Water is a unique chemical which is needed by nearly all living things. It provides homes or habitats for a vast number of animals, plants and micro-organisms which live in freshwater or the salty oceans and seas.

Water as a habitat

We can think of our pond as consisting of a number of zones:

• The land surrounding the pond
• The shallow water, where plants grow, near the pond's edge
• The air above the pond
• The surface of the pond, where air and water meet
• The open water, this is too deep for rooted plants to grow
• The bottom of the pond

Within each of these zones there are many smaller places for animals to live. Some animals may bury themselves in the mud, others may live under rocks, others may live attached to a leaf of a certain plant. These are examples of micro-habitats or very small habitats

Water surface zone

Many plants and animals have special adaptations for survival in this zone, for example many insects can walk on water. Other animals, especially flying insects, can become accidentally trapped on the surface and become food for pond animals.

Many animals need to go to the surface, or live just below it, to get their supplies of oxygen.

Water plant zone

In a shallow pond, this zone may include the whole pond, in larger lakes it may only cover the area nearer the banks. This zone contains all the rooted plants from Rushes to Elodea. Along with these plants are many animals, some which use the plants for food, others use them for shelter or protection.

Open Water Zone

In a larger pond or lake there are usually areas where there are no rooted plants. This is because light will reach the deeper water. This zone will have plankton near the surface and many animals feeding in it.

Mud zone

The bottom of a pond may be made up of mud (fine particles of soil or sand and organic materials. Although it may appear as if there is little alive in the mud, this region is where the scavengers and decomposers will be found along with their predators. Typical organisms found around the mud include tadpoles, worms, yabbies, mussels, snails, fungi and microscopic organisms, especially bacteria.

Some organisms are found buried in the mud, others live on top of it. Many filter feeders are found in this zone, they filter the detritus from the water or from the mud.

Detritus: Fine particles made up of the wastes or dead remains of plants and animals. These particles will sink to the bottom and become food for many decomposers.

Farm animals and farming

Farming has had some positive and many negative effects on Australia's freshwater bodies and the animals they contain.The creation of farm dams has led to more habitats becoming available for some aquatic plants and animals. This has been extended to the commercial breeding of many fish and yabbies. Farm animals, like sheep and cattle, have hard hooves, which trample the banks of dams, lakes and rivers. This leads to erosion with soil, etc being washed into the water as a result. This can increase the cloudiness of the water leading to less plant growth which, in turn, can decrease animal numbers. Farmers use a variety of chemicals, from fertilisers to increase plant growth to pesticides for controlling harmful organisms. Sometimes some of these chemicals can run-off into freshwater.

Pesticides

In high levels, pesticides may kill some organisms. In lower levels, certain pesticides have been shown to accumulate in the tissues of plants and animals and be passed up the food chain. This means that the top order carnivores can build up high levels of these pesticides. This process is called biomagnification.

DDT was a very commonly used insecticide. Its use, in controlling mosquitoes, has led to many lives being saved around the world but its properties, including its non-biodegradabilty and its solubility in fat, has led to it killing many larger birds and still being found in the tissues of many animals despite the fact that its use has been banned in many countries for many years. It was even found in the bodies of Antarctic penguins.

Fertilisers

Fertilisers, like sewage, can stimulate the growth of aquatic plants, especially algae which can grow and reproduce very rapidly. During the day algae photosynthesise but little of the oxygen produced will dissolve in the water due to its low solubility. At night, all the organisms in the pond use up oxygen in the process of respiration. If there is a large amount of algae there may not be enough oxygen for all organisms to survive. Larger fish are the first to die. This process is worsened as algae and other organisms die and are broken down. The decomposition process uses up even more oxygen. This process is called eutrophication.

Other problems

Many wetland areas (e.g. swamps) have been drained to create farms. Rivers have had dams and weirs built in them to control their flow for irrigation or flood control.

Obstacles such as these can prevent fish from migrating up and down the river, for example, to breed.

Increased erosion and run-off has meant that streams which had gravel beds are now covered in silt. This will prevent certain native fish from breeding.

Water temperatures in rivers and streams are changed by the release of water from dams, especially in summer. This release of cold water may prevent certain native fish from breeding.

Flooding is necessary for the successful reproduction of some native organisms, for example, the Golden Perch and River Red Gums. Improved flood control has affected the numbers of these species.