Environmental Component: All of us know that the sun is the intimate source of energy for all activities in our biosphere. The electromagnetic from the sun supply energy which warms up the earth and the atmosphere to provide a favorable global temperature for the living organisms.
In addition, light plays a variety of roles in the living world. It is essential for photosynthesis, the process by which light is converted into usable chemical energy. It is involved in the transmission of information, for instance, it helps plants and animals to programme their life cycles, coordinates the opening of buds and flowers, dropping of leaves and a variety of other physiological processes. Variation in the amount of light generally affects the local distribution of plants. In animals light regulates reproduction, hibernation and migration and of course makes vision possible. All these biological phenomena are readily influenced by variation in the intensity, and by seasonal or diurnal variations of light.
In the following sections we will discuss briefly the properties of solar radiations, and their global distribution.
In FST-1 you have learnt about the electromagnetic spectrum which extends from gamma rays, x-rays, ultra-violet, visible, infrared to radio waves. The spectral distribution and the intensity of solar radiation incident on the earth surface is known. As shown in the radiations that strike the earth extend from near ultraviolet to beyond the red to infrared. You know that visible light is only one small part of electromagnetic spectrum.
Electromagnetic radiation has a dual nature of wave and particle. Among the photo biological phenomena caused due to light, many others by the particle nature. Please see section 1.1 Dual natures of matter and radiation, chemistry, class Xl-Xll. Electromagnetic radiation is form of energy. It propagates in the form discrete packets of energy called photons. The photons of each wavelength have different quanta of energy. The amount of energy of a particular wave depends upon its wavelength or frequency and can be expressed as:
E * u E= energy of wave
E= hu u= frequency of wave
h= Planck’s proportionality constant. It has a value of ~ 1.6 * 10 cal/ sec.
Look at the visible region of electromagnetic spectrum. The varying wavelengths of visible light are perceived different colours. Blue light has shorter wavelength, relatively higher frequency and high end high energy than red light which has higher wavelength relatively lower frequency lower energy. As indicated earlier, visible light has a spectrum of colours and each colour has a specific range of wavelengths. Scientists use special colour filters to obtain light of particular wavelength and study its effect on the various biological processes and behavior of the organisms.
The radiations which affect the photo biological phenomena lie between 300 nm and 900 nm.
You will see the region of ultra violet, visible and infrared regions and the photo biological phenomena caused by them. You may spend some time studying this figure. Before reading the next section we would like you to complete. You may find it useful to consult the glossary to remind yourself of some of these photo biological phenomena.
Solar Energy Input
We have mentioned earlier that the spectral distribution and the intensity of solar radiation incident on the earth’s surface are known. Of the enormous amount of energy that is radiation by the sun, only about one-half of 1 billionth of that amount is intercepted by the earth. Not all the solar radiation can penetrate the earth’s atmosphere; however, the amount of solar energy received at the top of atmosphere is constant. This energy is referred to as solar constant. It is defined as the rate at which solar radiation falls on a unit area is a plane surface, which is oriented perpendicular to the solar beam, when the earth is at mean distance from the sun. On an average the value of solar constant is 2 cal/cm/min.
As the solar radiation travels through the atmosphere it interacts with it and gets diminished in three different ways: by reflection, scattering and absorption. The result of interaction of 100 units of solar radiation with earth and atmosphere. About 30% of the total incoming solar radiation is reflected by clouds and a portion of it is back-scattered and lost in space. About 19% of it is directly absorbed by oxygen, ozone, water, ice crystals and suspended particles. This absorbed radiation is converted into heat energy and the air is warmed to some extent. The remaining 51% is absorbed or reflected by earth’s surface that is converted to heat. Thus a total of 70% of the radiation absorbed by earth and atmosphere is involved in the functioning of our biosphere.
The earth has a variety of surface—rough, smooth, ice-covered, or water-covered and areas with different types of vegetation. The amount of radiation absorbed or reflected depends upon the nature of surface features i. e. topography of the area. The percentage of reflectivity of the incident radiation in meteorology is called albedo.
Albedo of snow covered landscapes is higher than vegetated landscapes or water column. Freshly fallen snow typically has an albedo between 75 to 95%. Ocean waters have low albedo and therefore they appear darker than the adjacent continental land masses. Rough surface have low albedo than smooth surfaces. Also the light coloured surface reflects more than dark surfaces. Reflectivity also depends upon the angel of incident radiation. The surfaces that are less perpendicular to the sun’s rays are more reflective than surfaces that make almost a right angle with the incoming solar radiation.
