Productivity

Every ecosystem needs a steady supply of solar energy to keep working.

Primary production is the organic matter (biomass) that plants make by photosynthesis on a unit of area over a stated period. It is expressed as weight (g m⁻²) or energy (kcal m⁻²).

Productivity is the rate of biomass production, expressed as g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹, so that different ecosystems can be compared.

Gross primary productivity (GPP) is the rate at which organic matter is made in photosynthesis. Plants use a considerable part of it in respiration.

Net primary productivity: GPP − R = NPP, where R is the respiration loss. NPP is the biomass available to heterotrophs (herbivores and decomposers).

Secondary productivity is the rate at which consumers form new organic matter.

Primary productivity depends on the plant species of an area, environmental factors, nutrient availability and the photosynthetic capacity of plants, so it differs between ecosystems.

The annual NPP of the whole biosphere is about 170 billion tons (dry weight) of organic matter.

Oceans cover about 70 per cent of the surface but produce only 55 billion tons of it; the rest is produced on land.