With minor exceptions, the biosphere is powered by the sun. The average amount of solar energy reaching Earth's surface (the insolation) in the United States ranges from 1250 to
2750 kcal/m2 • day in January to 5250 to 7000 kcal/m2 • day in July. For the entire year, averaged over all regions of the country, the input is about 3940 kcal/m2 • day. Much smaller quantities originate from nonthermal sources. Geothermal energy derives from radioactive decay in the Earth and contributes about 0.5% of the solar input. Tidal friction extracts energy from the kinetic energy of the Earth-sun-moon system and is about 0.0017% of solar. A portion of the wind's energy comes from the kinetic energy of Earth's rotation. Oxidation of reduced inorganic minerals transported to the biosphere from deep in Earth's crust by hot springs and deep ocean vents provide energy for unique ecosystems adjacent to them. (However, this source is not completely independent of solar input, as it relies on oxygen produced mostly in the photosphere by solar energy.) These are the ultimate sources of energy for the biosphere. However, sensible heat and mechanical forms of energy cannot be used to form carbohydrates from CO2. Organisms require high-quality energy in the form of photons or chemical bond energy in order to drive the reactions that produce biomass from inorganic precursors.
Most of the solar energy arriving at Earth's surface is converted to sensible heat or drives the processes of evaporation, wind, and waves. About 0.8%, however, is captured by photosynthetic conversion to organics. This autotrophic activity supplies practically all of the energy for the biosphere. The autotrophs responsible for the initial generation of fixed carbon from CO2 are called producers in ecological terminology. The rate at which carbon is fixed in an environment is called its primary productivity. It can be measured either in energy terms (kcal/m2 • day), or equivalently, in terms of biomass production rate (e.g., g/m2- yr). The basis area (m2) refers to the area of the land. Biomass is usually measured as the total dry weight of the organisms under consideration. Carbohydrates have about 4.1 kcal/g (17.2 kJ/g).
Note that much of the activity of chemoautotrophs is not "primary" in the sense that the reduced minerals they are oxidizing were produced by other organisms. For example, hydrogen sulfide and ammonia are produced by anaerobic bacteria in marsh sediments. These can then be used by chemoautotrophic bacteria to fix CO2. The energy for the anaerobes comes from organics that probably originated from plants. Other sources of these minerals, such as deep ocean vents or volcanoes, are abiotic in origin.
Primary productivity has two main components. Gross primary productivity (GPP) is the total amount of carbon fixed by the autotrophs. GPP may also be called the rate of assimilation. Only part of the gross productivity is available to other components of the ecosystem because the autotrophs themselves respire, consuming some of the fruits of their own labor. Net primary productivity (NPP) is the rate at which biomass or energy is accumulated by the autotrophs. It is equal to the difference between gross productivity and respiration. In practice, the net productivity is measured by harvesting and respiration measured by CO2 production.
Gross primary productivity can be computed from measurements of net primary productivity and respiration. In some systems, such as the ocean, GPP is strongly correlated to the chlorophyll content. Chlorophyll can be measured by solvent extraction followed by spectrophotometry. Then one can apply a factor known as the assimilation ratio, which is the ratio of carbon fixation rate to chlorophyll concentration. For the ocean the assimilation ratio is fairly constant at 3.7 g of carbon fixed per hour per gram of chlorophyll. However, this ratio can vary widely for other ecosystems. Plants that are adapted
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