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 the
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
sulphur and
nitrogen formed from burning of fossil fuels in cars, factories, 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
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 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 must have access to freshwater to grow bigger. 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 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
thumb|200px|[[Grande Ronde River flowing through
Hilgard Junction State Recreation Area in
Eastern Oregon. An important concern for hydrological ecosystems is securing minimum
streamflow, especially preserving and restoring
instream water allocations.]]
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,
[Gordon L., D. M. (2003). Land cover change and water vapour flows: learning from Australia. Philosophical Transactions: Biological Sciences , 358 (1440), 1973-1984.] 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.
Freshwater withdrawal
Freshwater withdrawal is the quantity of water removed from available sources for use in any purpose.
Water drawn-off is not necessarily entirely consumed and some portion may be returned for further use downstream.
See also
