Primary Productivity: It All Depends on Who You Ask

Vicki A Benge

Life on Earth exists as we know it thanks to the process -- whereby green plants and certain other organisms utilized light energy to convert carbon dioxide and water into carbohydrates and oxygen -- known as photosynthesis. Ecologists attempt to measure the biomass[1] created through this process and the carbon remaining for consumption. Thus, reference may be made to the primary productivity of an ecosystem or a particular region.

What do ecologists mean when they refer to the primary productivity of an ecosystem or a particular area? The answer received depends on who one asks. For example, biologist Sylvia S. Mader, author of the Essentials of Biology textbook defines primary productivity as "the amount of biomass produced primarily by photosynthesis" and as "the rate at which producers capture and store energy as organic nutrients over a certain length of time."[2]

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In Biology, 7th ed., primary productivity is defined as "the amount of energy produced by photosynthetic organisms in a community."[3]

From a University of Michigan lecture on "The Flow of Energy" the definition for primary production is given as "the synthesis and storage of organic molecules during the growth and reproduction of photosynthetic organisms." The lecture goes on to state that "over time, primary production results in the addition of new plant biomass to the system."[4]

Scientists attempt to measure the net primary productivity and analyze and compare the rates among the various ecosystems and regions of the Earth.

NASA defines net primary productivity as "how much carbon dioxide vegetation takes in during photosynthesis minus how much carbon dioxide the plants release during respiration (metabolizing sugars and starches for energy)."[5]

Oregon State University offers a simpler way of defining the entire process by stating that the producers themselves consume a part of the immediate energy from photosynthesis. Additionally photosynthetic organisms use the carbon products they create for growth and reproduction. What is left over is, "net primary production," and is available for consumption by other members of the ecosystem.[6]

According to scientists, the aquatic ecosystems - estuaries, swamps, and marshes, - along with tropical rainforests and coral reefs, have the highest levels of primary productivity; followed by (2) temperate deciduous forests; (3) temperate grasslands, lakes, streams; (4) beaches; (5) tundra; (6) open oceans; and (7) deserts. Note that coral reefs are among the top producers; yet open oceans are near the bottom in productivity. This is due to the open waters lacking concentrated supplies of nutrients.[7]

On a world map, an observer will notice that primary productivity overall remains highest near the equator where tropical forests are found, and lowest in the higher latitudes, where tundra ecosystems prevail. The higher the productivity, the more food available, thus, more biomass is readily found in areas with the highest levels of primary productivity. Look at three map images, provided by the NASA Earth Observatory, and note the differences in productivity, as it relates to the changes of the seasons.

Footnotes and References:
[1] Note that biomass = the number of organisms X their weight.
[2] Mader, Sylvia S., Essentials of Biology, New York: McGraw-Hill, 2007.
[3] Raven, Peter H., and George B. Johnson, Jonathan B. Losos, Susan R. Singer, Biology, 7th ed., New York: McGraw-Hill, 2005.
[4] University of Michigan: The Flow of Energy: Primary Production
[5] NASA: Net Primary Productivity: Global Maps
[6] Oregon State University: Ocean Productivity
[7] Mader; ibid.

See also Ecosystems, Page 1 and Page 2.