2.2.23 (HL)—Primary productivity

Primary productivity is the rate at which producers fix energy into new organic matter.

It is a rate, so area and time matter. Light, nutrients, temperature and water can limit fixation; high standing biomass does not necessarily mean high current productivity.

A mature forest may hold large biomass but add little each year, while a young fertilised crop can have faster annual production.

Carbon fixed per area per time directly measures the rate.

Productivity is not the same as biomass; one is a rate, the other is an amount present.

A field estimate can harvest replicate plots of known area at two times, dry samples to constant mass and calculate change in biomass per area per time; controls or corrections are needed for grazing, death and movement. A laboratory estimate can expose matched photosynthesizing samples to defined light or nutrient conditions and measure biomass, oxygen or carbon-dioxide change. Report productivity in units such as kg C m⁻² yr⁻¹ and distinguish the rate from standing biomass.