In ecosystem carbon budgeting, which statement best describes GPP?

• A. Net carbon fixed after respiratory losses.
• B. Carbon stored in soils and oceans.
• C. Total carbon fixed by photosynthesis.
• D. Carbon lost as heat during metabolism.

Answer: (C)

GPP is the total carbon fixed by photosynthesis in an ecosystem over a given time. It represents the gross influx of carbon into the plant system as light energy is converted into organic matter, without subtracting any losses from respiration. This is different from net primary production, which subtracts plant respiration to reflect the carbon actually remaining in plant biomass. Carbon stored in soils and oceans are pools, not the flux of carbon fixed by photosynthesis, and carbon lost as heat during metabolism is part of respiration that reduces what remains after GPP when you move to NPP. So the best description is that GPP equals the total carbon fixed by photosynthesis.