AP® Environmental Science · Unit 1 · Topic 1.8
Primary Productivity
Primary productivity is the rate at which plants make new biomass through photosynthesis. Net primary productivity is what's left after the plant subtracts its own respiration costs.
Plants capture light energy and build sugars through photosynthesis. The total amount of chemical energy they create per unit time is gross primary productivity, GPP. But plants are alive — they have to run their own cells, which costs energy. They burn some of that captured energy in respiration (R) just to survive. What's left over, the energy actually banked as new growth — new leaves, stems, roots — is net primary productivity: NPP = GPP - R. NPP is what's available to feed everything else in the ecosystem, from insects to apex predators.
Productivity is a rate, measured per unit area per unit time, like kcal per square meter per year. That's different from standing biomass, which is just a snapshot of how much plant material exists right now. A rainforest builds huge amounts of tissue per year because warmth, sunlight, and rainfall keep photosynthesis running fast all year. A tundra has short growing seasons and cold temperatures, so its rate of new growth stays low even though some tundra plants can persist for years. Fast turnover can beat large standing stock: a thin layer of phytoplankton can out-produce a dense forest per year, because the plankton reproduce and die rapidly, cycling through many generations while the forest adds biomass more slowly.
This sets up everything after it in the unit. NPP is the energy budget that trophic levels draw from, and only about ten percent of it transfers to the next level up, which is why energy pyramids shrink so fast. In water, productivity depends on light penetration — it peaks near the surface and drops toward zero by about 30 meters, once sunlight can no longer drive photosynthesis faster than respiration burns energy.
Key terms
- Gross primary productivity (GPP)
- The total rate at which plants convert light energy into chemical energy through photosynthesis.
- Net primary productivity (NPP)
- GPP minus the energy plants use for their own respiration; the energy available to the rest of the food web.
- Standing biomass
- The total mass of living plant material present in an area at one point in time.
- Respiration (R)
- The process by which plants burn stored chemical energy to power their own life functions.
Watch it explained
- How it worksHow does gross primary productivity split into what the plant burns and what it keeps?
- Worked exampleFind the net primary productivity when GPP is 1000 kcal per square meter and respiration is 400.
- Common mistakeWhy isn't the biggest standing biomass the same thing as the highest productivity?
- Going furtherWhat happens to primary productivity in a lake as you go from the surface down to 30 meters?
Check yourself
End of topic.