Freshwater
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Greyhood
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[[Image:Water surface lake.jpg|right|thumb|255px|The surface of a freshwater lake in daylight.]]
A term that refers to bodies of water such as [[pond]]s, lakes, rivers and streams containing low concentrations of dissolved [[salts]] and other [[total dissolved solids]]. Freshwater is an important [[renewable resource]], necessary for the survival of most terrestrial [[organisms]], and is required by humans for [[drinking water|drinking]] and [[agriculture]], among many other uses. The [[UN]] estimates that about 1.2 billion people (18 percent of the world's population) lack access to safe drinking water.[http://www.un.org/News/Press/docs/2006/sgsm10378.doc.htm]
==Numerical definition==
Freshwater is defined as water with less than 0.5 parts per thousand dissolved [[salt]]s.<ref>{{cite web | url=http://www.groundwater.org/gi/gwglossary.html | title=Groundwater Glossary | date=[[2006-03-27]] | accessdate=2006-05-14}}</ref> Freshwater bodies include [[lake]]s and [[pond]]s, [[river]]s, some bodies of [[groundwater|underground water]] and many kinds of man-made freshwater bodies, such as canals, ditches and reservoirs. The ultimate source of freshwater is the precipitation of [[Earth's atmosphere|atmosphere]] in the form of [[rain]] and [[snow]].
{| style="margin-left:auto; margin-right:auto;"
|-
! style="background:#B8E0F6" colspan="4" | Water salinity based on dissolved salts in parts per thousand (ppt)
|-
! style="background:#87CEFA" | '''Freshwater'''
! style="background:#87CEFA" | [[Brackish water]]
! style="background:#87CEFA" | [[Saline water]]
! style="background:#87CEFA" | [[Brine]]
|-
! style="background:#00BFFF" | < 0.5
! style="background:#00BFFF" | 0.5 - 35
! style="background:#00BFFF" | 35 - 50
! style="background:#00BFFF" | > 50
|}
==Water distribution==
Access to unpolluted freshwater is a critical issue for the survival of many species, including humans, who must drink freshwater in order to survive. Only three percent of the water on [[Earth]] is freshwater in nature, and about two-thirds of this is frozen in [[glacier]]s and [[polar ice cap]]s. 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 (North America)|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. <ref>{{cite book |last= Gleick |first= Peter |coauthors=et al. |editor= Stephen H. Schneider |title= [[Encyclopedia of Climate and Weather]] |publisher= Oxford University Press |year= 1996}}</ref> 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 [[Tonicity#Hypotonicity|hypotonic]] environment for aquatic organisms. This is problematic for some organisms, whose cell membranes will burst if excess water is not excreted. Some [[protist]]s accomplish this using [[Contractile vacuole]]s, while [[freshwater fish]] excrete excess water via the [[kidney]].<ref>{{cite web | url=http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/VertebrateKidneys.html | title=Vertebrate Kidneys | date=[[2002-11-03]] | accessdate=2006-05-14}}</ref> 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 [[Fish migration|migrate]] between freshwater and [[seawater|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.
== See also ==
*[[Desalination]]
*[[Limnology]]
*[[List of countries by freshwater withdrawal]]
*[[List of countries by total renewable water resources]]
*[[Seawater]]
*[[Water (molecule)]]
*[[Water cycle]]
*[[Water resources]]
*[[Water crisis]]
== References ==
{{reflist}}
==External links==
* [http://www.freshwaterlife.org/ Freshwater life.org]
* [http://www.fba.org.uk/ Freshwater Biological Association]
* [http://www.fba.org.uk/pdf/ReviewOfFreshwaterEcology.pdf Review of freshwater ecology in the UK]
* [http://www.environment-agency.gov.uk/ UK Government Environmental Agency website]
* [http://www.pondconservation.org.uk/ Pond Conservation]
* [http://www.pondnetwork.org.uk/ UK National pond monitoring network]
[[Category:Aquatic ecology]]
[[Category:Liquid water]]
[[Category:Hydrology]]
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