Fresh water
Fresh water is
water with less than 0.5 parts per thousand dissolved salts.
Fresh water may be found in
lakes,
rivers, and bodies of
groundwater. The ultimate source of fresh water is
rain (or, to be more precise, the precipitation of
atmospheric water vapor, see also
hydrologic cycle).
Access to unpolluted fresh water is a critical issue for the survival of many species, including humans, who must drink fresh water in order to survive. Only 3% of water on the
Earth is fresh water, and over two thirds of this is frozen in
glaciers and polar ice caps (see
water resources).
Many species can live in fresh water, including freshwater fish species. However, fresh water creates a
hypotonic environment for aquatic organisms. This is problematic for some organisms, whose cell walls will burst if excess water is not excreted. Some
protists accomplish this using
contractile vacuoles,
while freshwater
fishes 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, some
fishes have the ability to
migrate between fresh water and
seawater.
*
Desalination*
Limnology*
Seawater*
Water (molecule)
