Which of the following defines Gross Primary Productivity (GPP)?
Gross primary productivity, or GPP, is the total rate at which primary producers capture energy and fix inorganic carbon before subtracting their own respiratory losses. In photosynthetic ecosystems, it is commonly expressed as carbon fixed per unit area per unit time and corresponds broadly to total photosynthesis at the ecosystem scale. Plants and algae use part of this fixed carbon in cellular respiration to power maintenance, ion transport, repair, and growth. The remainder is net primary productivity, so NPP equals GPP minus autotrophic respiration. NPP is the new producer biomass available for growth, reproduction, herbivores, and detrital pathways. Energy transferred to the next trophic level is smaller still because consumers do not eat or assimilate all production. Carbon used by consumers is therefore not GPP. Measurements may use gas exchange, oxygen change, eddy covariance, remote sensing, or light-dark bottle methods, each with assumptions. The word “gross” is crucial: it means the entire photosynthetic capture before the producers’ metabolic costs are deducted.
Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 17