Fresh water

For other uses, see Fresh water (disambiguation).

Pearl Shoal Waterfall, western Sichuan, China.

Freshwater refers to naturally occurring water on the surface such as bogs, ponds, lakes, rivers and streams, and underground in aquifers and underground rivers. Freshwater is characterized by having low concentrations of dissolved salts. The term specifically excludes seawater and brackish water.

Systems

Scientifically, freshwater habitats are divided into lentic systems, which are the stillwaters including ponds, lake,swamps and mires; lotic systems, which are running water and groundwater which flows in rocks and aquifers. There is, in addition, a zone which bridges between groundwater and lotic systems, which is the hyporheic zone, which underlies many larger rivers and can contain substantially more water than is seen in the open channel. It may also be in direct contact with the underlying groundwater.

Source

The source of almost all freshwater is precipitation from atmosphere in the form of mist, rain and snow. A very small proportion is emitted from active volcanoes. Freshwater falling as mist, rain or snow contains materials dissolved from the atmosphere and material from the sea and land over which the rain bearing clouds have traveled. In industrialized areas rain is typically acid because of dissolved oxides of sulfur and nitrogen formed from burning of fossil fuels in cars, lorries, trains and aircraft and from the atmospheric emissions of industry. In extreme cases this causes acid rain which can has caused severe pollution of lakes and rivers in parts of Scandinavia, Scotland, Wales and the United States.

In coastal areas freshwater may contain significant concentrations of salts derived from the sea if windy conditions have lifted drops of seawater into the rain-bearing clouds. This can give rise to elevated concentrations of sodium, chloride, magnesium and sulfate as well as many other compounds in smaller concentrations.

In desert areas, or areas with impoverished or dusty soils, rain bearing winds can pick up sand and dust and this can be deposited elsewhere in precipitation and causing the freshwater flow to be measurably contaminated both by insoluble solids but also by the soluble components of those soils. Significant quantities of iron may be transported in this way including the well documented transfer of iron rich rainfall falling in Brazil derived from sand-storms in the Sahara in northern Africa. This effect can cause unwelcome contamination with dust from the Chernobyl disaster being spread across Europe in rain clouds

Very large amounts of freshwater are locked up in the ice-caps of the world, especially over Antarctica and Greenland

Source

The source of almost all freshwater is precipitation from atmosphere in the form of mist, rain and snow. A very small proportion is emitted from active volcanoes. Freshwater falling as mist, rain or snow contains materials dissolved from the atmosphere and material from the sea and land over which the rain bearing clouds have traveled. In industrialized areas rain is typically acid because of dissolved oxides of sulfur and nitrogen formed from burning of fossil fuels in cars, lorries, trains and aircraft and from the atmospheric emissions of industry. In extreme cases this causes acid rain which can has caused severe pollution of lakes and rivers in parts of Scandinavia, Scotland, Wales and the United States.

In coastal areas freshwater may contain significant concentrations of salts derived from the sea if windy coSource

The source of almost all freshwater is precipitation from atmosphere in the form of mist, rain and snow. A very small proportion is emitted from active volcanoes. Freshwater falling as mist, rain or snow contains materials dissolved from the atmosphere and material from the sea and land over which the rain bearing clouds have traveled. In industrialized areas rain is typically acid because of dissolved oxides of sulfur and nitrogen formed from burning of fossil fuels in cars, lorries, trains and aircraft and from the atmospheric emissions of industry. In extreme cases this causes acid rain which can has caused severe pollution of lakes and rivers in parts of Scandinavia, Scotland, Wales and the United States.

In coastal areas freshwater may contain significant concentrations of salts derived from the sea if windy conditions have lifted drops of seawater into the rain-bearing clouds. This can give rise to elevated concentrations of sodium, chloride, magnesium and sulfate as well as many other compounds in smaller concentrations.

In desert areas, or areas with impoverished or dusty soils, rain bearing winds can pick up sand and dust and this can be deposited elsewhere in precipitation and causing the freshwater flow to be measurably contaminated both by insoluble solids but also by the soluble components of those soils. Significant quantities of iron may be transported in this way including the well documented transfer of iron rich rainfall falling in Brazil derived from sand-storms in the Sahara in northern Africa. This effect can cause unwelcome contamination with dust from the Chernobyl disaster being spread across Europe in rain clouds [1]

Very large amounts of freshwater are locked up in the ice-caps of the world, especially over Antarctica and Greenlandnditions have lifted drops of seawater into the rain-bearing clouds. This can give rise to elevated concentrations of sodium, chloride, magnesium and sulfate as well as many other compounds in smaller concentrations.

