Measurement Productivity of Ecological Factors (complete information)

Productivity may be measured during any reasonable period of time. Beause of essential metabolic differences between day and night, however, the 24-hour day is the smallest practicable unit.

Similarly, because of seasonal changes in the environment and in community populations, the measurement of annual production is probably most useful. Since primary production is basic and concerns the capture of energy by plants, it will be considered first.

Primary Production

Various methods are employed for measuring primary productivity, each procedure having certain advantages and disadvantages. Further work in evaluating and improving these methods or developing ones is desirable.

A common procedure for analyzing aquatic habitats, dating back to Gaarder and Gran (1927), is to suspend during daylight hours equal samples of green phyto-plankton, ordinary in separable mixed with bacteria and zooplankton, in both transparent and blackened bottles at the same depth at which obtained, Photo-synthesis of course does not occur in the blackened bottle, and there is loss of oxygen, resulting from respiration, R, and decomposition, E + D + W. In the transparent bottle, photosynthesis occurs in addition to respiration and decomposition, bringing a production of carbohydrates.

There will either be an increase in oxygen concentration, or the loss of oxygen will not be as great as in the blackened bottle. The difference in the final oxygen content of the two bottles will be a measure of gross production: I_n.

If the oxygen content of the water is measured at the beginning of the experiment, the loss of oxygen in the blackened bottle subtracted from the difference in oxygen content of the two bottles at the end of the experiment will represent the net productivity.

This net productivity may also be determined from the difference in the oxygen content of the transparent bottle between the beginning and the end. The obtain net production for an entire daily cycle, the consumption of oxygen for respiration and decomposition over 24 hours must be substracted from the groes photosynthetic output during daylight hours.

One may use the amount of carbon dioxide absorbed during a period of times as a measure of photosynthesis if correction is made for the carbon dioxide given off in respiration and decomposition. Changes in the amount of C0₂ in the water may be calculated from the differences in pH of the hydrogen-ion concentration. As an illustration of the use of radioisotopes, net production during the daylight hours may be measured by introducing a known amount of 11C0₂ into a volume of water where the amount of carbon dioxide already present is known. The amount of 11C absorbed by the phytoplankton can be accurately determined by use of counters applied to phytoplankton collected and dried at the end of the period. Then the proportion of the radioactive carbon absorbed to the amount into introduced can be applied to the total CO_z initially present to get the total amount absorbed.

Since nitrogen and phosphrous are metabolized more rapidly by plants in the manufacture of food during the growing season than they are regenerated from decomposing material, the rate and extent of the depletion of nitrates and phosphates is freely circulating bodies of water or in the epilimnion of stratified lakes serve as an index of an amount or organic matter produced.

The rate of accumulation and regeneration of these substances is the hypolimnion from the dead organisms that sinks into it may also be used to get an approximation of primary production. These measurements are not exact since they do not account for the repeated regeneration and neutrilization of the substances in the photic zone during the season or their transference to and storage in the bodies of animals.

The rate of photosynthesis varies in relation to the amount of chlorophyll present and to light intensity. The amount of chlorophyll in the standing crop phytoplankton may be determined photometrically for all depths and calculated in terms of unit area of surface.

Determination of the rates of photosynthesis and oxygen use in streams and ponds may be made by direct measurement of changes in the concentration of oxygen and carbon dioxide in the water as between day and night.

Because of photosynthesis, there is no increase in oxygen concentration during the day time. At night the oxygen loss gives a measure of the rate of respiration and decomposition and this presumably remains the same throughout the 24-hour daily cycle. Adding the average hourly night loss to the average daily gain during the day and multiplying by the hours of daylight gives the total gross production for the 24-hours day.

To obtain the net production for the entire day, the hourly loss at night must be multiplied by 24 and subtracted from the total gross production. Corrections need to be made, however, for possible diffusion of oxygen from air into the water, particular at night when oxygen concentration in the water is lowered, and diffusion of the water during the day, if super saturation occurs. Additional correction will also be necessary for important of oxygen form groundwater and surface drainage and export of oxygen and carbon dioxide downstream by swift currents. I

Secondary Production

When an animal species is represented by a low overwintering population, or an immature stage, the maximum biomass obtained in each generation this the approximate not production is for that generation. However, it does not account for continued reproduction and growth of individuals after the maximum biomass of the population is attained, nor does it account for excreta natural deaths, or the kill of predators.

