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Name
Class
CHAPTER 24
Date
Ocean Water
2 Life in the Oceans
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• How does marine life affect the chemistry of ocean
water?
• Why are plankton called the foundation of life in the
ocean?
• What are the major zones of life in the ocean?
How Does Marine Life Affect Ocean
Chemistry?
Marine organisms are organisms that live in the
ocean. Most marine organisms depend on two main factors to survive: sunlight and the nutrients in ocean water.
Changes in these factors affect the survival of marine life.
Marine organisms help to balance the chemical
composition of ocean water. They remove certain
nutrients and gases from the ocean. They also return
nutrients and gases to the ocean. For example, some
marine organisms absorb carbon, hydrogen, oxygen, and
sulfur. Marine organisms that use photosynthesis release
oxygen into the water.
The chart below shows an example of how marine
organisms recycle nutrients.
A marine organism absorbs and stores
nutrients from the ocean.
READING TOOLBOX
FoldNotes Make a key-term
fold to learn the key terms in
this chapter. Write each key
term on the front of a tab. As
you read this section, write
the definition of each term
under the correct tab.
LOOKING CLOSER
1. Explain How do bacteria
help recycle nutrients?
The organism dies.
Bacteria break down the dead organism and
release nutrients into the water.
UPWELLING
In general, marine organisms get nutrients near the
surface of the ocean. When the organisms die, they sink
to lower depths. These dead organisms decay and release
nutrients back into the deep water.
Nutrients can return to the surface through a process
called upwelling. Upwelling is the movement of deep,
cold, nutrient-rich water to the surface. When wind blows
along a coastline, surface water moves away from the
shore. Then, more deep water rises to replace the surface
water that moved offshore.
Critical Thinking
2. Identify Relationships
How can the movement of
wind currents affect ocean
chemistry?
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
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Ocean Water
Name
SECTION 2
Class
Date
Life in the Oceans continued
Movement of surface water
LOOKING CLOSER
3. Describe In what
direction is the surface
water in this picture moving?
Surface winds
Upwelling
Offshore movement of surface water causes upwelling.
MARINE FOOD WEBS
READING CHECK
4. Explain Why do most
marine organisms live near
the surface of the water?
LOOKING CLOSER
5. Identify Give two
examples of nekton.
Most marine organisms need sunlight and nutrients.
For this reason, most marine organisms live in the upper
100 m of water. Organisms called plankton live in this
sunlit zone. The term plankton refers to all the tiny
organisms that float in the waters of lakes and oceans.
Plankton form the base of ocean food webs. Small
marine organisms eat plankton. Larger marine animals
eat the small marine organisms. These larger animals fall
into two groups: nekton and benthos. The table below
describes nekton and benthos.
Group
Characteristics
Examples
Nekton
swim actively in open water
fish, dolphins, squid
Benthos
live on the ocean floor
oysters, sea stars, crabs
What Are Ocean Environments?
Scientists divide the ocean into two basic environments,
or zones: the benthic zone and the pelagic zone. The
benthic zone is the region near the bottom of the ocean.
The pelagic zone is the upper region above the benthic
zone. The amount of sunlight, the water temperature, and
water pressure determine which organisms live in these
zones.
BENTHIC ZONES
The benthic zone is divided into five smaller zones.
The table on the next page describes the depths and
characteristics of these zones.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
368
Ocean Water
Name
SECTION 2
Class
Date
Life in the Oceans continued
Benthic Zones
Zone
Characteristics
Organisms
Intertidal zone
shifting tides, breaking waves;
constantly changing water depth
seaweed, mussels
Sublittoral zone
has the most organisms of any
benthic zone; always underwater;
on the continental shelf
sea stars, sea
lilies
Bathyal zone
begins at the continental slope;
extends to a depth of 4,000 m;
little or no sunlight;
few photosynthetic organisms
octopuses, sea
stars, brachiopods
Abyssal zone
depth of 4,000 m to 6,000 m;
no sunlight
sponges, worms
Hadal zone
deeper than 6,000 m;
unexplored by humans
unknown, but
probably few
living things
LOOKING CLOSER
6. Explain Why do few
organisms that carry out
photosynthesis live in the
bathyal zone?
PELAGIC ZONES
The pelagic zone lies above the benthic zone. The
pelagic zone is divided into two smaller zones: the neritic
zone and the oceanic zone.
The neritic zone has plenty of sunlight, mild
temperatures, and low water pressure. These are ideal
conditions for marine life. Nekton are common in the
neritic zone. These nekton include many fish and other
types of seafood that humans eat.
The oceanic zone stretches into the deep waters past
the continental shelf. The oceanic zone is divided into
four zones based on depth. The epipelagic zone is the
upper area of the oceanic zone. It gets some sunlight
and contains many marine organisms. The mesopelagic,
bathypelagic, and abyssopelagic zones are the deeper
areas of the oceanic zone. The amount of marine life in
the pelagic zone decreases as depth increases.
PELAGIC ENVIRONMENTS
Oceanic zone
Epipelagic zone
Neritic zone
l zone
Sublittora
READING CHECK
7. Describe the neritic zone.
al
Intertnide
zo
EN
VI
RO
N
Ba
thy ME
N
al
zo TS
ne
Mesopelagic zone
LOOKING CLOSER
8. Identify Which pelagic
zone is the farthest from the
ocean surface?
BE
NT
HI
C
Bathypelagic zone
Abyssopelagic zone
H
zo adal
ne
Abyssal zone
This diagram shows the locations of marine environments.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
369
Ocean Water
Name
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Date
Section 2 Review
SECTION VOCABULARY
benthic zone the bottom region of oceans and
bodies of fresh water
benthos organisms that live at the bottom of
oceans or bodies of fresh water
nekton all organisms that swim actively in open
water, independent of currents
pelagic zone the region of an ocean or body of
fresh water above the benthic zone
plankton the mass of mostly microscopic
organisms that float or drift freely in the
waters of aquatic (freshwater and marine)
environments
upwelling the movement of deep, cold, and
nutrient-rich water to the surface
1. Organize Complete the concept map below using the terms neritic zone, benthic
zone, oceanic zone, and pelagic zone.
the ocean environment
is divided into
is divided into
2. Explain How do marine organisms affect the chemistry of ocean water?
3. Explain What is the role of plankton in ocean food webs?
4. Summarize In your own words, describe the process of upwelling.
5. Make Predictions How would life in the ocean change if upwelling decreased?
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
370
Ocean Water