Aquatic habitat is a body of water in which certain organisms live naturally. In other words, aquatic habitats are habitats or places that relates to lives in water. Organisms that live in water are called aquatic organisms. Examples of aquatic organisms are fish, crabs, toads and plants.


Types of Aquatic Habitats

There are three types of aquatic habitats. These are marine or salt water habitats, estuarine or brackish water habitats and fresh water habitats.



Marine habitats refer to aquatic habitats which contain salt water. Marine habitats include the oceans, lakes, shores and the open seas.

Characteristics of Marine Habitat The marine or salt water habitat has the following characteristics:

  1. Salinity: Salinity is defined as the degree of saltiness or concentration of salt solution in oceans. The marine habitats have a high salinity and its average salinity is put at 35.2 per 1000. In other words, the average salinity of the ocean is 35.2 parts of salts by weight per 1000 parts of water.
  2. Density: The density of marine water is high, hence many organisms can float in it. While the density of ocean water is about 1.028, that of fresh water is 1.00. So, the density of ocean water is higher than that of fresh water.
  3. Pressure: Water pressure increases in depth at the rate of one atmosphere for every ten metres. In other words, pressure varies from one atmosphere at the surface level to about 1000 atmosphere at the greatest depth. This is why animals in marine habitats have features which enable them to adapt especially at the deep level of the sea.
  4. Size: Marine habitats represent the largest of all the habitats. The ocean alone occupies over 70% or 360 million square kilometres of the earth’s total area of 510 million square kilometres. Examples of oceans are Atlantic ocean, Indian ocean, Pacific ocean (the largest) etc.
  5. Currents: Currents are always produced by wind at the surface of the ocean. Currents are also produced down the ocean as a result of certain variations such as salinity and changes in temperature.
  6. Tides: Tides are the alternate rise and fall of the surface of the ocean approximately twice a day. This alternate rise and fall in water level is due to the gravitational effects of the moon and sun.
  7. Oxygen concentration: The concentration of oxygen in the ocean is highest at the surface while it decreases with depth, and in the very deep parts of the oceans there is practically no oxygen.
  8. Hydrogen ion concentration: Salt water is known to be alkaline in nature with pH of about 8.0-9.0 near the surface.
  9. Waves: Waves are movement of surface waters of the oceans and it can take any direction and are caused by winds. Waves bring about the mixing of sea water especially on the surface of the ocean.
  10. Light penetration: Light penetrates the ocean water only to a maximum depth of 200 metres. Therefore, plant life is limited to the upper layers of the ocean where light can penetrate. Penetration of light depends on the water turbidity.


Major Zones of the Marine Habitats

The major ecological zones of the marine habitats include:

  1. 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.
  2. Intertidal or Neritic zone: This zone which is also called planktonic or euphotic zone is only exposed at low tide or covered by water at high tide. It has high photosynthetic activities because of abundant sunlight. There is also fluctuation of the water temperature.
  3. Litoral or Subtidal zone: This zone is about 200m deep. It is constantly under water ., it has abundant sunlight and therefore abundant nutrients.
  4. Benthic zone: Benthic zone is also under water and is about 500m deep. It has low light penetration and low nutrients.
  5. 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.
  6. Hadal or Aphotic zone: This is the deepest zone of the marine habitat. It is over 7000m deep. It forms the floor or bed of the ocean. There is no light penetration and no photosynthetic activities.

On the basis of depth or light penetration or vertical zoning of marine habitat, there exist three major zones. These are euphotic, disphotic and aphotic zones.

  • Euphotic zone: This is an area which is directly connected with sunshine. Producers, consumers and decomposers are present here. There is enough light penetration for photosynthesis to take place.
  • Disphotic zone: Disphotic zone is a region of dim light. Consumers and decomposers are also found here. Light penetrates the water but the intensity is too low for photosynthesis to take place.
  • Aphotic zone: This represents the bottom or bed of the seas and oceans. It is characterised by cold dark water without light penetration and very few living organisms are found in this zone.


Distribution of Organisms in Marine

Habitats and their Adaptive Features The organisms in marine habitats include plants and animals.


