Methods by which the Primary Production of an Ecosystem can be measured | Essay

The primary production of an ecosystem can be measured by following methods:

1. O₂ Productions or Light and Dark Bottle Method:

The light-and-dark-bottle method, commonly used in aquatic environments, is based on the assumption that the amount of oxy­gen produced is proportional to gross production, since one mole­cule of oxygen is produced for each atom of carbon fixed.

Two bottles containing a given concentration of phytoplankton are sus­pended at the level from which the samples were obtained.

One bottle is black to exclude light; the other is clear. In the light bottle a quantity of oxygen proportional to the total organic matter fixed is produced by photosynthesis. At same time the phytoplankton is using some of the oxygen for respiration. Thus the amount of oxygen left is proportional to the amount of fixed organic matter remaining after respiration or net production. In the dark bottle, oxygen is being utilized but is not being produced.

Thus the quantity of oxygen utilized, obtained by subtracting the amount of oxygen left at the end of the tune (usually 24 hours) from the quantity at the start, gives a measure of respiration. The amount of oxygen in the light bottle added to the amount used in the dark bottle provides an estimate of total photosynthesis or gross production.

A modification of this method involves the whole aquatic eco­system, which becomes the light bottle in day-time and dark bottle in night-time. The oxygen content of the water is taken every 2 to 3 hours during a 24-hour period.

The rise and fall of the oxygen during the day and night is plotted as a diurnal curve. For obtain­ing a correct estimate for the oxygen production of plants, the oxy­gen exchanged between air and water and between the water and bottom must be estimated and deducted.

2. Carbon Dioxide Assimilation:

This method is suitable for estimation of production of trees trial ecosystems. It involves the measurement of the uptake of carbon dioxide and its release in respiration.

In this method a sample of community, which may be twig and its leaves, a seg­ment of the tree stem, the ground cover and soil surface, or even a portion of total community, is enclosed in a plastic tent. Air is drawn through the enclosure, and the carbon dioxide concentration of the incoming and outgoing air is measured with an infrared gas analyzer.

The assumption is that any carbon dioxide removed from the incoming air has been incorporated into organic matter. A similar sample is enclosed within a dark bag. The amount of carbon dioxide produced in the dark bag is a measure of respira­tion. In the light bag, the quantity of carbon dioxide would be equivalent to photosynthesis minus respiration. The two results added together indicate gross production.

3. Chlorophyll Method:

An estimate of the production of some ecosystems (e.g., aquatic ecosystems) can be obtained from chlorophyll and light data. This method involves the determination of chlorophyll-a content of the plant per gram or per square metre, which under reasonably favourable conditions is the same.

Because the quan­tity of chlorophyll in aquatic (and terrestrial) communities tends to increase or decrease with the amount of photosynthesis (which varies at different light intensities), the chlorophyll per square metre indicates the food manufacturing potential at the time. Chlorophyll content in planting is accounted with the help of colori­meter (Fig. 4.9).

4. Harvest Method:

This method is widely used in terrestrial ecosystems for esti­mating production. It is most useful for ecosystems for estimating production of cultivated land and range, where production starts from zero at seeding or planting time, becomes maximum at har­vest, and is subject to minimal utilization.

In brief, harvest method involves the clipping or removal of vegetation at periodic intervals, drying to a constant weight, and expressing that weight as biomass in grams per square metre per year. The caloric value of the material can also be determined by bomb calorimeter.

Then the biomass is converted to calories and the harvested material expressed as kilocalories per square metre per year. To be accurate, plant material must be sampled through­out the growing season and the contribution of each individual species determined.

Because plants of different age, size, and species make up the forest and shrub ecosystems, a modified harvest method known as dimension analysis is used. This involves the measurement of height, the diameter at breast height (DBH), and the diameter growth rate of the trees in a sample plot.

A set of sample trees is cut weighed, and measured, usually at the end of growing season. The height to the top of the tree, DBH, depth and diameter of crown and other parameters are taken. Total weight, both fresh and dry, of the leaves and branches are determined, as is the weight of the trunk and limbs. Roots are excavated and weighed.

By various calculations the net annual production of wood, bark, leaves, twigs, roots, flowers, and fruits is obtained. From this information the biomass and production of the trees in the sample unit are estimated and then summed for the whole forest.

5. Radioactive Tracers Method:

The most recent method of determining production involves the measurement of the rate of up take of radioactive carbon (¹⁴C) by plants. It is a most sensitive method to measure net photo­synthesis under field conditions.

This method involves the- addition of a quantity of radioactive carbon as a carbonate (¹⁴CO_s) to a sample of water containing its natural phytoplankton population. After a short period of time, to allow photosynthesis to take place, the plankton material is strained from the water, washed, and dried. Then radioactive counts are taken, and from them calculations are made to estimate the amount of carbon dioxide fixed in photosynthesis.

In a similar way, radioactive phosphorus (³²P) has been used for determining the direction of energy flow in an ecosystem.