1. Specific Heat:
Water is capable of storing tremendous quantities of heat energy with a relatively small rise in temperature. In this property, it is exceeded only by ammonia, liquid hydrogen and lithium. Thus water is said to have a high specific heat which can be defined as the number of calories necessary to raise one gram not water one degree centigrade. The specific heat of water is given the value of 1.
Because such great quantities of heat are absorbed before the temperature of natural waters such as ponds, lakes and seas can be raised one degree centigrade, they warm up slowly in the spring and cool off just as slowly in the fall.
This tends to maintain a relatively constant temperature of aquatic habitats and prevents wide seasonal fluctuations in the temperature of ponds, lakes and seas and also moderates the temperatures of local or worldwide environments. The property of high heat capacity of water is functionally important to aquatic organisms.
2. Latent Heat:
Water possesses the highest heat of fusion and heat of evaporation, collectively called latent heat, of all known substances that are liquid at ordinary temperatures. Thus, large quantities of heat energy must be removed before water can change from a liquid to a solid state, and conversely, it must absorb considerable heat before ice can be converted to the liquid state.
It takes approximately 80 calories of heat to convert one gram of ice to a liquid state when both are at 0°C. This is equivalent to the amount of heat needed to raise the same quantity of water from 0°C that 80°C. The value of latent heat of melting of water is thus 80.
Evaporation occurs at interface between air and water at all ranges of temperature. It also requires considerable amounts of heat, as 536 calories of heat are needed to overcome the attraction between molecules and convert one gram of water at 100°C into vapour. This is as much heat as is needed to raise 536 grams of water 1°C. Thus, the value of latent heat of water is 536.
The properties of latent heat of water are important not only because they moderate the temperature of the biosphere, but also because they play a basic role in the evaporation of water and its precipitation (condensation) as rain and as dew in the hydrological (water) cycle.
3. Thermal Conductivity:
Although water is a poor thermal conductor compared to metals, among the common liquids it is excellent: for example, the conductivity value for silver is 1.10; for water, 0.0125; for alcohol, 0.00048 and for benzene, 0.00033.
4. Expansion before Freezing:
The unusual relationship between temperature and density of water (section 10 2) has a great ecological significance. At the freezing point (O°C) water expands markedly and ice always floats on the top of a lake or stream, and it is very unusual for an aquatic system ever to freeze solid, unless it is very small. This phenomenon protects the aquatic biota from sudden freezing and consequent death due to it.
The viscosity of water is also high because of the energy contained in the hydrogen bonds. Viscosity of water can be visualized best if one observes it or any other liquid flowing through a glass tube or a clear plastic hose. The liquid moving through the tube behaves as if it consisted of a series of parallel concentric layers flowing one over another.
The rate of flow is greater at the centre, but because of the amount of internal friction between layers, the flow decreases toward the sides of the tube. This phenomenon can be observed along the side of any stream or river with uniform banks. The water along the banks is nearly still, while the current in the centre may be swift.
This resistance between the layers is called lateral or laminar viscosity. In another kind of viscosity, called eddy viscosity, water masses pass from one layer- to another and create turbulence both horizontally and vertically.
Viscosity is the source of frictional resistance to objects including organisms moving through the water. Since this resistance is 100 times that of air, animals must expend considerable muscular energy to move through the water. A mucous coating on fish, frogs and certain other aquatic organisms reduces surface tension. Streamlining of body does likewise. In fact the body forms of some aquatic organisms have evolved under the stresses of viscosity.
The faster an aquatic organism moves through the water, the greater the stress placed on the surface and greater the volume of water that must be replaced in a given time. Replacement of water in the space left behind by the moving animal adds additional drag on the body. An animal streamlined in reversed direction, with a short, rounded front and a rapidly tapering body and meets least resistance in the water. The acme of such streamlining is the sperm whale.
The viscosity of water allows organisms to swim using relatively simple movements. It also protects the aquatic animals and plants from the mechanical disturbances.
6. Surface Tension:
Within all substances including water particles of the same matter are attracted to one another. Molecules of water below the surface are symmetrically surrounded by other molecules. The forces of attraction are the same on one side of the molecule as on the other. But at the water’s surface the molecules exist under a different set of conditions.
Below is a hemisphere of strongly attractive similar water molecules; above is the much smaller attractive force of the air. Since the molecules on the surface are drawn into the liquid, the liquid surface tends to be as small as possible, taut-like the rubber of an inflated balloon. This is surface tension, which is greatest among all common liquids, except mercury. The surface tension of water is important in the lives of aquatic organisms.
For example, the water surface is able to support small objects and animals, such as water striders and water spiders that run across the pond surface. To other organisms surface tension is a barrier, whether they wish to penetrate into the water below or escape into the air above. For some, the surface tension is too great to break; for others it is a trap to avoid while skimming the surface to feed or to lay eggs.
