Freshwater 230001 222348796 2008-06-28T21:37:22Z Greyhood 7227106 /* See also */ {{otheruses}} [[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]] [[ca:Aigua dolça]] [[da:Ferskvand]] [[de:Süßwasser]] [[et:Magevesi]] [[es:Agua dulce]] [[eo:Nesala akvo]] [[fa:آب شیرین]] [[fr:Eau douce]] [[ko:민물]] [[id:Air tawar]] [[he:מים מתוקים]] [[lv:Saldūdens]] [[lb:Séisswaasser]] [[mk:Питка вода]] [[nl:Zoetwater]] [[ja:淡水]] [[no:Ferskvann]] [[nn:Ferskvatn]] [[nds:Söötwater]] [[pl:Woda słodka]] [[pt:Água doce]] [[ru:Пресная вода]] [[simple:Fresh water]] [[fi:Makea vesi]] [[sv:Sötvatten]] [[ta:நன்னீர்]] [[vi:Nước ngọt]] [[uk:Прісна вода]] [[zh:淡水]]