Biology Now, 2e

382 ■ CHAPTER 21 Ecosystems

ECOLOGY

Nutrients from abiotic

sources are absorbed

by producers, then

passed to consumers

and decomposers.

Energy is captured by producers, then passed

through consumers and decomposers. At each

level, most energy is lost through heat loss.

Decomposers extract

nutrients from dead

consumers and

producers and return

them to the abiotic

world to be used again.

Consumers

Producers

Decomposers

Heat loss Heat loss

Heat loss

Figure 21.5

Energy flow and nutrient cycling

Unlike energy, which moves up and out of ecosystems, nutrients are constantly

cycled between the abiotic and biotic worlds. Important nutrients for the biotic

world include carbon (C), potassium (K), phosphorus (P), and nitrogen (N).

Q1: How is a decomposer different from a more typical consumer?

Q2: What is the difference between how carbon is brought into the

biotic portion of the ecosystem, and how other nutrients, such as

phosphorus, are brought in?

Q3: Describe all the points at which heat is lost in this figure.

Figure 21.6

Average chlorophyll concentration

in the oceans

Phytoplankton are most abundant in high

latitudes, along coastlines and continental

shelves, and along the equator in the Pacific

and Atlantic Oceans (yellow)—but are scarce in

remote oceans (dark blue).

Chlorophyll concentration (mg/ml)

0.01 .01 1 10

made by producers. Without decomposers,

nutrients could not be repeatedly reused, and

life would cease because all essential nutrients

would remain locked up in the bodies of dead

organisms. In this way, decomposers are the

“cleaners” of an ecosystem. Bacteria and fungi

are important decomposers in the ocean, as are

hagfishes, worms, and others.

Ecologists and earth scientists use the

term “nutrient cycle” to describe the passage

of a chemical element through an ecosystem

(Figure 21.5). The nutrient cycle and the flow

of energy are two of four processes that link

the biotic and abiotic worlds in an ecosystem.

These ecosystem processes also include the

water cycle (see Figure 18.12) and succession,

the process by which the species in a community

change over time (as discussed in Chapter 20).

A Multitude of Measurements

For over 100 years, researchers around the globe

have studied ecosystems containing phytoplank-

ton. Boyce tapped into that wealth of research to

document past and present levels of phytoplank-

ton in the oceans.

The amount of phytoplankton biomass in a

given area can be estimated by the concentra-

tion of chlorophyll found there (Figure 21.6).

For decades, nearly all ocean studies have used

chlorophyll concentration as a reliable metric of

phytoplankton biomass. Chlorophyll concentra-

tion is measured by detecting the color of water.

Water takes on deeper shades of green as the

amount of chlorophyll increases. When there is

no chlorophyll, water appears clear.

Ideally, Boyce would have used satellite data

to detect chlorophyll and thus phytoplank-

ton concentrations, since satellites today take

high-resolution color measurements of the ocean

surface. Yet Boyce planned to review phytoplank-

ton levels over the past 100 years, and high-quality

satellite data have been available for only the last

decade. He needed another source of data.