Plants in Marine Habitats

  1. Sea weeds: They possess hold-fast for attachment. They also possess mucilagenous cover to prevent dissociation. They have divided leaves, floating devices or air bladder for bouyancy.
  2. Algae, e.g. sargassum: Algae possess chlorophyll for photosynthetic activities, small size or large surface area for drifting or floating.
  3. Sesuvium: Sesuvium possesses thick leaves or reduced leaves for water conservation.
  4. Planktons, e.g. diatoms: They possess air spaces in their tissues, rhizoids (fake feet) for attachment to rocks and air bladder for buoyancy.


Animals in Marine Habitats and their Adaptive Features

  1. Barnacles: Barnacles have protective mantle or pad for attachment or anchorage to rock shore, cilia for feeding, shell which prevents dessication and mantle which retains water.
  2. Cartilagenous fishes: Cartilagenous fishes like shark and dogfish have the ability to retain urea in their body to cope with high salinity.
  3. Bony fish: Fishes like tilapia and herring drink salt water to cope with high salt content of the ocean. They also possess salt secreting glands in their gills or eyes to enable them maintain osmoregulation or salt balance.
  4. Shrimps: They possess powerful claws or chelipods for seizing or holding food or prey.
  5. Crabs: Crabs are capable of burrowing fast into the mud to protect them against predators, strong waves or tides.
  6. Periwinkles: They possess lungs for breathing and foot for attachment.
  7. Starfish: They possess tube feet which enable them to hold on to rock shores, and hard shell which prevents dessication or drying up.


Food Chain in Marine Habitat

A typical food chain in marine habitat could be up to three or four trophic levels. The phytoplanktons, e.g. diatoms serve as the major producers which support the food chain. Some examples of food chain include:

  • Diatoms — > Zooplanktons -> Tilapia -> Shark
  • Diatoms -> Crabs -> Tilapia


Factors Affecting Marine Habitats

Some of the major factors affecting marine habitats are temperature, sunlight, wind, density, pH and salinity. These factors have been explained under the characteristics of marine habitat.



Estuarine habitat is a body of water formed at the coast as a result of the action of tides which mix salt water from the sea with fresh water from the land. The mixing of salt water and fresh water results in the formation of a brackish water. This brackish water is what is called estuarine.


Types of Estuaries

Estuary is found in the following bodies of water:

  1. Delta: A delta is where a river divides into many channels before entry into the ocean or sea. Brackish water or estuary (delta) is formed at the mouth of a river as it enters the sea.
  2. Lagoon: Lagoon is a body of ocean water that enters into the land through a canal and therefore has the opportunity of mixing with fresh water from rivers and streams.
  3. Bay: Bay is a little or small portion of the sea water which enters into the land and mixes up with fresh water from rivers and streams. It should be noted that a lagoon is bigger than a bay and it may be long enough to join the sea at another end while bay is very small and not long enough to rejoin the sea in another end.


Characteristics of Estuarine Habitats

The followings are the characteristics of the estuarine habitats:

  1. Fluctuation in salinity: Salinity fluctuates in this habitat. Salinity is lower at the mouth of a river and gets higher towards the sea. Salinity is also affected by season. While rainy season reduces salinity due to addition of fresh water, dry season increases it.
  2. Turbidity: Turbidity of estuarine habitat increases especially during the rainy season when lots of debris are brought down by rivers to the habitat. This high turbidity also reduces the rate of photosynthesis and respiration by organisms.
  3. Shallowness of water: Unlike the sea water which is deep, the water in estuarine habitat is very shallow.
  4. Low species diversity: The estuarine habitat has low diversity of species compared to marine habitat. Common plant species are phytoplanktons algae, marsh vegetation etc while animal species are crabs, oysters, lobsters, fishes etc.
  5. Water is affected by tides: Sea water usually flows rapidly into estuaries at high tides and rushes back into the ocean at low tides.
  6. High level of nutrients: The estuarine habitat contains abundant nutrients especially the organic detritus which form the bulk of producers in the habitat.
  7. Low oxygen content: Oxygen content of estuarine habitat is generally very low and as a result, much of the microbiological activities are anaerobic.


