Inspire Biology

The area of the spray zone is dry most of the time. It is only during high tides that

this part of the shoreline is sprayed with salt water, and few plants and animals are

able to live in this environment. The high-tide zone is under water only during high

tides. However, this area receives more water than the spray zone, so more plants

and animals are able to live there.

The mid-tide zone undergoes severe disruption twice a day as the tides cover and

uncover the shoreline with water. Organisms in this area must be adapted to long

periods of air and water. The low-tide zone is covered with water unless the tide is

unusually low, and is the most populated area of the intertidal zone.

Open ocean ecosystems

As illustrated in

Figure 24

on the next page, the zones in the open ocean include the

pelagic (puh LAY jihk) zone and benthic zone. All the water in the open ocean that is

not in the intertidal zone or very near the bottom is considered to be the

pelagic zone.

The area to a depth of about 200 m of the pelagic zone is the

photic zone,

also called the

euphotic zone. The photic zone is shallow enough that sunlight is able to penetrate. As

depth increases, light decreases. Autotrophic organisms in the photic zone include

surface seaweeds and plankton. Animals in the photic zone include many species of

fish, sea turtles, jellyfish, whales, and dolphins. Many of these animals feed on plank-

ton, but others feed on larger species.

Below the photic zone lies the

aphotic zone,

an area where sunlight is unable to pene-

trate. This region of the pelagic zone remains in constant darkness and generally is cold,

but there is thermal layering with a mixing of warm and cold ocean currents. Organ-

isms that depend on light energy to survive cannot live in the aphotic zone.

The deepest region of the pelagic zone is often referred to as the

abyssal zone.

The

abyssal zone generally includes areas at depths greater than 4,000 m. Water in such

areas is very cold. Most organisms in this zone rely on food materials that drift down

from the zones above. However, on the seafloor along the boundaries of Earth’s plates,

hydrothermal vents spew large amounts of hot water, hydrogen sulfide, and other

minerals. In these locations scientists have found bacterial communities that can use the

sulfide molecules for energy. These organisms are at the bottom of a food chain that

includes invertebrates, such as clams and crabs, and vertebrates, such as fishes.

Get It?

Describe

environmental variation in intertidal zones.

WORD ORIGIN

photic

comes from the Greek word

photos,

meaning

light.

ACADEMIC VOCABULARY

constant

occurring all of the time or continually

There is a constant flow of cars on the highway

Module 3 • Communities, Biomes, and Ecosystems