Practice question

Gross primary productivity is the total rate of carbon dioxide fixation by autotrophs before accounting for their respiration. Photosynthetic producers use light energy to reduce CO2 into organic compounds, while chemoautotrophs use chemical energy for carbon fixation. Some of the fixed carbon is oxidized in autotrophic respiration, releasing CO2 and supplying energy for maintenance and biosynthesis. The remainder is net primary productivity, expressed as NPP = GPP − Ra. Total ecosystem respiration, RE, includes both autotrophic respiration and heterotrophic respiration by consumers and decomposers. RH specifically denotes heterotrophic respiration. Consequently, neither NPP nor respiration terms represent the full amount initially fixed. Rates are normally reported per unit area or volume per unit time, such as grams of carbon per square metre per year. GPP may be estimated through oxygen changes, CO2 exchange, isotopes, or ecosystem models. The “gross” label identifies carbon at the moment of biological capture, before any respiratory subtraction, which directly matches the stated total.