Question

Suppose you get 50 calories from the broccoli and 200 from the chicken.     Assume that 50% of the chicken’s energy came from the grain and 50% from the grubs.

What was the likely net primary productivity of the producers(s) that gave you the energy in the broccoli?    In the chicken?

Answer 1

Net primary productivity (NPP) of the producers (i.e. plants) can be defined as the gross primary production (GPP) by the plants eliminating the loss due to respiration or mass production. NPP is the total biomass available for the consumers.

hence, NPP = GPP - plant respiration (R).

so, Plant cellular respiration involves this reaction of C6H12O2 --> 6 CO2 + 6 H2O.

Here, the radioactive form of carbon, C14 (14CO2), is generally used for monitoring carbon uptake and fixation.

Also conversion of the results between oxygen and carbon methods can be used by multiplying the oxygen values by 0.375 to put them into carbon equivalents (the factor comes from differences in atomic mass).

So, assume R = 42 mg/L/hr

Now,  1 calorie = 4.184 Joules

Therefore, Energy obtained from broccoli i.e. GPP = 50 * 4.184 = 209.2 J= 0.2 KJ

So, NPP = GPP - R = 209.2 - 42 = 167.2 J (Broccoli)

As, the net productivity is found at the level of the producer so in chickens, It is calculated from the 50% energy received from the grain.

Energy obtained from grains in chicken (GPP) = 200 * 50/100 = 100 calories = 100 * 4.184 = 418.4 J = 0.4 KJ

Hence, NPP = GPP - R = 418.4 - 42 = 376.4 J (Chicken)

Note that these values are obtained by comparative quantification and will vary according to the change in value of R.