We have learnt that earth and atmosphere receive solar radiation, absorb a part of it and get warmed up. We also know that during night earth cools down. So where does the energy of radiation absorbed by the earth go? Actually, the absorbed radiation in turn is continually re-radiation from the earth as heat in the form of infrared radiation and is sent off to outer space continually. If it had not re-radiation the air temperature would rise steadily day by day. Larger percentage of the infrared radiation is absorbed by the atmosphere, water vapours, carbon dioxide and some other gases and so they impede its loss to space. Part of it re-radiates back to earth surface and helps in maintaining global temperature. The value of solar radiation at sea level is approximately 1.5 gc/sq cm/min.
The spectral distribution of solar radiation is greatly altered as it passes through cloud cover, water and vegetation. Short ultraviolet radiation is abruptly reduced by ozone layer in the outer atmosphere. Visible light is broadly reduced by absorptions in the atmosphere. Spectral distribution of solar radiation above the atmosphere, after it passes through the atmosphere and reaches earth during clear weather, and under plan cover.
In this section we will learn about the instruments used for measuring solar energy, light intensity and duration of light.
Measurement of solar energy input
A variety of instruments have been designed to measure the energy of solar radiations of all wavelengths as well as of a particular range of wavelengths. Pyrenometer measures the energy of sunlight of short wavelengths, indirect sunlight and scattered sky light radiation. The receiver of radiation A has alternate black and white strips. These act as hot and cold thermocouple junctions respectively. This arrangement is enclosed in a spherical glass bulb which shields the receiving surface from disturbances by wind. But the glass container limits the wavelength response to the range of 280 nm to 3,000 nm.
When the receiver is placed in the sunlight the temperature of black strips, increases because it absorbs all radiations falling upon it. But the white strips remain cool as they reflect all incoming radiation. Thus if we connect a thermocouple between these strips, a thermo-emf develops as shown in the margin. Galvanometer shows a deflection indicating this emf. In this way the radiation absorbed by the receiver can be measured. Normally, the galvanometer is calibrated to give a nearly linear voltage response to incident radiation absorbed by the receiver.
Radiometer measures the flux of energy of all wavelengths received on a single surface of the receiver. There are also instruments that can measure the difference between the downward incident solar radiation and upward streams of reflected re-radiation and gives us the net value of radiation. This is called net radiometer. It has two exposed surfaces one upward facing and another downward facting. The receiver A is a long thin blackened parts of this strip are connected to a thermocouple.
Since the black receiver is not enclosed within a glass cover, it responds to all wavelengths. However, exposed receiver may not give us very accurate value because its surface may transfer heat to air by convection and conduction. Moreover, the moisture can also influence it. To avoid such disturbances the receiver is either ventilated at a constant rate or it is covered by suitable material which transparent to visible as well as infrared radiations.
At meteorological department of India, Pune, solar radiation is measured by a thermoelectric pyrenometer. The sensor is made up of blackened copper constant thermopile. When exposed it gives rise to a thermoelectric current proportional to the incident radiation. The current is fed to a continuously recording millivoltmeter.
Measurement of Light Intensity
How do you measure the intensity of a source of light falling on a surface? You know that light spreads out uniformly in all directions from a source. The amount of light shining on a unit area decreases with increasing distance. This decrease is equal to the square of the distance away from the source.
Light intensity is measured in lux. A lux is the amount of illumination shed on a square metre of curved surface, one metre from a standard candle. Previously, the unit foot candle was used. A foot candle is the amount illumination shed on a square foot candle.
Photometer measures the intensity of light. The metallic plate A is a photo which emits electrons from its surface when light of sufficient frequency impinges. The emission of electrons emerging from the irradiated surface constitute the current is measured by a sensitive ammeter which is calibrated to give the amount generated as the Value of light intensity in metre candle.
Duration of Light
Sunshine recorder measures the duration of sunshine. The recorder consists essentially of a glass sphere of about 10 cm in diameter mounted concentrically in a spherical metallic bowl which itself is mounted on a marble base. The inner surface of the bowl is flanged to take 3 sets of special cards for use at different periods of the year. A semi-circular brass bar supports the blow and sphere and has degrees of latitudes engraved on it. It can be moved and set to any latitude in its range. The rays of the sun when focused sharply on the card burn a trace on the card gradually. The length of the burn indicates the duration of sunshine, which is read off the hour marks printed on the cards.
Periodic Variations in Light—Diurnal and Seasonal
We know that rotation of the earth on its axis accounts for day-to-night variations in the amount of radiations falling at a given place and seasonal variations occur due to the orbiting of the earth around the sun. Since the earth’s equatorial plane is inclined to its orbit at an angel of 23.27’, the rays of the sun do not fall vertically on all parts of the earth. From March 22 nd to Sept. 22 nd, the North Pole is inclined towards the sun. So the most intense solar beam is focused on the North Pole the sun shines for 24 hours of the day. While on South Pole it is dark for six months and the southern hemisphere has winter season. The opposite situation exists on the poles from September 24 to March 20 when the northern hemisphere has winter season and southern hemisphere has summer season.
