Introduction:

Life on Earth relies upon water. Water includes 99% of our own bodies and covers 71% of earth's surface. It provides different functions ranging from the transport function by serving as solvent for most chemicals to serving habitat to several organisms. Several organisms also depend on water for certain phases of their life. Water is the significant issue for the survival of all living organisms. Some can use salt water but several organisms comprising great majority of higher plants and most mammals should have access to fresh water to live.

Hydrobiology is the science of life and life processes in water. Much of modern hydrobiology can be viewed as a sub-discipline of ecology but the sphere of hydrobiology includes taxonomy, economic biology, industrial biology, morphology, physiology etc. The one distinguishing aspect is that all relate to aquatic organisms. Much work is closely related to limnology and can be divided into lotic system ecology (flowing waters) and lentic system ecology (still waters).

Physical and Chemical Nature of Freshwater:

a) Color and Turbidity: Frequently it is the color of freshwater or how clear or hazy water is that is the most obvious visual characteristic. Though both color and turbidity decrease the amount of light penetrating water and can have important impact on algae and macrophytes. Some algae in particular are extremely dependent on water with low color and turbidity. Several rivers draining high moor-lands overlain by peat have the very deep yellow brown color caused by dissolved humic acids.

b) Organic constituents: One of the principal sources of elevated concentrations of organic chemical constituents is from treated sewage. Dissolved organic material is most usually measured using either Biochemical oxygen demand (BOD) test of Chemical oxygen demand (COD) test. Organic constituents are important in river chemistry for the effect that they have on dissolved oxygen concentration and for impact that individual organic species may have directly on aquatic biota.

c) Metals: The wide range of metals may be found in rivers from natural sources where metal ores are present in rocks over which the river flows or in aquifers feeding water in river.

- Iron: Iron, generally as Fe+++ is the common constituent of river waters at very low levels. Higher iron concentrations in acidic springs or the anoxic hyporheic zone may cause visible orange/brown staining or semi-gelatinous precipitates of dense orange iron bacterial floc carpeting river bed.

- Zinc: Zinc is usually related with metal mining, particularly Lead and Silver mining but is also a component pollutant related with the variety of other metal mining activities and with Coal mining.

- Heavy metals: Lead and silver in river waters are usually found together and related with lead mining. Silver is very toxic even at very low concentrations but leaves no visible evidence of its contamination. Lead is also extremely toxic to freshwater organisms and to humans if water is used as drinking water. As with Silver, Lead pollution is not visible to naked eye. Important levels of copper are unusual in rivers and where it occurs source is most likely to be mining activities, coal stocking, or pig farming.

d) Nitrogen: Nitrogenous compounds have a variety of sources including washout of oxides of nitrogen from the atmosphere, some geological inputs and some from macrophyte and algal nitrogen fixation. However for many rivers in the proximity of humans, the largest input is from sewage whether treated or untreated. The nitrogen derives from breakdown products of proteins found in urine and faeces. The differing forms of nitrogen are relatively stable in most river systems with nitrite slowly transforming into nitrate in well oxygenated rivers and ammonia transforming into nitrite/ nitrate. The management of river chemistry to avoid ecological damage is particularly difficult in the case of ammonia as a wide range of potential scenarios of concentration, pH and temperature have to be considered and the diurnal pH fluctuation caused by photosynthesis considered.

e) Phosphorus: Phosphorus compounds are usually found as relatively insoluble phosphates in river water and, except in some exceptional circumstances, their origin is agriculture or human sewage. Phosphorus can encourage excessive growths of plants and algae and contribute to eutrophication. If a river discharges into a lake or reservoir phosphate can be mobilised year after year by natural processes. As the plants die in the late summer they fall into the cool water layers underneath - the hypolimnion - and decompose.

f) Arsenic: Geological deposits of arsenic may be released into rivers where deep ground-waters are exploited as in parts of Pakistan. Many metalloid ores such as lead, gold and copper contain traces of arsenic and poorly stored tailings may result in arsenic entering the hydrological cycle.

g) Solids: Inert solids are produced in all montane rivers as the energy of the water helps grind away rocks into gravel, sand and finer material. Much of this settles very quickly and provides an important substrate for many aquatic organisms. Many salmonid fish require beds of gravel and sand in which to lay their eggs. Many other types of solids from agriculture, mining, quarrying, urban run-off and sewage may block-out sunlight from the river and may block interstices in gravel beds making them useless for spawning and supporting insect life.

pH: pH in rivers is affected by the geology of the water source, atmospheric inputs and a range of other chemical contaminants. pH is only likely to become an issue on very poorly buffered upland rivers where atmospheric sulphur and nitrogen oxides may very significantly depress the pH as low as pH4 or in eutrophic alkaline rivers where photosynthetic bi-carbonate ion production in photosynthesis may drive pH up above pH10

Pressure, Density and Buoyancy:

• The pressure on a lake dwelling organism is the weight of water column above it and also the weight of atmosphere.
• The absence of animal life from deep water is ordinarily a consequence of low oxygen supply or low temperature rather than pressure.
• Water is most dense at 4 degree centigrade dissolved salts increase the water density. Few algae and protozoa are capable of living in salty lakes.
• Buoyancy varies with density of water and is influenced by factors affecting density.

Temperature:

• Thermal properties of water are best demonstrated by freshwater environment.
• Seasonal and diurnal temperature variations are evident in these environments than in marine environments.
• Difference in day and night temperatures remains more conspicuous in the shallow waters.
• Thermal stratification is observed more frequently in the lakes of tropical countries. Hence, lakes are classified into 3 types- Tropical lakes, Temperate lakes and Polar lakes

Light:

• Light affects freshwater ecosystem really.
• Freshwater have a lot of suspended particles that affect light to reach to bottom and therefore affects productivity.
• Shallow lake receives light greatly to depth and therefore more abundant growth than deep lakes.
• Light also manages orientation and changes in position of attached species and their nature of growth and it also causes diurnal planktonic species migration.

Oxygen:

• Chemically pure water is biologically inhabitable. Therefore, oxygen is the necessary chemical component which remains dissolved in water.
• Aquatic environment which remain in contact with atmosphere is abundant in oxygen concentration.
• Atmospheric oxygen reaches to water either by diffusion or by water movements.
• Aquatic plants supply water with oxygen.
• Oxygen is utilized in respiration by aquatic animals and in dead organisms decomposition.

Carbon dioxide:

• Aquatic vegetation and phytoplankton needs carbon dioxide for photosynthesis.
• It is produced due to respiration and decomposition.
• It gets dissolved in water and forms bicarbonic acid which affects water pH.
• Photosynthesis is the main cause for drain of carbon dioxide.

Water distribution:

Out of all the water on Earth, merely 2.75% is fresh water, comprising 2.05% frozen in glaciers, 0.68% as groundwater and 0.011% of it as surface water in lakes and rivers. Freshwater lakes, most particularly Lake Baikal in Russia and Great Lakes in North America, have seven-eighths of this fresh surface water. Swamps have most of the balance with only the small amount in rivers, most particularly Amazon River. Atmosphere has 0.04% water. In areas with no fresh water on the ground

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