on Earth’s surface interacts with the above ingredients in plant cells through the
auspices of the green pigment chlorophyll. The radiant solar energy is converted
into chemical form as a series of carbohydrates. This storage allows plants a
steady supply of energy for respiration, the processes that keep the plant alive.
Of incredible importance to our planet is the oxygen released as a byproduct; the
bulk of our atmospheric oxygen supply was produced by photosynthesis.
Plants manufacturing carbohydrates from solar energy are known as primary
producers. Once fixed into chemical form, solar energy is available for further
use by other organisms. Some of the primary producers are consumed by animals
that use the carbohydrates to sustain their life processes and these animals are
known as primary consumers. Of course, some animals eat other animals thereby
gaining the energy the primary consumers gained from plants; in this case the
animals are known as secondary consumers. Plants and animals are intertwined
by energy and mass pathways known as food chains. This is, perhaps, oversimpli-
fication in that most ecosystems are complex webs of recycled energy and matter.
The relationships do not represent perfect usage of energy. At every step,
about 90 percent of the energy is lost. Ironically, any top predator like an eagle
or a shark is several steps away from the capture of solar energy by plants, and is
dependent on the existence of huge amounts of biomass to survive. Humans have
survived because of their ability to either eat plants directly or eat animals that
have eaten plants.
Geographers are interested in global net primary productivity (NPP). It is the
measure of photosynthesis minus respiration and is commonly given as kilograms
of carbon per square meter per year. NPP is geographically varied by climate and
landscape environments. Tropical rainforests have the greatest net primary produc-
tivities, on the order of 2.2 kg/m^2 /yr, whereas tundra manages only 0.14 kg/m^2 /yr.
Although some upwelling areas of the oceans are quite productive, there are large
stretches that are unproductive. For example, the stable waters in the subtropical
highs might have NPPs of .002 kg/m^2 /yr. Moreover, these numbers are only for
the top waters into which solar energy can penetrate.
Thehydrologic cycleis the most massive of the cycles on the planet with about
400,000 cubic kilometers leaving the oceans each year. Water is a part of all Earth’s
life so that its circulation around the planet is of major importance to the sustenance
of life.
Oxygen is plentiful in the atmosphere and is well known for its use by the ani-
mal life on our planet. Like other components of our atmosphere, oxygen
has evolved over time. In Earth’s earliest atmospheres, the two- and three-atom
oxygen so common now could not exist in quantity because of the propensity of
unmitigated solar energy to break apart these molecules. Two things happened
to increase the oxygen supply. They were (1) the formation of the ozone (O 3 ) layer
Biogeochemical Cycles 29