In desert areas, or areas with impoverished or dusty soils, rain bearing winds can pick up sand and dust and this can be deposited elsewhere in precipitation and causing the freshwater flow to be measurably contaminated both by insoluble solids but also by the soluble components of those soils. Significant quantities of iron may be transported in this way including the well documented transfer of iron rich rainfall falling in Brazil derived from sand-storms in the Sahara in northern Africa. This effect can cause unwelcome contamination with dust from the Chernobyl disaster being spread across Europe in rain clouds [1]

Very large amounts of freshwater are locked up in the ice-caps of the world, especially over Antarctica and Greenland

Numerical definition

The surface of a freshwater lake in daylight.

Freshwater can be defined as water with less than 500 parts per million (ppm) of dissolved salts..

Water salinity based on dissolved salts in parts per thousand (ppt)
Freshwater	Brackish water	Saline water	Brine
< 0.5	0.5 – 30	30 – 50	> 50

Water distribution

Water is a critical issue for the survival of all living organisms. Many can use salty water but many organisms including the great majority of higher plants and most mammals, including humans, must have access to freshwater to survive. Some terrestrial mammals, especially desert rodents appear to survive without drinking but they do generate water through the metabolism of cereal seeds and they also have mechanisms to conserve water to the maximum degree.

Only three percent of the water on Earth is freshwater, and about two-thirds of this is frozen in glaciers and polar ice caps. Most of the rest is underground and only 0.3 percent is surface water. Freshwater lakes, most notably Lake Baikal in Russia and the Great Lakes in North America, contain seven-eighths of this fresh surface water. Swamps have most of the balance with only a small amount in rivers, most notably the Amazon River. The atmosphere contains 0.04% water. In areas with no freshwater on the ground surface, freshwater derived from precipitation may, because of its lower density, overlie saline ground water in lenses or layers.

Aquatic organisms

Freshwater creates a hypotonic environment for aquatic organisms. This is problematic for some organisms with pervious skins or with gill membranes, whose cell membranes may burst if excess water is not excreted. Some protists accomplish this using contractile vacuoles, while freshwater fish excrete excess water via the kidney. Although most aquatic organisms have a limited ability to regulate their osmotic balance and therefore can only live within a narrow range of salinity, diadromous fish have the ability to migrate between freshwater and saline water bodies. During these migrations they undergo changes to adapt to the surroundings of the changed salinities; these processes are hormonally controlled. The eel (Anguilla anguilla) uses the hormone prolactin, while in salmon (Salmo salar) the hormone cortisol plays a key role during this process.

Many sea-birds have special glands at the base of the bill through which excess salt is excreted. Similarly the marine Iguanas on the Galapagos islands excrete excess salt through a nasal gland and they sneeze out a very salty excretion.

Freshwater as a resource

Freshwater is an important natural resource necessary for the survival of all ecosystems. The use of water by humans for activities such as irrigation and industrial applications can have adverse impacts on down-stream ecosystems . Chemical contamination of freshwater can also seriously damage eco-systems.

Pollution from human activity, including oil spills, also presents a problem for freshwater resources. The largest oil spill that has ever occurred in freshwater was caused by a Shell tank ship in Magdalena, Argentina, on January 15, 1999, polluting the environment, drinkable water, plants and animals.

Agriculture

Changing landscape for the use of agriculture has a great effect on the flow of freshwater. Changes in landscape by the removal of trees and soils changes the flow of freshwater in the local environment and also affects the cycle of freshwater. As a result more freshwater is stored in the soil which benefits agriculture. However, since agriculture is the human activity that consumes the most freshwater, this can put a severe strain on local freshwater resources resulting in the destruction of local ecosystems. In Australia, over-abstraction of freshwater for intensive irrigation activities has caused 33% of the land area to be at risk of salination.

Limiting resource

Freshwater is a renewable and changeable, but limited natural resource. Freshwater can only be renewed through the process of the water cycle, where water from seas, lakes, rivers, and dams evaporates, forms clouds, and returns to water sources as precipitation. However, if more freshwater is consumed through human activities than is restored by nature, the result is that the quantity of freshwater available in lakes, rivers, dams and underground waters is reduced which can cause serious damage to the surrounding environment.

See also

• Desalination
• Freshwater ecology
• Lake
• Limnology
• List of countries by freshwater withdrawal
• List of countries by total renewable water resources
• River
• Seawater
• Water (molecule)
• Water cycle
• Water resources
• Water crisis

References

• Brian D. Richter, R. M. (2003). Ecologically sustainable water management: managing river flows for ecological integrity. Ecological Applications , 13 (1), 206-224.
• Robert B. Jackson, S. R. (2001). Water in a changing world. Ecological Applications , 11 (4), 1027-1045.

Further reading

• Moustakas, A. & I. Karakassis. How diverse is aquatic biodiversity research?, Aquatic Ecology, 39, 367-375

External links

• Freshwater life.org
• Freshwater Biological Association
• Review of freshwater ecology in the UK
• Fritsch Collection of Algal Illustrations
• UK Environment Agency website
• Pond Conservation
• UK National pond monitoring network
• U.S. Geological Survey