Distribution of Plants and Animals in Estuarine Habitats


Plant Species and their Adaptive Features

  1. Planktons (diatoms): They possess air spaces in their tissues, rhizoids or false feet for attachment to rock shores and air bladder for bouyancy.
  2. Algae: They possess chlorophyll for photosynthetic activities and small size or large surface area for floating.
  3. Red mangrove (Rhizophora racemosa): It has silt roots which grow down from the stem into the soft mud and develop numerous rootlets which have air spaces for conducting air to the tissues of the roots. The roots also provide support and prevent plants from being washed away by the tides. Again the seeds of red mangrove germinate while they are still on the parent plant thereby preventing the seedlings from being carried away by water current.
  4. White mangrove (Avicennia nitida): It has pneumatophores or breathing roots for exchange of gases.


Animals Species and their Adaptive Features

  1. Mosquito larvae and pupae: These organisms possess breathing trumpets for gaseous exchange.
  2. Crabs: They can burrow fast into the mud against predators, strong waves or tides.
  3. Water snails and shrimps: These animals can burrow into the mud when the tide is going out, thus escaping the periodic dilution of its external medium.
  4. Worms: They have strong protective and impermeable covering against high salinity.
  5. Mud skippers: These animals have fins adapted for crawling when on land and for swimming when in water.
  6. Fishes: Fishes like Tilapia have fins for movement and swim bladder for bouyancy.


Food Chain in Estuarine Habitat

A typical food chain in an estuarine habitat may have up to three, four or five trophic levels. The phytoplanktons such as diatoms and detritus form the basic producers which support the food chain.

Some examples of food chains in the estuarine habitat are:

Detritus -> Worms -> Snails -> Birds

Diatoms -> Shrimps -> Fishes


Factors Affecting Estuarine Habitats

The factors which affect estuarine habitats are common to aquatic habitats and these include temperature, wind, relative humidity, light and pH.



Fresh water habitat is a body of water formed mainly from inland waters and contain very low level of salinity. Examples of fresh water habitats are rivers, ponds, streams, springs and lakes.


Types of Fresh Waters

Fresh waters are classified on the basis of their mobility. Based on this, two types are identified. These are:

  1. Lotic fresh waters: These include all running waters which can flow continuously in a specific direction. In other words, these are flowing or running waters, e.g. rivers, springs and streams.
  2. Lentic fresh waters: These include standing or stagnant waters. These waters do not flow nor move. Examples of lentic fresh waters are lakes, ponds, swamps and dams.


Characteristics of Fresh Water Habitats

The following characteristics are associated with fresh water habitats:

  1. Low salinity: Fresh water habitats normally contain very low level of salts. It has about 0.5% of salt compared to about 3.5% for sea water.
  2. Small in size: Fresh water habitat is usually very small compared to the ocean water which is about 75% of the earth surface.
  3. Variation in temperature: The temperature of fresh water habitat usually varies with season and depth.
  4. High concentration of oxygen content: Oxygen is usually available in all parts of the fresh water especially in the surface of the water.
  5. Shallowness of water: Most fresh water habitats are very shallow hence sunlight can easily penetrate through the water to the bottom.
  6. Seasonal variation: Some fresh water habitats like streams and rivers normally dry up during the dry season while others have their volume reduced. The volume of water in rivers also increases during the rainy season. Turbidity and fast flow of rivers are also high during the rainy season than in dry season.
  7. Currents: Currents can affect the distribution of gases, salts and small organisms in fresh water habitats such as rivers and streams.


Major Ecological Zones of Fresh Water Habitats

The zones of a lentic of fresh water habitat, e.g. lake are similar to those of the marine habitats but there are no supratidal and inter-tidal zones.

There are two major zones in a lentic fresh water habitat. These are littoral and benthic zones.

  1. Littoral zone: Littoral zone is the shallow part of fresh water habitat. It contains several plants and animals. The littoral zone has rooted vegetation at its base. It has the highest level of primary production because sunlight can easily penetrate the zone, hence photosynthetic activities are common. Plants associated with this zone include Spirogyra, Chlamydomonas, water lettuce, water fern, duckweed, diatoms and sedges. Animals associated with this zone include water fleas, water snails, flatworms, frogs, toads, water skaters, ducks. snakes, crocodiles, tadpoles, Hydrae, and hippopotamus.
  2. Benthic zone: Benthic zone is the deepest parts of the lentic fresh water habitat. The benthic zone does not have rooted vegetation like the littoral zone although flowering plants may occur at its surface. Plants associated with the benthic zone have well developed root system in the mud. These plants include water lily, water arum, ferns, crinum lily, commelina and grasses. Animals associated with the benthic zone include protozoa, rotifers, Hydrae, Tilapia fish, mud fish, cat fish, leeches, caddish fly larvae, larvae and pupae of mosquito, water snail, water spider, crayfish, water scorpion, water boatman and water bugs.


