Unit 6: The diversity of the ecosystems                                      1   2   3

3. Aquatic ecosystems

There are two types of aquatic environment:

- Marine environments (saltwater), such as oceans and seas.

- Continental waters (freshwater), such as wetlands, rivers, lakes, etc.

3.1. Marine ecosystems

Most living things in the ocean are found near the surface of the water. This is because:

- water is a much more effective light filter than air and so sunlight can only illuminate the upper layers. Photosynthesis cannot take place in water deeper than 100 m, because any light that reaches these areas is too weak.

- water pressure increases with depth and temperature decreases, so not many organisms can live in water deeper than 200 m.

These factors, together with the presence or absence of water currents and the saline concentration of the water, determine the different marine biological zones.

a) Marine biological zones

The following marine biological zones are found in seas and oceans:

- According to their distance from the coast:

- Neritic zone (coastal waters): this is the area which extends from the shore to the edge of the continental shelf. It is less than the 10% of the oceans but contains 90% of marine species.

- Oceanic zone (open-sea): It is the area beyond the continental self.

- According to their depth:

- Pelagic zone: this is the area of open sea which extends from the surface to over 200 m depth. Sunlight penetrates here (euphotic zone). Planckton, most part of fish and marine mammals live in it.

- Bathyal zone: This is the area which goes from 200 m depth to about 2000 m depth. The amount of light is very little (disphotic zone).

- Abyssal zone (deep-sea): this area goes from 2000m depth to the very bottom of the ocean. There is no light at all (aphotic zone). It is inhabited by animals adapted to the extreme pressure and total darkness that exist there.

b) Marine organisms

Organisms which live in marine ecosystems are classified according to how they live:

- Benthonic organisms or benthos live in or on the sea bed. They can be either permanently fixed to a structure or able to move around freely. (E.g. seaweeds, starfish, sponges, etc.)

- Pelagic organisms are deep-sea living things. There are different types of pelagic organisms:

- Nektonic organisms or nekton, which can actively swim, and do not depend on water currents to move around, for example, fish, and most marine mammals.

- Planktonic organisms or plankton, which float in water currents, and are mainly made up of microscopic organisms, for example, algae and protozoa. However, plankton can also contain invertebrates and other medium-sized organisms, such as larvae.

There are no forests or visible vegetation in the sea, except for the algae which can be seen near the shore. So, what do deep-sea organisms feed on?

Deep-sea organisms feed on plankton. This is partially composed of microscopic photosynthetic algae, which are eaten by other microscopic marine organisms. These marine organisms are then eaten by larger animals. This process forms a food chain which leads eventually to consumers at the highest trophic levels, such as the giant squid or the shark.

c) Main marine ecosystems

-  Intertidal areas

These are areas along the coastline that are affected by the movement of the waves. They are submerged at high tide and exposed at low tide. Many creatures live in the sand and the rocks.

• On rocky coastlines, there are often many types of algae and small snails that eat the algae. Animals that attach to the rocks such as barnacles and mussels can be common.  Sea urchins and sponges live in areas that are usually covered with water. There are sometimes small pools of water between rocks that remain even after the tide goes out. These are called tide pools and they are often filled with many different animals, plants, and algae.

• On sandy beaches, the intertidal zone is often home to animals that live in burrows within the sand such as clams and worms. Crabs scurry across the sand and usually have burrows too. Large waves often crash along sandy beaches, so living in a burrow offers some protection.

- Shallow ocean (neritic zone).

At the bottom of the ocean, in areas shallow enough for sunlight to reach them, there are thousands of types of living things.

• On sandy ocean floor there are crabs, starfish and snails. Some fish and sharks prefer to live near the bottom swimming just above the sand. Other animals live buried in the sand or mud, such as clams, worms, and sea urchins. Sea grass, which looks like a lawn of tall grass, can also grow in sand and mud.

• On rocky ocean floor and the water is cold, kelp forests can form. The huge kelp (large brown algae) creates an underwater forest where many different animals live. Sea lions swim between the kelp and catch fish. Sea otters eat invertebrates like urchins and abalone

• In warm water, coral reefs grow on rocky areas of the shallow ocean floor. They are normally near the coast, and are usually found in the tropics. Corals reefs are home to a wide variety of living beings. Many species of colorful fish can be found swimming through reefs. Invertebrates like sponges, molluscs, sea fans, worms, crabs, and echinoderms are common too.

- Open ocean (oceanic zone).

It reaches from coasts to the middle of the ocean. The living things that live here need to have a way to float or swim. The area of the open ocean where sunlight shines through the water is called the photic zone. Most life in the open ocean is found in the photic zone.

• Animals, protists, algae, and bacteria that float in ocean water are called plankton. They are moved around the open ocean by surface currents and wind.  Phytoplankton is the part of plankton that perform photosynthesis like diatoms and other algae. They are the beginning of food chains. Zooplankton are animals and one-celled protozoa that drift in the ocean water.

• In the open ocean there are many types of swimmers (nekton) including fish, whales, and sharks. Some fish, such as herring and tuna, swim in shoals while others swim alone.  In the parts of the open ocean below where light can penetrate, there are fish and other animals like giant squid, but not algae.

- The deep ocean (Abyssal zone).

It is very cold, under high pressure, and always dark. Animals in the deep ocean are unique and not too abundant. They are carnivores (hunters or scavengers). Food is scarce and comes from other animals or the carcasses that rain down from the open ocean waters above. Most animals are highly adapted. They have large gaps to ingest any possible food. A great example of this is the deep-sea anglerfish. Other deep sea animals include the giant squid, black swallower, tripod fish.

But there is an environment in the deep sea where life is more abundant, the hydrothermal vents. While most of the water in the deep ocean is close to freezing, the water at hydrothermal vents is very hot. It is heated by volcanic activity at tectonic ocean ridges. These vents emit gases rich in chemical elements that certain types of bacteria are able to turn into the energy they need to survive. Many other types of living things including fish, shrimp, giant tubeworms, mussels, crabs, and clams thrive in this environment as well.

Video: Aquatic biomes (Discovery Science)

a. Sardines

b. Sea anemones

c. Jellyfish

d. Crabs

e. Microscopic algae

f.  Whales