into three main groups based on their pigments:\ftftx\q32 1``i 

• Green algae (Chlorophyta) – found in shallow waters.
• Brown algae (Phaeophyta) – found in deeper waters (e.g. kelp).
• Red algae (Rhodophyta) – found in the deepest regions where light is dim.
• Examples: Kelp, Sargassum, Ulva (sea lettuce), Gelidium.

3. Seagrasses

• These are true flowering plants (angiosperms) that grow completely submerged in shallow marine waters, especially near coastlines.
• They have roots, stems, and leaves and reproduce using flowers and seeds.
• Seagrasses help stabilize sediments and provide breeding grounds for fish and other marine animals.
• Examples: Eelgrass (Zostera), Turtle grass (Thalassia), and Manatee grass (Syringodium).

4. Mangroves

• These are woody plants that grow along coastal shorelines and estuaries, where saltwater mixes with freshwater.
• They have aerial roots, prop roots, and salt-excreting leaves to survive in salty, waterlogged soils.
• Mangroves help protect coastlines from erosion and serve as nurseries for many marine animals.
• Examples: Red mangrove (Rhizophora), Black mangrove (Avicennia), White mangrove (Laguncularia).
  
   

5. Floating Marine Plants

• Some plants float freely on the surface of the sea, absorbing sunlight for photosynthesis.
• They often occur in calm ocean areas or near the shore.
• Examples: Sargassum (floating brown algae), Duckweed (in brackish areas).

Animals in Marine Habitats and their Adaptive Features

Food hain in the Marine Habitat

A food chain is the sequence of feeding relationships among living organisms in which energy and nutrients are transferred from one organism to another.

In a marine habitat, it shows how energy flows from tiny plant-like organisms to large sea animals.

Examples of Marine Food Chains

1. Simple Marine Food Chain
    Phytoplankton → Zooplankton → Small Fish → Big Fish → Shark
   
    
2. Another Example
    Seaweed → Periwinkle → Crab → Octopus → Dolphin
   
    
3. Deep-Sea Example
    Chemosynthetic bacteria → Worms → Small fish → Squid → Whale

Factors affecting Marine Habitat

Several factors — both living (biotic) and non-living (abiotic) — influence the distribution, survival, and behavior of organisms in this habitat.

Abiotic (Non-living) Factors

1. Salinity
2. Temperature
3. Light intensity
4. Water pressure
5. Oxygen concentration
6. Nutrient availability
7. pH level (acidity)
8. Tides and currents
9. Depth of water
10. Type of sea floor or substrate

Biotic (Living) Factors

1. Competition
2. Predation
3. Parasitism
4. Mutualism
5. Availability of food
6. Human activities

Human (Anthropogenic) Factors

1. Pollution
2. Overfishing
3. Habitat destruction
4. Climate change
5. Shipping and noise pollution

ESTUARINE HABITATS

An estuarine habitat is a coastal water body where freshwater from rivers mixes with saltwater from the sea. It is a transition zone between marine and freshwater habitats, and it supports a wide variety of plants and animals adapted to changing salinity levels. It is also called Brackish Water.
Examples: Niger Delta (Nigeria), River Thames Estuary (U.K.), Mississippi Estuary (U.S.A.).

TYPES OF ESTUARIES

Estuaries are classified based on how they are formed and the shape of their water bodies where freshwater and seawater mix. The three common types are:

1. Delta Estuary

It is formed where a river deposits sediments (mud, sand, and silt) at its mouth faster than the sea can carry them away. The accumulated deposits build up and divide the river into several small channels called distributaries.
 The entire area often takes the shape of a fan or triangle, hence the name “delta.”
Examples: Niger Delta (Nigeria), Mississippi Delta (U.S.A.)

2. Lagoon Estuary

It is formed when a sandbar or barrier island separates a section of the sea from the open ocean, trapping seawater inside.  Freshwater from rivers or streams then flows into the lagoon, mixing with the trapped seawater to form brackish water.  Lagoon estuaries are usually shallow and calm, making them ideal breeding grounds for many aquatic animals.
Examples: Lekki Lagoon (Nigeria), Epe Lagoon (Nigeria)

3. Bay Estuary

It is a little or small portion of sea water which enters into the land and mixes up with fresh water from rivers and streams. During high tides, seawater enters the bay, and during low tides, freshwater from the river dominates, creating a mixing zone of salt and fresh water. Bay estuaries are generally deep, wide, and influenced by tides.
Examples: Bonny River Estuary (Nigeria), Chesapeake Bay (U.S.A.)

It should be noted that a lagoon is bigger than a bay and it may be long enough to join the sea at the other end while a bay is very small and not long enough to re-join the sea at the other end.

Characteristics of Estuarine Habitats

1. Brackish water (a mixture of fresh and salt water).
2. Variable salinity – changes with tides and rainfall.
3. High nutrient content from river deposits.
4. Rich biodiversity – supports many species of plants and animals.
5. Tidal influence – water level rises and falls with tides.
6. Soft, muddy or sandy bottom due to sediment deposits.
7. High turbidity – water often appears cloudy because of suspended particles.
8. Abundant plankton that form the base of the food chain.
9. Sheltered environment – ideal for breeding and nursery grounds for fish.

