Net Primary Productivity Calculator (NPP from GPP, Biomass or Light–Dark Bottles)
Calculate net primary productivity (NPP) from GPP and autotrophic respiration, GPP and carbon-use efficiency, biomass change with litterfall, or light and dark bottle oxygen readings. Gives g C/m²/yr, dry matter, CO₂ fixed, O₂ released, t C/ha, area totals, NEP and the closest ecosystem type.
How it works
NPP = GPP − Ra · NPP = GPP × CUE · NPP (harvest) = (B_end − B_start + litterfall + herbivory) ÷ years × carbon fraction · NEP = NPP − Rh · CO₂ = C × 44/12 · Bottles: GPP = (light − dark) × 375 ÷ PQ, R = (initial − dark) × 375 × RQ, in mg C/m³ per incubation
Gross primary productivity (GPP) is all the carbon that plants or algae fix by photosynthesis. The plants burn part of it straight away in their own respiration (autotrophic respiration, Ra); what is left is net primary productivity (NPP), the carbon that builds new leaves, wood and roots and feeds every consumer. The share of GPP that ends up as NPP is the carbon-use efficiency, often near 50%, so if you only know GPP you can estimate NPP with that ratio. In the field NPP is often measured by the harvest method: weigh the living biomass at two dates, then add back whatever was produced but lost in between (litterfall and grazing), otherwise the result is too low. Dry plant matter is roughly 47% carbon (the IPCC default), so dry mass is multiplied by the carbon fraction. Each gram of carbon corresponds to 44/12 = 3.67 g of CO₂ taken from the air and roughly 2.67 g of O₂ released. Subtracting heterotrophic respiration (soil microbes and animals) gives net ecosystem productivity (NEP), which says whether the site is a carbon sink or source. For lakes and the sea, the light and dark bottle method measures oxygen: the dark bottle shows respiration only, the light bottle shows photosynthesis minus respiration, and their difference is gross production. Oxygen is converted to carbon with 12/32 = 0.375 and the photosynthetic quotient (about 1.25 for phytoplankton). The ecosystem table uses the Whittaker and Likens means, which are in dry matter; the carbon column applies your carbon fraction.
Worked example
With the defaults, GPP is 2,500 g C/m²/yr and autotrophic respiration 1,300 g C/m²/yr. NPP = 2,500 − 1,300 = 1,200 g C/m²/yr, so the carbon-use efficiency is 1,200 ÷ 2,500 = 48%. At 47% carbon that is 1,200 ÷ 0.47 = 2,553 g dry matter/m²/yr, closest to a tropical rain forest (mean about 2,200). The CO₂ fixed is 1,200 × 44/12 = 4,397 g CO₂/m²/yr and about 3,197 g of O₂ is released. Per hectare that is 12.00 t C/ha/yr or 44.0 t CO₂/ha/yr (3.29 g C/m² per day), and the 10 ha area fixes 120.0 t C (439.7 t CO₂) a year.
Sources: Field et al. (1998), global NPP 104.9 Pg C/yr — BioNumbers · Biome productivity table (Whittaker & Likens means) — Portland State University · IPCC 2006 Guidelines, Vol. 4 Ch. 4: carbon fraction 0.47 · Light and dark bottle productivity calculations — Kennesaw State University
Frequently asked questions
What is the difference between GPP and NPP?
GPP is the total carbon fixed by photosynthesis. NPP is what remains after the plants subtract their own respiration, so NPP is always smaller than GPP, typically about half of it.
What is the global average NPP?
Field and colleagues (1998) estimated about 105 Pg of carbon a year for the whole biosphere, split almost evenly between land (about 56 Pg) and oceans (about 49 Pg). That works out to roughly 430 g C/m²/yr on vegetated land and about 140 g C/m²/yr in the oceans.
How do I convert NPP from dry matter to carbon or CO₂?
Multiply dry matter by the carbon fraction (0.47 by IPCC default, 0.45 is also common) to get grams of carbon, then multiply carbon by 44/12 = 3.67 for grams of CO₂.
What is net ecosystem productivity (NEP)?
NEP = NPP − heterotrophic respiration, the carbon released by decomposers and animals. A positive NEP means the ecosystem is storing carbon (a sink); a negative NEP means it is losing carbon (a source).
How does the light and dark bottle method work?
Identical bottles of water are filled, one sealed in the dark and one left in the light at the sampling depth. After a few hours, initial − dark oxygen is respiration, light − initial is net production and light − dark is gross production.
Why does the harvest method need litterfall and herbivory?
Leaves that fell and plant matter that was eaten were produced during the period but are no longer there to weigh. Without adding them back, NPP is underestimated, sometimes by a third or more in forests.
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