Biology Now, 2e

Here and Gone ■ 381

Figure 21.4

Nutrients cycle within and beyond the ocean ecosystem

In the ocean, phytoplankton absorb nutrients from the abiotic world. Within the biotic world, these

nutrients are cycled among consumers, beginning with zooplankton and moving up through trophic

levels. The nutrients then return to the abiotic world when dead or dying organisms are broken down by

decomposers into their constituent elements.

Q1: Which organisms are the producers in this ecosystem?

Q2: How do nutrients flow from the abiotic to the biotic components of the ecosystem?

Q3: How do nutrients flow from the biotic to the abiotic components of the ecosystem?

Nutrients

Phytoplankton

Decomposers

Zooplankton

Photosynthesis

Cellular

respiration

CO 2

CO 2

C

N

Si

P

Fe

Carbon (C) is captured via

photosynthesis. Nitrogen

(N), phosphorus (P), iron

(Fe), and silicon (Si) are

absorbed from the water

by phytoplankton.

As individuals die at each

level—from phytoplankton

to top predators like

tuna—their remains drift

to the ocean floor, where

decomposers consume

them and return nutrients

to the water.

Nutrients acquired by

zooplankton from

phytoplankton are

passed up the energy

pyramid to the next

trophic levels.

of time. Phytoplankton, for example, require the

nutrients nitrogen, phosphorus, iron, and silicon

for growth. When zooplankton eat phytoplank-

ton, they take up those nutrients, and so on up

the food chain (Figure 21.4).

Nutrients are eventually returned to the

abiotic world when decomposers break down

the dead bodies of other organisms. In some

ecosystems, decomposers break down 80

percent of the biomass, or biological material,