There is also horizontal variation in the distribution of radiation on the earth. Because the earth is nearly spherical in shape, parallel beam of incoming sunlight does not fall vertically on all parts of the earth. It strikes lower latitudes more directly than higher latitudes. Therefore, at higher latitudes the incident radiation falls obliquely on the surface, travels more through the atmosphere, and spreads over a greater area thus is less intense than the vertical beam falling on or near the equator.
Light and Distribution
We have mentioned in the beginning that the variation in the amount of light generally affects the global and local distribution of plants and animals. Light plays a great role in species composition and development of vegetation. We have already discussed the global variation of light intensity. Let us study the caused of variation in light climate in terrestrial and aquatic ecosystem. In order to provide a comprehensive idea of light climate of any locality, information on the following three aspects needs to be provided: i) Intensity or amount of light per unit area per unit time, ii) The quality or wavelength composition, and iii) Photoperiod or duration.
Significant local variation in the light in the terrestrial ecosystems results due to the interception of light by vegetation. In a forest, tall trees with fully expanded canopy receive maximum sunshine and absorbs a major portion of the incident light especially in the red and blue regions. The under shrub and herb layers receive only light filtered through the tree canopy from above. In a thick forest the light interception by the multistoreyed vegetation is very efficient and on the forest floor light intensity may be only 1 % of the incident solar radiation received at the top of the canopy.
Due to selective absorption, spectral light quality changes as it passes through the tree canopies. Yet, we find that some plant species are adapted to functioning in such low light intensities. On the basis of relative preference for natural growth in bright or diffused light the plants have been classified into two categories—sciophytes and helophytes. Some plants are more rigid in their preference for shade or bright light. These are termed as obligate sciophytes and obligate helophytes respectively. There are some helophytes that can also grow in shade but not so well. These are called facultative sciophytes. Similarly, the sciophytes that can also grow in bright light are called facultative helophytes.
Plants can survive only when the total energy harnessed in photosynthesis exceeds that used in respiration. The intensity of light at which energy harnessed through photosynthesis is just sufficient to meet the energy requirement of respiration is called light compensation point. In deep shade, under trees the amount of light is not enough to carry on photosynthesis to satisfy the immediate need of the plants. Therefore, they lose leaves and usually branches. The leaves in a tree canopy are arranged in a way so as to function above light compensation point.
So far we have discussed the influence of light on the distribution of species in terrestrial ecosystem. Let us see the distribution of light in aquatic ecosystems.
Vertical stratification of light intensity with increasing depth and accompanying changes in its spectral quality occur due to its limited penetration in water. Generally, less than 1% of sunlight can penetrate up to a depth of about 30 metre in a water body. The light penetration can be further reduced by suspended material in water like silt, clay, and solute content, plankton which significantly decrease the intensity of light and change the light quality. Often, plants such as duckweed which float freely on the surface of water can intercept the incoming light. Beside, pure water absorbs light at a rapid rate and causes profound changes in its spectral distribution. Red and blue wavelengths are mostly filtered out and the remaining predominantly greenish light penetrates to a greater depth.
We know that the amount of photosynthesis is directly related to the intensity of light and thus the light compensation point in water reaches at a certain depth which is called compensation depth. The zones above and below this depth are called photic zone and aphetic zones respectively.
A lake has three zones. The littoral zone includes all those areas where light penetrates to the lake bottom and where aquatic plants, aquatic animals and decomposers grow. The profundal zone includes areas too deep to be penetrated by light useful for photosynthesis. This area is below the compensation depth. The limnetic zone is open, sunlit water above the profundal zone. It extends outwards from the littoral zone. Tiny free floating plankton especially phytoplankton dominate in this area.
Light is an important factor in regulating the distribution pattern of marine organisms. The accompanying various depths to which light penetrates in the ocean.
Activity like breeding and migration in animals; flowering, seed germination in plants are regulated by the length of daily period of light and darkness. This behavioral phenomenon is known as photoperiodism. For example, plants such as radish, potatoes and spinach bloom when the light duration is more than 12hours/day. Such plants are called long day-plants.
Cereal, tobacco, dahelia and many other plants bloom when light duration is less than 12 hrs/day. These are called short day plants. Such responses show that plants have built-in mechanisms for measuring the duration of illumination and darkness and hence flower in specific seasons.
Similar photoperiodic responses are absorbed in animals. These may be diurnal, lunar or annual. Reproduction and migration in birds are such annual photoperiodic responses. From such responses it seem that distribution of some plants and animals may be restricted because the necessary photoperiodic stimulus is available only at certain latitudes.
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