If caught in the surface tension, the insect may flounder on the surface. The imagoes of mayflies find surface tension a handicap in their efforts to emerge from the water. Slowed down at the surface, these insects become easy prey for trout.
Likewise, surface tension is the force that draws liquids through the pores of the soil and the conducting network of plants. Aquatic insects and plants have evolved structural adaptations that prevent the penetration of water into the tracheal systems of the former and the stomata and internal air spaces of the latter.
No other compound can be compared to water as a solvent. So many different substances can be dissolved in it that it is know n as the universal solvent. More things, in fact, can be dissolved in water than in any other liquid. This is especially true for inorganic chemicals which split, or dissociate to form electrically charged entities termed ions. Ionization influences most electrical phenomena and many chemical phenomena of solutions.
It is probable that all natural elements are soluble in water, at least in trace amounts, and that they are all found in natural water at some place or other on the earth’s surface. In addition, many organic chemicals are water-soluble. Thus, water is the main medium by which chemical constituents are transported from one part of an ecosystem to the other.
It is the only medium by which these constituents can pass from the abiotic portion of the ecosystem into the living portion. Even in the driest of terrestrial environments, nutrient materials pass into the roots of plants in aqueous solution; when air is breathed by animals, oxygen is dissolved in water at the surface of the long before it can cross the mucous membrane and be absorbed by the blood.
Water is a buoyant medium. Organisms can exist in it without specialized supportive structures such as are needed by organisms that inhabit terrestrial environments.
9. Light Penetration of Water:
Water is a transparent medium. Its transparency enables the penetration of light to the depths where it is ultimately absorbed. Water absorbs light, transforming radiant energy into heat. But it does not absorb all wave lengths equally both ends of the visible light spectrum, especially the red end, are selectively absorbed, so that as one descends into a body of water the colour changes from white to bluish to a dull blue-green.
Light may be diffused or absorbed by particles in the water such as sediments, detritus, animals and plants. The more particles in the water, the greater the degree of diffusion and absorption.
Figure 10.2 indicates the amount of light transmission in different bodies of water under various conditions of turbidity. The zone upto which light rays penetrate is called photic zone and below this zone there is complete darkness.
The most significant abiotic effect of light penetration is heating of the water. However, the most important biotic function of light is its role in photosynthesis. In contrast to the air of terrestrial systems, light is absorbed by water very much faster, so light is an important limiting factor in an aquatic ecosystem. In order to survive, a plant must fix more energy than it utilizes in its respiration.
That is, the ratio between gross primary production and respiration must be greater than one. There is a level in the water at which the mean daily light flux allows a rate of production equal to that of respiration. This is termed the light compensation level. Above the compensation level, autotrophs can produce sufficient food for themselves and the animals associated them.
Below the compensation level, all energy must be derived from organisms living and feeding above the compensation level and sinking below it, either as part of their normal behaviour or after death.
The compensation level fluctuates under various circumstances. It tends to be higher in winter than in summer because the total radiation flux is lowest in the winter. In highly turbid areas, due to light-scattering and great diffusion of light, the compensation level raises upto the surface of water.
Organisms living at sea level experience a pressure of about 15 psi, which is defined as 1 atm (760 mm of Hg). Pressures increase with increased depth of water at the rate of one atmosphere (1 atm) for every 10 meters of descent. Organisms inhabiting the floor of deep-sea areas at depth of 10,500 meters are exposed to hydrostatic pressure of about one ton per square centimeter. Pressure influences solubility, ionic dissociation, and surface tension, – and water is slightly compressible with increased pressures.
Salinity has been defined as “the total amount of solid material in grams contained in one kilogram of the water, when all the carbonate has been converted into oxide, bromine and iodine replaced by chlorine and all organic matter completely oxidized. All types of natural waters contain various amounts of different salts (ions) such as Na, K, Mg, CI, SO4, PO4, CO3, HCO3, NO3, etc., and all these salts are responsible for the saltiness, salinity or salt content of water.
The salinity of marine water is rather constant being about 3 5%. The salinity of fresh water varies greatly. Some salt lakes may have a salinity of 25% to 30% which greatly restricts life in them.
Table 10-1. Comparison of some of the principal ions found in different kinds of water:
WaterNaKCaMgClSO4CO3Total per liter
1. Soft water0.016—0.010.00050.0190.0070.0120.065
2. Hard fresh0.0210.0160.620.0140.0410.0250.1160.301
4 Great salt lake65.543.760.0654.47110.0813.04—197.51
Salinity of water acts as an important limiting factor for the distribution of a number of species of plants and animals. Certain animals such as, spider crab, Maia, etc., can tolerate only narrow -fluctuations in salinity of water and are known as stenohaline animals.
While some animals such as Mytilus, Aplysia, etc., can withstand wider ranges of salinity and are called euryhaline animals. However, there are certain animals, such as, Anguilla, Salmon, etc., which are both stenohaline and euryhaline.