Question

In order to estimate the gross primary productivity (GPP) of a pond containing phytoplankton and zooplankton, a student collected water samples from the pond in two bottles labelled X and Y. The dissolved oxygen (DO) of the samples immediately after collection was $8$mg $m{l}^{-1}$. The DO content when measured after $24$ hours of incubation was as follows:
Bottle X: $2$ mg $m{l}^{-1}$ after incubation in dark
Bottle Y: $18$ mg $m{l}^{-1}$ after incubation in light
GPP of the pond is?

• A. $6$
• B. $10$
• C. $16$
• D. $20$

Answer 1

Answer: (C)

$16$

GPP or Gross Primary productivity is referred as the total rate of photosynthesis including the organic matter used up in respiration during the measurement period.

When the bottles X and Y containing pond water are kept in dark and light respectively, there is decline in Dissolved oxygen (DO) in bottle X because phytoplankton and zooplankton will not be able to photosynthesise and the oxygen will be used up in respiration whereas, there is increase in DO in bottle Y because phytoplankton and zooplankton will photosynthesise in presence of light resulting in production of oxygen.

So, from the information given,

Respiratory activity can be calculated as : Initial DO - DO after incubation in dark = 8 - 2= 6 mg/ml

Net photosynthetic activity can be calculated as : DO after incubation in light - Initial DO = 18 - 8= 10 mg/ml

GPP = Net photosynthetic activity + Respiratory activity = 10 + 6 = 16.

So, the correct answer is C.