Plants in Estuarine Habitat and their Adaptive Features

Animals and their Adaptive Features

Food Chain in the Estuarine Habitat

A typical food chain in this habitat has three, four or five trophic levels. The phytoplanktons such as diatoms and detritus  form the basic producers which supports the food chain.

Example 1: Phytoplankton → Zooplankton → Small Fish → Big Fish → Bird (e.g., Heron)

Example 2: Mangrove leaves → Detritus → Worms → Crabs → Fish → Man

Example 3: Algae → Periwinkle → Crab → Fish → Human

Factors affecting Estuarine Habitat

Salinity – Varies with tides, freshwater inflow, and evaporation, influencing the types of organisms present.
Tides – Control the mixing of salt and freshwater and affect the availability of nutrients and oxygen.
Temperature – Affects the metabolic activities of aquatic organisms.
Freshwater Inflow – Determines the nutrient level and salinity balance in the estuary.
Sedimentation – Deposition of mud, sand, and silt affects water clarity and the type of bottom-dwelling organisms.
Oxygen Concentration – Influences the survival of aquatic plants and animals.
Pollution – From industrial waste, oil spills, and agricultural runoff can harm aquatic life.
Turbidity – High turbidity reduces light penetration, affecting photosynthesis in aquatic plants.
Human Activities – Dredging, reclamation, and construction alter estuarine environments.
Wind and Currents – Aid in the mixing of water and distribution of nutrients.

FRESH WATER HABITAT

A freshwater habitat is an aquatic environment where the water has very low salt content (less than 0.05% salt). It includes rivers, lakes, streams, ponds, and wetlands. Freshwater habitats support a wide range of plants and animals adapted to low salinity conditions. Examples: rivers, ponds, streams, springs and lakes. 

Types of Freshwater Habitats

1. Lentic habitats – Still or slow-moving water bodies, e.g., lakes, ponds, swamps.
2. Lotic habitats – Fast-flowing water bodies, e.g., rivers and streams.

Characteristics of Freshwater Habitats

• Low salt concentration (fresh water)
• Variable water flow (fast in rivers, slow in lakes)
• High dissolved oxygen in fast-flowing areas
• Temperature changes with seasons and depth
• Supports both aquatic plants and animals
• Usually has different zones based on depth, light, and oxygen

Major Ecological zones of Fresh Water

In Lenthic fresh water, there are two major zones: the Littoral and the Benthic zone

Littoral Zone: The littoral zone is found near the shore of the water body, where sunlight penetrates easily, allowing rooted aquatic plants(water lettuce, water fern, duck weed, etc) and algae to grow. This zone is rich in oxygen and provides food and shelter for many organisms, including small fish, insects, snails, crabs, and amphibians. It is also an important area for breeding and feeding, making it highly productive.

Benthic Zone:   It is located at the bottom of the water body. It receives little or no sunlight in deeper areas, and oxygen levels may be low, especially in muddy sediments. This zone is inhabited by decomposers, such as bacteria and fungi, and bottom-dwelling animals, including worms, insect larvae, and some fish. The benthic zone plays a crucial role in recycling nutrients from decaying plants and animals, making it essential for the overall health of the freshwater ecosystem. Plants associated with this zone has well developed root system in the mud. E.g. water arum, ferns, water lily, crinum lily, etc. 

In Lothic Fresh Water Habitat, we have two zones: 

Lotic habitats, such as rivers and streams, are characterized by flowing water and are divided into zones based on flow speed, depth, and light availability. The pool zone is the deeper, slower-moving section of the river where water flow is gentle. Sunlight penetrates well in this zone, allowing aquatic plants, algae, and rooted vegetation to grow. It provides a suitable habitat for fish, amphibians, and invertebrates, and serves as an area for resting and feeding.

The rapid zone, also called the riffle zone, consists of shallow, fast-flowing water over rocks and coarse substrates. The water is well-aerated due to turbulence, providing high levels of dissolved oxygen, which is ideal for many invertebrates and oxygen-demanding fish. Organisms in this zone are adapted to cling onto rocks or swim strongly against the current. Both the pool and rapid zones play important roles in maintaining the health and biodiversity of river ecosystem. 

Adaptive Features of some Organisms in Fresh Water Habitat

Food Chain in Freshwater Habitat

Example 1: Phytoplankton → Zooplankton → Small Fish → Big Fish → Bird (e.g., Kingfisher)

Example 2: Algae → Snail → Crab → Fish → Human

Example 3: Aquatic plants → Insect larvae → Frog → Snake → Bird

Factors affecting  Fresh Water Habitats

Abiotic (Non-living) Factors

1. Water temperature
2. pH level
3. Dissolved oxygen
4. Light availability
5. Water flow (current speed)
6. Sedimentation
7. Mineral and nutrient content
8. Turbidity (water clarity)
   
    

Biotic (Living) Factors

1. Competition among organisms
2. Predation
3. Parasitism
4. Mutualism
5. Availability of food
6. Human activities (pollution, habitat alteration, introduction of invasive species)