Net and gross primary productivity?

Question

a question on my ecology review sheet is :"Describe a method to find the net and gross primary productivity in a freshwater pond over a 24 hour period." However, the book never mentioned actual methods of finding them. any ideas?

Answer 1

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