Measuring Gross Primary Productivity (GPP) using Light and Dark Bottle methods: Sample Question

Question

The following question is taken from my Cliffnotes AP Biology 4th edition practice exam and it deals with the light and dark method for measuring gross primary productivity.

In a classroom investigation, students filled two bottles with pond water containing only photosynthesizing organisms. They used a dissolved oxygen (DO2) sensor to measure the amount of dissolved oxygen in each bottle. One bottle was put under a light. The second bottle was wrapped with aluminum foil to block all light and was put under the same light. AFter 24 hours, pond water DO2 in the two bottles was again measured. Average values for DO2 from all students are provided in the following table.

Calculate the gross primary productivity for the observed sample. Express your answer in mg fixed carbon/L/day to the nearest hundredth.

Okay, so I know that net primary production (NPP) is the difference between gross primary production (from photosynthesis) minus Autotrophic respiration (Ra):

NPP = GPP - Ra

I also understand that the light bottle is measuring NPP, because it allows for both photosynthesis and cellular respiration. The dark bottle measures Ra because it only allows for respiration. Therefore, GPP can be calculated by rearranging the formula:

GPP = NPP + Ra

Calculating the difference in the final and initial values for both bottles, I get the following:

Light: 0.15 mg O2 / L <-- NPP

Dark: -0.08 mg O2 / L <-- Ra

Next, I plugged these into the formula:

GPP = 0.15 - 0.08 = 0.07 mg O2 / L

Using the conversion formulas provided on the College Board's AP Bio Appendix (essentially just multiplying the mg O2 / L value by 0.374128 to get mg Carbon fixed / L), I get the following:

0.07 mg O2 / L * 0.374128 = 0.02618896 mg C fixed / L

But according to my review book's solution, I was supposed to add 0.08, not subtract it. This seems a bit strange to me. Is this a mistake in the book or am I not understanding something?

Answer 1

The dynamics are:

Light bottle: GPP - Ra = 0.15
Dark bottle : Ra = 0.08

Then GPP = 0.15 + 0.08.

The amount of O2 used in the respiration is 0.08 mg O2 / L, not -0.08 mg O2 / L.

Answer 2

If you think about what is going on in the light bottle, this should make more sense. Oxygen is being produced by photosynthesis, but respiration is using up the oxygen being produced. Therefore, the total amount is NPP. So if you want to find out how much oxygen was originally created through photosynthesis (GPP), you need to put those losses due to cellular respiration back into the light bottle (in other words, add the R amount to the light bottle amount).