Lotic Fresh Water Habitat

In a lotic fresh water habitat e.g. rivers, there exist two zones. These are:

  1. Pool zone: In this zone, water is relatively slow and calm.
  2. Rapid zone: In this zone, water is very fast. The lotic fresh water habitat is not as stratified as the lentic fresh water habitat.


Adaptive Features of Some Organisms in Fresh Water Habitat

Some Plants and their Adaptive Features

  1. Water lily (Nymphaea): The plant has air bladders, expanded shape and light weight which keep it afloat. It has long petioles attached at the centre of leaf blade which prevent them from being drawn under water by the current.
  2. Water hycinth (Ipomea grassipis): They have cavities and intercellular air spaces which give them the ability to float or maintain buoyancy on water.
  3. Spirogyra: The plant has mucilagenous cover which protect them in water.
  4. Water lettuce (pistia): Water lettuce has hairs on their leaves which help them to trap air and enable them to float.
  5. Hornwort (Ceratophyllum): The plants have submerged or thin dissected leaves which increase their surface area to sunlight and gaseous exchange.
  6. Water weed (Elodea): The plant has a long and flexible submerged petiole which enables it to swing with water currents.


Some Animals and their Adaptive Features

  1. Protozoa: These animals possess contractile vacuoles which enable them to carry out osmoregulation in water.
  2. Tilapia fish: They have swim bladders which enable them to float (buoyancy) in water. They also have gills for respiration.
  3. Duck: It has webbed digit on its feet for easy locomotion and serrated beak for sieving food in water into its mouth.
  4. Lung fish (Protopterus): The animals obtain oxygen through the gills but when the water dries out during the dry season, they dig into the mud and breathe with lungs until the rain comes again.
  5. Hydra: It has slippery surface, hooks and suckers for attachment to water particles.
  6. Pond skaters (Gerris): This animal has long legs with which it skates on water surface.
  7. Water boatman: This animal can carry bubbles of air with it as it goes below the water surface to the bottom and use these as their air supply (respiration) under water.


Food Chain in Fresh Water Habitat

Diatoms, Spirogyra, detritus, water lily and most of the flowering plants are the major producers in fresh water habitat. Some examples of food chains are:

  • Diatoms -> Fish fry  -> Tilapia
  • Detritus -> Worm -> Shrimps -> Birds
  • Spirogyra -> Tadpoles -> Carps -> King fishers


Energy flow in freshwater habitat

  1. Energy from sun/radiant energy/solar energy is received.
  2. It is then absorbed/trapped by green plants/algae/phytoplanking (of fresh water/pond/stream/river, etc).
  3. This is used in photosynthesis, to synthesize/manufacture organic substances.
  4. Zooplankton/fish/animals of the pond would now depend or feed on the green plants for food.
  5. The feeding is direct if they are herbivores as the plants are eaten by herbivores.
  6. The feeding is indirect if they are carnivores as herbivores are eaten by carnivores.
  7. When plants or animals/organisms die or when animals discharge their droppings/wastes, they are decomposed via the action of saprophytes/fungi/bacteria-decomposers/micro-organisms.
  8. By sodaing, energy is lost.


Factors Affecting Fresh Water Habitats

Factors which affect fresh water habitats are both biotic and abiotic factors.

Biotic Factors

  • plants/producers.
  • animals/consumers.
  • parasites.
  • decomposers/saprophytes/microorganisms.
  • predators.


Abiotic Factors

  • temperature.
  • sunlight/light.
  • wind.
  • inorganic compounds.
  • turbidity/suspended particles /transparency.
  • topography.
  • dissolved oxygen.
  • pH/dissolved CO.
  • water current/currents.
  • rainfall/precipitation.