142 CHAPTER 6 Ecosystems and Evolution
fishes, turtles, and whales. Benthos are bottom-dwelling
organisms that fix themselves to one spot (sponges and
oysters), burrow into the sand (worms and clams), or
simply walk about on the bottom (crawfish and aquatic
insect larvae).Freshwater Ecosystems
Freshwater ecosystems include lakes and ponds (standing-
water ecosystems), rivers and streams (flowing-water
ecosystems), and marshes and swamps (freshwater
wetlands). Specific abiotic conditions and character-
istic organisms distinguish each freshwater ecosystem.
Although freshwater ecosystems occupy only about 2 per-
cent of Earth’s surface, they play an important role in
the hydrologic cycle: They help recycle precipitation that
flows into the ocean as surface runoff. (See Chapter 5
for a detailed explanation of the hydrologic cycle.) Large
bodies of fresh water help moderate daily and seasonal
temperature fluctuations on nearby land regions, and
freshwater habitats provide homes for many species.
Zonation is characteristic of standing-water
ecosystems. A large lake has three zones: the litto-
ral, limnetic, and profundal zones (see What a Scientist
Sees). The littoral zone is a produc-
tive, shallow-water area along
the shore of a lake or pond.
Emergent vegetation, such as
cattails and bur reeds, as well as
several deeper-dwelling aquatic
plants and algae, live in the lit-
toral zone. Animals here include
frogs, turtles, worms, crayfish and other crustaceans,
insect larvae, and many fishes. The limnetic zone is the
open water beyond the littoral zone—that is, away fromAquatic Ecosystems
LEARNING OBJECTIVES
- Summarize the important environmental factors
that affect aquatic ecosystems. - Describe the various aquatic ecosystems,
giving attention to the environmental
characteristics of each.
T
he most fundamental division in aquatic
ecology is probably between freshwater and
saltwater environments. Salinity, which is the
concentration of dissolved salts (such as
s odium chloride) in a body of water, affects the kinds of
organisms present in aquatic ecosystems, as does the
amount of dissolved oxygen. Water greatly i nterferes with
the penetration of light, so floating aquatic organisms
that photosynthesize must remain near the w ater’s sur-
face, and vegetation anchored to lake floors or streambeds
will grow only in relatively shallow water. In addition, low
levels of essential nutrient minerals limit the number and
distribution of organisms in certain aquatic environ-
ments. In this section, we discuss freshwater ecosystems
only; because the immense marine environment is so
critical to the environmental well-being of Earth, we
devote an entire chapter to it (see Chapter 11).
Aquatic ecosystems contain three main ecological
categories of organisms: free-floating plankton, strongly
swimming nekton, and bottom-dwelling benthos. Plank-
ton are usually small or microscopic organisms. They
tend to drift or swim feebly, so, for the most part, they are
carried about at the mercy of currents and waves. Plank-
ton include phytoplankton, photosynthetic algae and cya-
nobacteria that form the base of most aquatic food webs,
and zooplankton, animal-like organisms that feed on algae
and cyanobacteria and are in turn consumed by newly
hatched fish and other small aquatic organisms. Nekton
are larger, more strongly swimming organisms such as
standing-water
ecosystem A
body of fresh water
surrounded by land
and whose water
does not flow; a lake
or a pond.Human population growth and industrial expansion in
tropical countries may spell the end of tropical rain forests
during the 21st century. Biologists know that many rainforest
species will become extinct before they are even identified
and scientifically described. (See Chapter 13 for more dis-
cussion of the ecological impacts of rainforest destruction.)
- What is a biome?
- How do you distinguish between temperate
rain forest and tropical rain forest? between
savanna and desert?