Only a tiny fraction of the planet's abundant water is available to us as freshwater. About 97.4% by volume is found in the oceans and is too salty for drinking, irrigation, or industry (except as a coolant).
Most of the remaining 2.6% water is freshwater and locked up in ice layers or glaciers or it's too deep underground to be reached or too salty to be used.
Thus, only about 0.014% of the earth's total volume of water is easily available to us as soil moisture, usable groundwater, water vapor, and lakes and streams.
Freshwater ecosystems are things such as rivers, lakes or ponds. They do not include the sea, or oceans which have salt water in them. They contain lots of different types of animals and plants; from fish and ducks to tiny water beetles and worms, and from algae to water lillies.
When a river becomes polluted, it means that there are things in the water that should not be there (such as chemicals, objects or extra heat). For instance, if something is spilt into the river, either accidentally or on purpose, the river has been polluted, and the substance put into the river is called a pollutant.
There are a number of different ways in which a river can become polluted, and there are many different things which can be pollutants. Some low levels of pollutants are not harmful to the area where plants and animals live together (the ecosystem), because the river is able to get rid of them itself, either by diluting them (making them weaker by adding water), or washing them away. When the river does this it is called natural repair, because the river is repairing the damage that the pollutants have done. Areas of water such as ponds or lakes, where the water does not move much, suffer more from pollution than rivers, as they are not able to get rid of the pollutants very well.
Rivers can also be polluted by chemicals released from factories or by chemicals and oils spilt on roads. These chemicals are pollutants, and this is called chemical pollution. Sometimes when there are too many pollutants in the water, the river or stream cannot get rid of all of them by natural repair. When this happens, the pollutants begin to affect the wildlife that is in the water, either by killing it or by making plants and animals grow differently.
Another type of pollution called
organic pollution has a different effect on the wildlife in the
river. The main causes of organic pollution are sewage works and
farms. When organic pollution is released into the river, it
is broken down (eaten) by millions of tiny bacteria and worms.
Whilst breaking down this pollution, the bacteria and worms use up
oxygen, much like we do when we breathe. When the amount of organic
pollution is high, there is less oxygen in the water. This leaves
animals, such as water beetles and fish, with not enough enough
oxygen to survive and they begin to die.