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