Primary Production by Autotrophs | Ecosystem

The following points highlight the top four methods for measurement of primary production by Autotrophs. The methods are: 1. Harvest Method 2. Carbon Dioxide Assimilation Method 3. Oxygen Production Method 4. Radio-Isotope Method.

Method # 1. Harvest Method:

The common and at the same time oldest method of measuring primary pro­duction is the Harvest method. In this method the plants grown on a particular field are clipped at ground level and their weight is taken. This is done at periodic intervals.

After drying to constant weight the harvest is then ex­pressed in terms of Biomass or mass per unit area per unit time (the unit of mass= gram, unit of area=meter, and unit of time =year). And now if the exact caloric content of the material is known the biomass can easily be converted and ex­pressed in terms of energy.

Though widely used, the harvest me­thod has a number of limitations. In this method the roots of the plant are not taken into consideration and as a result considerable error is introduced.

The me­thod fails to account for the amount of the material consumed by herbivores and also the energy utilized by the autotroph in its own metabolic activity. However it can­not be denied that harvest method can throw some light on the gross estimate or gross production.

Method # 2. Carbon Dioxide Assimilation Method:

The incorporation of CO₂ in photo­synthesis or its loss in respiration under natural condition can be determined by using infra-red gas analyzer. With the help of the analyzer it is possible to measure the amount of CO₂ entering or leaving an air­tight chamber of known area or volume.

In this method it is assumed that any CO₂ taken from the incoming air has been in­corporated into organic matter during the period of observation thereby indicating both the amount and rate of photosyn­thesis.

As there had been respiratory acti­vity during the period of observation the use of infra-red gas analyzer alone cannot be full proof. So a comparable study is established using a light-tight container (where no photosynthesis but respiration alone will take place).

The amount of CO₂ released from this chamber in a given period will be an index of the amount and the rate of respiratory activity. This value when added to that obtained in the light exposed chamber will give the total or gross production of the system. The use of infra-red gas analyzer is not feasible in an aquatic ecosystem.

Method # 3. Oxygen Production Method:

In aquatic ecosystem the usual proce­dure is the measurement of oxygen evolu­tion by the aquatic plants. In this method samples of water containing the autotrophs are collected from a given depth of pond, lake or ocean. The sample is then distri­buted into smaller and paired sample bottles—the capacity of each bottle being 125-300 ml.

One of the paired bottles is made up of light-transmitting material, permitting photosynthesis to take place while the other is made up of light-tight material to preclude photosynthesis but not respiration.

The bottles are then sus­pended usually at the same depth from which samples were collected and are allowed to incubate for a given period of time. At the end of this time the oxygen content of the light-transmitting bottle is determined either by standard chemical titration or by electromagnetic devices.

The value thus obtained is then plugged into the photosynthetic equation and the amount and rate of photosynthesis is determined. The normal consumption of oxygen in respiration is calculated from the light-tight bottle. By combining this amount with the value obtained in the light bottle, the total or gross production is approximated.

Method # 4. Radio-Isotope Method:

Another method to study the primary production of aquatic autotrophs is the radio-isotope method. The method is similar to the light-dark paired bottle tech­nique for ‘measuring oxygen. But here radio-active form of carbon known as carbon-14 (¹⁴C) is the extra material used.

Water samples containing autotrophs are collected from a desired level of the pond, lake or ocean and are then distributed into light-dark paired sample bottles. A known quantity of carbon-14 (¹⁴C) in the form of a bicarbonate (NaH¹⁴G0₃) is then intro­duced into the bottles and the bottles are resuspended at the same level of water from which the samples were taken.

The bottles are kept there for six hours. During the period of exposure the stable carbon present as CO₂ or HCO₃ as well as the unstable ¹⁴C are assimilated into carbohy­drate and incorporated into the proto­plasm of the autotrophs.

At the end of the incubation period the samples are filtered to capture the stable and radio-active car­bon that has been incorporated biologi­cally. After drying the filters are placed in a counting chamber to determine their level of radio-activity or in other words, the amount of radio-active carbohydrate produced.

Calculations are then made from photosynthetic formula depending upon the assumption that the uptake of ¹⁴C is proportional to stable carbon.