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Is there an Upper Limit to Primary Production?

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Let's briefly consider how much energy is in fact captured by autotrophs, and examine how efficient is the process of photosynthesis. Recall that the intensity of solar radiation reaching the earth's surface depends partly on location: the maximum energy intensity is received at the equator, and the intensity decreases as we move toward the poles. As we saw in the lecture on ecosystems, these differences have profound effects on climate, and lead to observed geographic patterns of biomes.
Furthermore, we know that only a small fraction of the sun's radiation actually reaches the earth’s surface. The atmospheric ozone layer absorbs nearly all ultraviolet light, which makes up about 9% of the sun's radiation. Of the remaining light, about half is in the infrared portion of the spectrum, and about half is in the visible portion. Most of the infrared light is absorbed
• by atmospheric carbon dioxide, water vapor, and water droplets (clouds). About 35% of the visible light is reflected back into space by clouds, ice, snow, oceans, and other particles. As a result of these losses to reflection and absorption, only about 1 - 4% of the energy reaching the earth's outer atmosphere is available at the earth's surface.
Of the light reaching the earth's surface, about 10 percent is ultraviolet, while visible and infrared wavelengths each comprise about 45 percent of the total. Plants can only use certain wavelengths of light. They strongly absorb light of blue and red wavelengths (hence their greencolor, the result of reflection of green wavelengths), as well as light in the far infrared region, and they reflect light in the near infrared region. Plants do not, then, use all of the light energy theoretically available to them On average, plant gross primary production on earth is about 5.83 x 106 cal m2 yr1
- This is about 0.06% of the amount of solar energy falling per square meter on the outer edge of the earth's atmosphere per year (defined as the solar constant and equal to 1.05 x 1010 cal m-2 yr1). After the costs of respiration, plant net primary production is reduced to 4.95 x 106 cal m-2 yr1 , or about 0.05% of the solar constant. Note that this is the "average" efficiency, and in land plants this value can reach ~2-3% and in aquatic systems this value can reach ~1%. This relatively low efficiency of conversion of solar energy into energy in carbon compounds sets the overall amount of energy available to heterotrophs at all other trophic levels.

francis1897 answered the question on February 27, 2023 at 12:25

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Is there an Upper Limit to Primary Production?

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