Productivity

Productivity refers to the rate at which biomass is generated in an ecosystem, expressed in units of mass per unit surface area (or volume) per unit time — for instance, grams per square metre per day (g/m²/day). The mass unit used may refer either to dry matter or to the mass of carbon fixed. Productivity is split into two related measures: gross primary productivity (GPP), which is the total rate of production of organic matter by photosynthesis, and net primary productivity (NPP), which is what remains after the plants themselves have used up a portion of that GPP for their own respiration (R). This relationship is expressed simply as GPP − R = NPP, and it is this net primary productivity that represents the biomass genuinely available for consumption by heterotrophs — herbivores, carnivores and decomposers.

Gross primary productivity depends on which plant species occupy a given area, along with a range of environmental factors, nutrient availability, and the plants' own photosynthetic capacity, which is why productivity varies widely between different types of ecosystems. Globally, the annual net primary productivity of the entire biosphere is estimated at approximately 170 billion tons (dry weight) of organic matter, of which the oceans contribute only around 55 billion tons, with the remainder coming from land-based ecosystems — a reflection of how comparatively unproductive open ocean water is relative to its enormous area. …