Essentials of Ecology

164 CHAPTER 8 Aquatic Biodiversity

rivers, streams, and inland wetlands). Although some

systems such as estuaries are a mix of saltwater and

freshwater, we classify them as marine systems for pur-

poses of discussion.

Figure 8-3 shows the distribution of the world’s

major oceans, coral reefs, mangroves, lakes, and rivers.

These aquatic systems play vital roles in the earth’s bio-

logical productivity, climate, biogeochemical cycles, and

biodiversity, and they provide us with fish, shellfish,

minerals, recreation, transportation routes, and many

other economically important goods and services.

Most Aquatic Species Live in Top,

Middle, or Bottom Layers of Water

Saltwater and freshwater life zones contain several ma-

jor types of organisms. One such type consists of weakly

swimming, free-floating plankton, which can be di-

vided into three groups, the first of which is phytoplank-

ton (“FY-toe-plank-ton,” Greek for “drifting plants”; see

bottom of Figure 3-14, p. 63), which includes many

types of algae. They and various rooted plants near

shorelines are primary producers that support most

aquatic food webs. See The Habitable Planet, Videos 2 and

3, at http://www.learner.org/resources/series209.html.

The second group is zooplankton (“ZOH-uh-plank-

ton,” Greek for “drifting animals”; see bottom of Fig-

ure 3-14, p. 63). They consist of primary consumers

(herbivores) that feed on phytoplankton and secondary

consumers that feed on other zooplankton. They range

from single-celled protozoa to large invertebrates such

as jellyfish.

A third group consists of huge populations of much

smaller plankton called ultraplankton. These extremely

small photosynthetic bacteria may be responsible

for 70% of the primary productivity near the ocean

surface.

A second major type of organisms is nekton,

strongly swimming consumers such as fish, turtles, and

whales. The third type, benthos, consists of bottom

dwellers such as oysters, which anchor themselves to

one spot; clams and worms, which burrow into the sand

or mud; and lobsters and crabs, which walk about on the

sea floor. A fourth major type is decomposers (mostly

bacteria), which break down organic compounds in the

dead bodies and wastes of aquatic organisms into nutri-

ents that can be used by aquatic primary producers.

Lakes

Rivers

Coral reefs

Mangroves

Figure 8-3 Natural capital: distribution of the world’s major salt water oceans, coral reefs, mangroves, and fresh-

water lakes and rivers. Question: Why do you think most coral reefs lie in the southern hemisphere?