If the population of the species is maintained at a more or less uniform level throughout the year, the mean biomass time the number of generations gives the net production, again with the exception of the factors mentioned above. Linderman considered the phytoplankton turnover, or the production of a new generation, to occur every week from May to September and every 2 weeks through the rest of the year, the zooplankton to replace itself bi-weekly through the year, Chaoborus to have three generations per year; midge flies, two; and various acquatic bettles and bugs, one generation per year. Juday (1940) estimated that the mean standing crop of both phytoplankton and zooplankton replaced itself every 2 weeks throughout the year. To obtain gross productivity, the respiratory rates of these animals must also be measured.

Although measurement of energy flow through the total phytoplankton, zooplankton, and soil organisms is often practical and sufficient, measurements are required on individual species of large size |^{n e} higher tropic levels before the total utilization of energy by the ^tropic level can be determined.

Light

Light is a complex physical factor, influencing the plants and animals to great extent. It supplies energy to the photosynthetic organisms and for so many other activities performed by animals and plants.

Intensity of light

The intensity of the light reaching the earth’s surface is dependent mainly upon two factors: (i) The angle of the incidence of the light rays, (ii) The actual amount of absorption of rays by various atmospheric layers.

The light rays having wavelength shorter than 2870A are absorbed by the gases in the earth’s atmosphere. Oxygen also absorbs ultraviolet radiation from the higher strata atmosphere forming ozone layer, so called ozone umbrella. The greatest intensity of sunlight occurs at positions on the earth’s surface and at times when sun is mostly overhead.

At high altitudes, the intensity of light is correspondingly reduced. For example, at 50°N latitudes, during the period of equinox in March and September (having 12 hours day) the intensity of sunlight is about one-half of what it is at equator. Besides latitudinal effect, other factors like’ moisture, clouds, dust in the atmosphere have profound and irregular effect in reducing the intensity of sunlight. Different forest communities develop variably according to the intensity of sunlight and likewise animals as well.

(a) In terrestrial ecosystem

The intensity of light have far-reaching effect as exemplified by the phenomenon-photoperiodism. In it, plants and also animals show response by exposure to specific duration of light (photoperiod). In the dense forest area, the intensity of the light is extremely reduced to minimum to the forest floor due to luxuriant growth of the trees.

(b) In aquatic ecosystem

All the light does not penetrate the deeper layers of water, amongst it, about 10% of the light falling on the surface of the water is reflected back. It is done by the inorganic materials suspended in water layers. As depth increases, the intensity of the light is likewise decreased. In the oceans, region from surface up to 80 metres depth is called euphotic zone (carrying out photosynthetic activities), the zone between 80 to 200 metres is dysphotic zone (where light is highly modified having violet and ultraviolet radiation) and last aphotic zone characterized by absence of photosynthesis and dominance of darkness.

Duration and Amount of Light

The total amount of light received by the organisms is determined both by its intensity and the duration of the period of irradiation. On the equator, day is 12 hours long but in the temperature region, the day grows longer as spring progresses. This effect is accelerated at higher latitudes and day becomes 24 hrs, long in the polar region during the summer. Usually, at moderately high latitudes, the increase in length of the day summer period has more effect on the total amount of the light received per day than the reductions in solar intensity due to greater angle of incidence.

Light in Water

Pure water absorbs light at a very rapid rate as compared to air and cause profound change in spectral distribution. In natural waters, suspended particles and dissolved materials cause a reduction in transparency and alteration in spectral composition. The suspended living organisms in water increase the extinction of light and thus modify their own environment. In fresh water of ponds, lakes, etc., photoplankton sometimes produces a noticeable reduction of light. A thick layer of algae in a pond may reduce the light supply to such a great extent that other plants in water beneath the algae also do not grow. In temperate and coastal seas, fine particles present tend to absorb or scatter blue component of light more strongly than occurs in pure water.