we get most of our fresh water from rivers and streams.
we get most of our fresh water from rivers and streams.
where do we get most of our fresh water?
rivers and streams
saline water in oceans, seas and saline groundwater make up about 97% of all the water on earth. only 2.5–2.75% is fresh water, including 1.75–2% frozen in glaciers, ice and snow, 0.5–0.75% as fresh groundwater and soil moisture, and less than 0.01% of it as surface water in lakes, swamps and rivers. freshwater lakes contain about 87% of this fresh surface water, including 29% in the african great lakes, 22% in lake baikal in russia, 21% in the north american great lakes, and 14% in other lakes. 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 fresh water on the ground surface, fresh water derived from precipitation may, because of its lower density, overlie saline ground water in lenses or layers. most of the world's fresh water is frozen in ice sheets. many areas suffer from lack of distribution of fresh water, such as deserts.
fresh water
elasmobranchs' ability to enter fresh water is limited because their blood is normally at least as salty (in terms of osmotic strength) as seawater through the accumulation of urea and trimethylamine oxide, but bull sharks living in fresh water show a significantly reduced concentration of urea within their blood. despite this, the solute composition (i.e. osmolarity) of a bull shark in fresh water is still much higher than that of the external environment. this results in a large influx of water across the gills due to osmosis and loss of sodium and chloride from the shark's body. however, bull sharks in fresh water possess several organs with which to maintain appropriate salt and water balance; these are the rectal gland, kidneys, liver, and gills. all elasmobranchs have a rectal gland which functions in the excretion of excess salts accumulated as a consequence of living in seawater. bull sharks in freshwater environments decrease the salt-excretory activity of the rectal gland, thereby conserving sodium and chloride. the kidneys produce large amounts of dilute urine, but also play an important role in the active reabsorption of solutes into the blood. the gills of bull sharks are likely to be involved in the uptake of sodium and chloride from the surrounding fresh water, whereas urea is produced in the liver as required with changes in environmental salinity. recent work also shows that the differences in density of fresh water to that of marine waters result in significantly greater negative buoyancies in sharks occupying fresh water, resulting in increasing costs of living in fresh water. bull sharks caught in freshwater have subsequently been shown to have lower liver densities than sharks living in marine waters. this may reduce the added cost of greater negative buoyancy.
bull shark