Define gross primary productivity (GPP) and net primary productivity (NPP).

• A. GPP is total photosynthetic energy capture per unit area per time; NPP = GPP minus plant respiratory losses.
• B. GPP is energy stored after respiration; NPP is energy captured.
• C. GPP is energy consumed by consumers; NPP is energy produced by plants.
• D. GPP and NPP are the same.

Answer: (A)

GPP is the total energy captured by photosynthesis per unit area per time. It represents all the energy plants convert from sunlight into chemical energy, basically the gross amount of carbon fixed. Plants then use part of that energy for their own metabolism through respiration, which consumes energy. When you subtract this respiratory cost from the total captured energy, you get NPP—the portion that remains as new plant biomass and is available to herbivores and decomposers. In other words, GPP is the input from photosynthesis, and NPP is the net amount that contributes to growth after plant energy use is accounted for. Units are often expressed as grams of carbon per square meter per year or an equivalent energy unit. For example, if GPP is 1000 g C m^-2 year^-1 and respiration is 350 g C m^-2 year^-1, NPP would be 650 g C m^-2 year^-1.