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8. Measurement of productivity

Several methods are used in measurement of productivity or rate of production. (see separate Practicals Handout on the topic).

Harvest method: This is the simplest and measuring the productivity of a water body such as fish pond by hervest at the end of the season. The productivity given is secondary productivity and indicates net productivity and also quite often fish production given is in net weight giving productivity value.

8.1 Oxygen measurement method:

Primary productivity can be measured from the amount of oxygen consumed by a volume of water in a fixed period of time; water for which productivity is to be determined is enclosed in sealed white and dark bottles (bottle painted dark so light would not enter). Do (dissolved oxygen) measurement of water is made at the beginning of the immersion period. The two bottles are then immersed in the water body concerned at the level from which the water is taken. The phytoplankton and other elements in the water produce oxygen in the water bottle, but some oxygen disappears due to respiration. The latter is measured from the readings of dark bottle, where only respiration takes place. Thus from the oxygen produced by photosynthesis of enclosed organism (representing a sample of the water body) can be known. However this oxygen production indicates net primary productivity only. From the DO difference in dark botfle oxygen consumed by the enclosed organisms can be obtained and when this respiration value is added to the oxygen production in the white bottle, a value for gross primary productivity is obtained.

8.2 Diel method:

Estimates of primary productivity can also be made from diel changes in oxygen, considering the day as the light bottle and night as the dark bottle. The increase in DO in the day time is net primary production and the decrease in the night is half the diel respiration. This can be added on to the day-time gain to obtain daily gross photosynthesis. This volume should normally be corrected for the loss or gain in oxygen due to concentration gradient over the day.

8.3 C14 method:

The most accurate method for determining productivity is the method of using radioactive carbon (C14) added as carbonate. Labelled carbonate is added into a bottle containing water with the phytoplankton and other organisms and after a short period of time the plankton is separated, dried and planchetted and the radioactive carbon fixed can be measured from the radioactive counts made. The productivity measured thus is net primary productivity as the carbon fixed in the tissues only are measured here.

In selecting a water body for aquaculture measurement of primary productivity and estimation of potential yield would be of great assistance in planning the culture activity. This would be specially done while evaluating water bodies (natural or man-made) for stocking(in extensive culture) and also for cage and enclosure culture.

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