Q.What is primary productivity ?
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Start your 14-day free trial to unlock the full solution →Primary productivity is the rate of biomass production by plants via photosynthesis; it varies with species and environment, and Net Primary Productivity = Gross Primary Productivity − respiratory loss.
Definition
Primary productivity is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis, expressed as g/m²/yr or kcal/m²/yr.
Gross vs Net Primary Productivity
- Gross Primary Productivity (GPP) — the total rate of production of organic matter (biomass) during photosynthesis.
- Net Primary Productivity (NPP) — the biomass left after the plants themselves use up part of GPP in respiration (R): NPP = GPP − R. NPP is the biomass actually available to heterotrophs (herbivores and decomposers) at the next trophic level.
(i) Why productivity varies in different ecosystems
Primary productivity depends on the plant species occupying an area and on several environmental factors:
- availability and duration of sunlight,
- temperature,
- availability of water (precipitation),
- availability of nutrients (soil fertility, minerals).
Different ecosystems differ in these factors — e.g. tropical rain forests, with abundant light, warmth, water and nutrients, are highly productive, while deserts and the open ocean, where water/nutrients are scarce, are far less productive. Even under similar climate, productivity differs with the plant species present and their photosynthetic efficiency.
(ii) Relationship between GPP and NPP
NPP = GPP − R. Gross primary productivity is the total organic matter fixed by photosynthesis; part of this is consumed by the plant's own respiration (R) for its metabolic needs. What remains after this respiratory loss is the net primary productivity — the biomass surplus actually available for consumption by the next trophic level.
*(OR alternative — function of a reservoir + carbon cycle: In a biogeochemical/nutrient cycle, elements move between the abiotic environment (the reservoir pool, e.g. atmosphere, soil, water, rock) and living organisms (the cycling/biotic pool) and back. The reservoir is the large storage pool, usually in the atmosphere or Earth's crust, that supplies nutrients to, and receives them back from, the living world, buffering the cycle over long time spans.
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