Practice question

Net primary productivity is the rate at which producers accumulate new organic matter after meeting their own respiratory costs. Gross primary productivity represents all carbon fixed or energy captured by autotrophs. Autotrophic respiration consumes part of that material to support cellular maintenance, active transport, tissue construction, and repair. The balance is therefore NPP = GPP − R, where R in this formula means respiration by primary producers, often written Ra. NPP supplies plant growth and reproduction and is the energy potentially available to herbivores and decomposers. Adding respiration would count respired carbon as retained production, while reversing the subtraction could yield a biologically meaningless negative value. Division by respiration is a ratio, not a production rate. This equation must also be distinguished from net ecosystem production, which subtracts both autotrophic and heterotrophic respiration from GPP. The carbon-balance mechanism explains the sign: photosynthesis adds organic carbon to producer biomass, whereas respiration oxidizes some organic carbon and returns carbon dioxide, leaving the difference as net production.