Fresh water ecosystem:

Fresh water is naturally occurring water on Earth's surface in ice caps, ice sheets, bogs, glaciers, ponds, rivers, lakes, and streams, and underground as groundwater in aquifers and underground streams. Fresh water is usually classified by having low concentrations of dissolved salts and other total dissolved solids. Term specially excludes seawater and brackish water though it does comprise mineral rich waters like chalybeate springs. The term sweet water has been utilized to explain fresh water in contrast to salt water. Freshwater ecosystems cover 0.80% of the Earth's surface and inhabit 0.009% of its total water. They produce almost 3% of its net primary production. Freshwater ecosystems have 41% of world's known fish species. Main zones in river ecosystems are determined by the river bed's gradient or by the velocity of the current. Faster moving turbulent water typically has greater concentrations of dissolved oxygen that supports greater biodiversity than slow moving water of pools.

Kinds of Freshwater:

There are three essential kinds of freshwater ecosystems:

i) Lentic: slow-moving water, comprising ponds, pools, and lakes.

ii) Lotic: rapidly-moving water, for instance streams and rivers.

iii) Wetlands: areas where soil is saturated or inundated for at least part of the time

Freshwater Habitats:

Methodically, freshwater habitats are divided in lentic systems that are still waters comprising lakes, ponds, swamps and mires; lotic systems, which are running water; and groundwater that flows in rocks and aquifers. There is, additionally, a zone that bridges between groundwater and lotic systems, which are the hyporheic zone that underlies several larger rivers and have considerably more water than is seen in open channel. It may also be in direct contact with the underlying underground water.

The freshwater Major Habitat Types (MHTs) reflect groupings of ecoregions with similar chemical, biological, and physical characteristics and are approximately equivalent to biomes for terrestrial systems. MHTs refer to dynamics of ecological systems and broad habitat structures which define them and such groupings can give the structured framework for analyzing and comparing diversity of life in freshwater systems. Due to large scale of ecoregions, all have patches of multiple habitat types. For instance, ecoregions in the large lakes habitat type can contain floodplains, swamps, and grassy savannas additionally to dominant lake habitat. Large Lakes are freshwater ecoregions which are dominated and stated by large lentic systems. This MHT includes large tropical, temperate, and polar lakes, in addition to Inland Seas included in analysis (Aral and Caspian). Ecoregions having deltas but not defined by specific deltaic fauna, like the Lower Mississippi ecoregion, are not considered Large River Delta ecoregions.

Montane Freshwaters are freshwater ecoregions made up of small streams, rivers, lakes or wetlands at higher elevations, in spite of latitude. These ecoregions comprise either high gradient, relatively fast-flowing streams, shallow, with rapids or complexes of high-altitude wetlands and lakes, and montane climatic conditions. Examples comprise Mount Nimba and Western Equatorial Crater Lakes in Africa and Orinoco Piedmont and Andes Mountains in South America.

Xeric Freshwaters and Endorheic (Closed) Basins are freshwater ecoregions dominated by endorheic aquatic systems or freshwaters which are found in semi-arid, arid, or dry sub-humid environments. These ecosystems tend to have detailed fauna adapted to ephemeral and intermittent flooding regimes or lower waters levels during certain times of the year. Examples comprise lower Nile River, or Death Valley ecoregion in the US.

Temperate Coastal Rivers are freshwater ecoregions dominated by numerous small to medium coastal basins in mid-latitudes (temperate). These ecoregions are classified by riverine ecosystems, but may also have small lakes, coastal lagoons, and other wetlands. Migratory species which spend part of their life cycles inside marine environments may inhabit these ecoregions. Though floodplains may take place along rivers inside this MHT, the dominant characteristics are various, small to medium-sized basins which drain to ocean,, instead of one large river predominating with the extensive fringing floodplain.

Temperate Upland Rivers are freshwater ecoregions which are dominated and stated by mid-latitude non-floodplain rivers, comprising headwater drainages and tributaries of large river systems. These rivers are classified by moderate gradients and absence of the cyclically flooded, fringing floodplain. Examples comprise Ozark Highlands and Ouachita Highlands in North America. Temperate Floodplain Rivers and Wetland Complexes are freshwater ecoregions which are dominated by the single mid-latitude large river system, comprising main stem river drainage and related sub-basins that are either currently or were historically classified by the cyclically flooded, fringing floodplain.

Tropical and Subtropical Coastal Rivers are freshwater ecoregions dominated by numerous small to medium coastal basins at low-latitudes (tropics). These ecoregions are classified by riverine ecosystems but may also have small coastal lagoons, lakes, and other wetlands. Though floodplains may take place along rivers inside this MHT, the dominant characteristics are several, small to medium sized basins which drain to ocean, instead of one large river predominating with the extensive fringing floodplain. This MHT also includes island ecoregions with these features. Examples comprise Kenyan Coastal Rivers and Mata Atlantica.

Tropical and Subtropical Floodplain Rivers and Wetland Complexes are freshwater ecoregions which are dominated by the single low-latitude large river system, comprising main stem river drainage and related sub-basins, that are either currently or were historically classified by the cyclically flooded, fringing floodplain. These ecoregions may also have wetland complexes made up of internal marshes, deltas, and/or swamps, related with main river system. Examples comprise Lower Congo, Lower Niger-Benue, Cuvette Central, Amazonas Lowland, and Orinoco-Llanos.

Polar Freshwaters are freshwater ecoregions including entire drainages; from headwaters to mouth, and found in high latitudes. Examples comprise Lena River in Siberia and the Yukon in Alaska. Oceanic Islands are freshwater ecoregions made up of one or more islands entirely enclosed by water, above high tide, and isolated from other important landmasses. These ecoregions are classified by freshwater biotas derived from marine ancestors. Examples comprise Fiji and the Hawaiian Islands.

Sources of fresh water:

The source of approximately all fresh water is precipitation from atmosphere, in the form of rain, mist and snow. Fresh water falling as mist, rain or snow has materials dissolved from atmosphere and material from sea and land over which rain bearing clouds have traveled. In industrialized areas rain is characteristically acidic due to dissolved oxides of sulfur and nitrogen formed from burning of fossil fuels in cars, trains, factories and aircraft and from atmospheric emissions of industry. In severe cases this acid rain results in pollution of lakes and rivers in parts of Scandinavia, Scotland, Wales and the United States. In coastal areas fresh water may have important concentrations of salts derived from sea if windy situation have lifted drops of seawater in rain-bearing clouds. This can give rise to elevated concentrations of sodium, chloride, magnesium and sulfate and several 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 somewhere else in precipitation and causing freshwater flow to be noticeably contaminated both by insoluble solids but also by soluble components of those soils. Important quantities of iron may be transported in this way comprising well-documented transfer of iron-rich rainfall falling in Brazil derived from sand-storms in the Sahara in north Africa.

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