It has following chemical properties.
Solubility of Gases in Water:
Most gases dissolve readily in water, most notably those that are essential for life. The concentration of any gas in water generally varies between zero and a theoretical maximum or saturation. The latter is the amount of gas that can be dissolved in water when the atmosphere and the water are in equilibrium with one another.
Except for waterfalls and very turbulent streams, the water in natural ecosystem is seldom in equilibrium with the atmosphere. A gas may show e deficit, if it is being utilized in the ecosystem faster than it is going into solution across the air water interface, or it may be supersaturated, if it is being produced in the ecosystem faster than it is being released from solution across the interface. The concentration of important gases may vary widely in any ecosystem, both horizontally and vertically.
The saturation level of any gas in water depends on several variables, most notably temperature, salinity, the concentration of the gas in the atmosphere, and its relative solubility in water. The greater the concentration of a gas its concentration in the water will tend to be, depending on its relative solubility in water.
One of the most critical factors in an aquatic environment is the amount of oxygen in the water because most living organisms (excepting anaerobic forms) require this gas for respiration.
In contrast to atmosphere, the oxygen becomes limiting factor for aquatic animals as the saturation concentration of oxygen in water is governed by temperature and salinity. As is evident in table 10.3, the lower the temperature, the greater the oxygen retaining capacity of water, whether it is fresh-water or sea water (Fig. 10.3).
In fresh-water areas of any depth and in the salt water habitat, there are three recognizable zones with regard to oxygen concentration a surface stratum, where the oxygen tends to be in equilibrium with the atmosphere above. Below this surface stratum of variable depth is an intermediate stratum where oxygen values fluctuate in accordance with existing factors.
Respiration, decomposition of organic materials (stagnant ponds) and stream pollution all tend to reduce the amount of available oxygen, while photosynthetic activity will often balance or more than balance oxygen loss.
The deepest layers of water will usually have a very low oxygen concentration in the deeper lakes and oceanic areas because the continual decomposition of organic debris, the respiration of organisms inhabiting these deeper water, and the complete absence of photosynthetic activity in these lower strata will tend to deplete the oxygen concentration. The deep stratum is entirely dependent upon the slow transport of oxygen from the overlying intermediate layer.
Because oxygen is needed for the respiration of all heterotrophs, it tends to be taken out of solution continuously by the organisms. The only means by which it can enter the water are via solution at the air-water interface or through photosynthesis by aquatic plants.
Oxygen is transferred to deeper water by diffusion or through circulation of the water. The rate of oxygen solution at the surface is highly variable, depending mainly on the turbulence of the water and the presence of waves spray and foam.
Some organisms (e.g., aquatic beetles, true bugs, fresh water snails, larvae of mosquitoes, whales, seals, pot poises, alligator, crocodiles. etc.) are independent of the oxygen concentration in water because they move to the surface and obtain atmospheric oxygen.
Ingenious devices such as air tubes or the ability to trap air bubbles in surface hairs or under wings cover are utilized by some of these forms to insure submersion for an interval of time. Some bacteria and animals (e.g., Teredo, Mytilus edulis, etc.) can live an aerobically by breaking down glucose, glycogen and other carbohydrates and obtain oxygen from this source.
Nitrogen is significantly less soluble in water than oxygen. But because it constitutes 78 per cent of the atmosphere, it still accounts for about 65 per cent of the dissolved gases at equilibrium. It is fairly inert chemically and does not react with water, although some bacteria, fungi, blue-green algae, and so on, can use it to satisfy their nitrogen requirements, and other bacteria can produce it through reduction of nitrate under conditions of very low oxygen concentration.
The decomposition of organic matter and the respiratory activity of aquatic plants and animals produce carbon dioxide. This gas is one of the essential raw materials necessary for photosynthetic activity by green plants. Carbon dioxide combines chemically with water to produce carbonic acid (H2CO3) which influences the hydrogen ion concentration (pH) of water.
Carbonic acid dissociates to produce hydrogen (H+) and bicarbonate (HCO3-) ions. The bicarbonate radical may undergo further dissociation forming more hydrogen (H+) and carbonate (C03-), as represented in following reactions:
The amount of free or uncombined carbon dioxide in water is of ecological importance: it governs the precipitation of calcium in the form of calcium carbonate (CaCO3). Calcium precipitates when temperatures and salinity are high and the amount of uncombined carbon dioxide is low. This means more carbonate (CO3-) is present to combine with the calcium cation (Ca++). These conditions exist in shallow tropical waters, where evaporation is high.
This raises the salinity and photosynthetic activity of plants and reduces the quantity of free carbon dioxide in water. The precipitation of calcium carbonate in tropical areas as the Bahamas explains the preponderance of thick calcareous shells of shallow water tropical molluscs, plankton and algae.
In deep oceanic water, temperatures are low and there are no- photosynthetic plants, consequently. The carbon dioxide content of the water is high. Deep water fauna (molluscs, crustaceans) possess very fragile skeleton because the precipitation of calcium carbonate is minimum.
The deeper layers of many bodies of water, including ponds, lakes, and some estuaries, may contain significant amounts of the toxic gas, hydrogen sulphide, which is released by decaying organic matter if concentration of the gas builds up, all life but anaerobic bacteria excluded from the area (e.g., deeper strata of Black sea).
Hydrogen-ion Concentration (pH):
One property of natural water is their acidity or alkalinity. About one water molecule in every 10,000,000 splits into two, producing a hydrogen ion (H+) and a hydroxy ion (OH–):
H2O – H+ + OH–
In pure water, there are equal numbers of H+ and OH– ions; it therefore, has a neutral reaction. Some natural waters, however, acquire an excess of H+ and are acidic, while others, with an excess of OH– are alkaline. For example, in fresh-water habitats such as acid bogs, swamps and drainage streams carrying water from these areas are acidic; contain pH value as low as 1.4.
Despite the high acidity, rich populations of acidophilic flora and fauna thrive under such conditions. On the other hand, certain lake waters may be quite alkaline, particularly in lime stone areas, where the pH may range from 10 to 12. Basophilic plants and animals are found in these areas.
In a vast majority of fresh-water localities pH value ranges between 5–5 and 8?5. Oceanic areas exhibit little change in pH values over vast areas because of the effective buffering action of anions such as HCO3– and CO3– surface waters of open ocean have pH values of 8.0 to 8.4, but deeper water is close to neutrality (pH 7 4 to 7 9). There is often a wider range of pH values in shallow marine water, estuaries and tide pools.