HUMAN BIOLOGY

prinCiples of eCology 465

What is a biogeochemical cycle?

• In a biogeochemical cycle, nutrients move from the environment
  to organisms, then back to their reservoir in the environment.


• Nutrients usually move slowly through the environment but
  rapidly between organisms and the environment.


• In the water cycle, the world ocean is the main reservoir.

taKe-Home message

biogeochemical cycle

Movement of nutrients from

the environment to organ-

isms, then back to a reser-

voir in the environment.

water cycle The cycling of

water molecules between

bodies of water (the res-

ervoirs) to the atmosphere

and back to Earth, where

water is available to

organisms.

Driven by solar energy, Earth’s

waters move slowly, on a vast scale,

from the ocean into the atmosphere,

to land, and back to the ocean—the

main reservoir. Water evaporating

into the lower atmo sphere initially

stays aloft in the form of vapor,

clouds, and ice crystals. It returns to

Earth as precipi tation, mostly rain

and snow. Ocean currents and pre-

vailing wind patterns influence this

global cycling.

introduction to Biogeochemical Cycles

Figure 24.9 Animated! In the water cycle, water moves from oceans to the atmosphere, the land, and back to

the ocean. Yellow boxes indicate the main reservoirs. Arrow labels identify the processes involved in the movement

of water between reservoirs, measured in cubic kilometers per year. (© Cengage Learning)

evaporation

from ocean

425,000

precipitation

into ocean

385,000

wind-driven water vapor

40,000

evaporation from

land plants

(transpiration)

71,000

precipitation

onto land

111,000

surface and

groundwater flow

40,000

Land

Atmosphere

Ocean

n Ecosystems depend on primary productivity. This is why

the availability of water, carbon dioxide, and the mineral

ions that serve as nutrients for producers have such an

important impact on ecosystems.

In a biogeochemical cycle, ions or molecules of a nutrient
are moved from the environment to organisms, then back
to the environment—part of which serves as a reservoir for
them. They generally move slowly through the reservoir,
compared to their rapid movement between organisms and
the environment. There are three types of biogeochemical
cycles, based on the part of the environment that has the
largest supply of the ion or molecule being considered. In
atmospheric cycles, much of the nutrient is in the form of a
gas. Sedimentary cycles move chemicals that do not occur as
gases, such as phosphorus. Such solid nutrients move from
land to the seafloor and return to dry land only through
geological uplifting, which may take millions of years. The
Earth’s crust is the main storehouse for these substances.
The global water cycle is a third type of biogeochemi-
cal cycle (Figure 24.9). In this cycle, oxygen and hydrogen
move in the form of water molecules.

24.4

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