Primary Productivity: Gross vs Net Ecosystem Production

Primary productivity is the rate at which producers convert light energy into chemical energy (biomass); gross primary productivity (GPP) is the total energy fixed by photosynthesis, and net primary productivity (NPP) is what remains after producers have used some for their own respiration: NPP = GPP − R. In brief: NPP is the energy available to consumers and decomposers, and it is the base of the whole food web. Productivity is measured in energy (kJ per m² per year) or biomass (g per m² per year). Tropical rainforests and estuaries are among the most productive ecosystems, and deserts and the open ocean are among the least.

Energy flow through ecosystems begins with primary productivity. This guide defines biomass and productivity, explains the difference between GPP and NPP, shows how they differ among ecosystems, discusses the factors that limit productivity, introduces secondary productivity, works through a calculation with real-style numbers and corrects common mistakes.

What are biomass and productivity?

Biomass is the total mass of living (or recently living) organic matter in an area at a given time. It is usually given as dry mass, because the water content varies, in units such as grams per square meter (g/m²) or tonnes per hectare. Biomass is a stock, a quantity at one moment.

Productivity is a rate: the amount of energy or biomass produced per unit of area per unit of time, for example in kJ/m²/year or g/m²/year. A forest may have a very large biomass, yet low productivity if it grows slowly. A fast-growing algal bloom may have a small biomass but high productivity.

What are gross and net primary productivity?

Gross primary productivity (GPP) is the total rate at which producers capture energy in photosynthesis (the organic matter made). A part of this energy is used by the producers themselves for respiration (R), which releases heat and CO₂. The rest is stored in new tissues, as growth and reproduction, which is net primary productivity (NPP):

NPP = GPP − R

NPP is the energy available to the next trophic level (consumers) and to decomposers. Respiration in plants typically uses 40 to 60 percent of GPP in forests, and less in rapidly growing young plants. Efficiency of energy capture is low: the NPP in the biosphere averages less than 1 percent of the incident sunlight on a global scale, and under the best conditions in crops it reaches 2 to 3 percent, since only about 45 percent of sunlight is photosynthetically active, much is reflected or absorbed as heat, and photosynthesis itself has a maximum efficiency of 4 to 6 percent. Global NPP is estimated to be about 100 to 120 billion tonnes of carbon per year, around half on land and half in the oceans.

How is productivity measured?

• Harvest method: measuring the dry mass of plants in sample plots at the start and end of a period. The change in biomass estimates NPP (it underestimates, since it ignores material eaten or shed).
• Gas exchange: measuring the CO₂ taken up in light (net photosynthesis) and the CO₂ released in the dark (respiration). GPP = net CO₂ uptake + respiration.
• Oxygen method (aquatic): comparing dissolved oxygen in clear (light) and dark bottles. Light bottle change = NPP; dark bottle change = −R; their difference = GPP.
• Satellites: estimate the “greenness” (chlorophyll) of vegetation, which is used to calculate productivity over large areas.

Productivity of different ecosystems

The values are classic estimates by R. H. Whittaker, and later values differ somewhat, but the ranking is robust. Although the open ocean has a low productivity per square meter, it covers about 65 percent of the Earth’s surface, so it contributes a large share of global NPP.

What limits primary productivity?

• Light: limits growth in deep water, in dense forest understories and at high latitudes in winter.
• Water: the main limit on land in dry regions.
• Temperature: affects enzymes of photosynthesis, and the length of the growing season.
• Nutrients: nitrogen, phosphorus and, in some oceans, iron. In the open ocean, the nutrients sink to the deep water with dead organisms, so the surface is poor in nutrients. In regions of upwelling, deep water rises to the surface, and productivity is high.
• Carbon dioxide: sometimes limits photosynthesis (see the article on limiting factors).
• Grazing and disease: remove biomass.

What is secondary productivity?

Secondary productivity is the rate at which consumers make new biomass from the food they eat. The energy a consumer takes in as food is divided into: energy lost in feces and undigested food, energy used for respiration (heat and movement) and energy stored as new biomass (growth and reproduction). Net secondary productivity is the energy stored: N = I − F − R, where I is intake, F the energy in feces and R the respiration. Typically, carnivores assimilate more of their food (80 to 90 percent) than herbivores (30 to 60 percent), and endotherms use more energy for respiration than ectotherms, so the latter produce biomass more efficiently.

Worked example: calculating GPP, NPP and efficiency

A temperate grassland receives 4 000 000 kJ of solar energy per square meter per year. The grass fixes 40 000 kJ/m²/year in photosynthesis (GPP). Respiration of the grass uses 22 000 kJ/m²/year. Grasshoppers eat 10 000 kJ/m²/year of the NPP, of which 3 000 kJ is lost in feces, 5 500 kJ is used in respiration.

1. NPP of the grass: 40 000 − 22 000 = 18 000 kJ/m²/year.
2. Efficiency of GPP: 40 000 ÷ 4 000 000 = 0.01, or 1.0 percent of the incident energy.
3. NPP as a percentage of GPP: 18 000 ÷ 40 000 = 0.45, or 45 percent.
4. Net secondary productivity of grasshoppers: 10 000 − 3 000 − 5 500 = 1 500 kJ/m²/year.
5. Efficiency of energy transfer from grass NPP to grasshopper production: 1 500 ÷ 18 000 = 0.083, or 8.3 percent (a little less than the 10 percent rule of thumb).
6. Assimilation efficiency of the grasshoppers: (10 000 − 3 000) ÷ 10 000 = 70 percent.

The calculations show where the energy goes at each step: much is lost as heat in respiration, and only a small part is passed on. The 1 500 kJ/m²/year available to the next level is only 0.0375 percent of the incident sunlight (1 500 ÷ 4 000 000), which explains why food chains are short. The numbers are invented for the exercise and are of a realistic order of magnitude.

How was productivity studied?

Raymond Lindeman published his study of energy flow in Cedar Bog Lake, Minnesota, in 1942, which introduced the trophic-dynamic concept and showed that energy is lost at each step. Eugene and Howard Odum measured the productivity of Silver Springs in Florida and of coral reefs in the 1950s. Satellite sensors since the 1980s have enabled global mapping of NPP, and eddy-covariance towers now measure the carbon exchange of ecosystems directly.

Which mistakes are common?

Mistake 1: Confusing biomass and productivity

Biomass is the amount present at a time. Productivity is the rate of production per unit of time.

Mistake 2: Saying that NPP is the energy captured by plants

The total captured is GPP. NPP is GPP minus the producers’ respiration.

Mistake 3: Believing that all NPP passes to consumers

Part is not eaten (roots, wood), and part goes to decomposers. Consumers get only a share of NPP.

Mistake 4: Forgetting the units

Productivity must be per unit area per unit time: for example kJ/m²/year.