Ocean Zones
The ocean is a huge environment. We have divided the ocean into zones
to make studying the life there easier and descriptions of habitats more
concise. The ocean has been divided many different ways including
horizontally, vertically, light availability, as well as whether an organism
lives in the water column, or along the sea floor.
Benthic & Pelagic Zones - . The benthic zone refers to the sea floor and
is divided into several regions. These regions correspond with the pelagic
zones adjacent to them. Listed from the shallowest zones at the
coastline to the deepest reaches of the ocean they are: littoral, sublittoral, bathyal, abyssal, hadal. Organisms along the
sea floor regardless of how deep it is are known as
benthic organisms. Benthic organisms are split into 3
main categories. Epiflora or epifauna refers to plants
or animals, respectively, which live on the surface of
the ocean floor. Infauna refer to animals who live
buried within the substrate on the sea floor, hidden
from predators, or harsh conditions. The pelagic
zone refers to the open water region of the ocean.
The pelagic zone is also divided into several smaller
more manageable zones. Listing the pelagic zones
from shallowest to deepest they are: epipelagic,
mesopelagic, bathypelagic, abyssalpelagic, and
hadalpelagic. Organisms who can be found
swimming, drifting, or floating in the water column
are known as pelagic organisms. We find plankton
that are microscopic in size to some that float at and
above the sea surface (neuston), to some that easily weigh more than a 1000 lbs. These plankton can be in one of two
categories phytoplankton, or plant-like plankton; or zooplankton, or animal-like plankton. Plankton are not the only
organisms in the pelagic zones however. We find many organisms who are considered strong swimmers and can
typically swim easily against an ocean current, nekton. Nekton inhabit all regions of the ocean from the deepest, darkest
regions, to the shallowest parts.
Neritic & Oceanic Zones - Along the shores of the continents it is shallow, but offshore it can be deeper than the
highest mountain on earth. Conditions in these areas can be very different and the organisms that live in each area are
different too. The oceans therefore have been split into two principle horizontal life zones: the neritic zone (Coastal) and
the oceanic zone (open sea) which are both considered
to be part of the pelagic zone.
The neretic zone includes shorelines worldwide and
can be a tough place to live. It represents less than
10% of all the oceans, but it contains 90% of all the
ocean species. This zone is the source of most of the
ocean’s productivity per acre. It consists of the
intertidal zone (littoral) and the subtidal zone (sublittoral). Animals and plants in these zones must be able to hold on
tight to rocks, or bury into the protection of the sand and substrate to avoid being washed out to sea with the tides,
storm surges, or constant wave action. Many are also well protected with spiney skin, shells, or camouflage since their
enemies may come from either land, sea, or air.
The oceanic zone includes the open water beyond the continental shelf. The oceanic zone contains around 90% of the
ocean area, but only about 10% of all the ocean species. It extends from the continental margin all the way to the next
continental margin. This vast zone has been split by depth as seen above as well as light and nutrient availability. Some
organisms will never see a photon of sunlight in their entire existence while others will never venture away from light at
the surface. Some even have the ability to produce their own light to ward off would be attackers, attract mates, or find
food. This zone while it may have light does not have any plants due to not having anything for the plants to anchor to.
Light Zones - The pelagic zone can be divided into three sub-zones based on the amount of light that is available. The
uppermost zone is the Euphotic zone (sunlight). This is the ocean zone that sunlight penetrates completely. Because
this zone gets sunlight we find plants, algae, and phytoplankton here. This is where photosynthesis occurs. This zone
extends from the ocean surface down to about 200m (660ft) Organisms in this zone are adapted to relatively warmer
waters, increased salinity fluctuation, and very little area to hide in. Due to the lack of hiding places many fish exhibit
countershading where they have a dark upper body and lighter lower body to make the body blend in with the ocean
environment when viewed from below or above the organism.
The zone beneath the euphotic zone is the Dysphotic zone (twilight). Some sunlight reaches this zone, but not enough
for any photosynthesis to occur. This zone reaches from about 200m- 1,000m (3,300ft) deep. Because less sunlight
reaches this zone the temperature starts to decline
quickly. This significant change in temperature that
occurs in this zone is known as the thermocline. There
are no plants or other producers in this zone due to the
lack of enough sunlight. Many organisms have thin
bodies to reduce their silhouette and avoid predators.
Other organisms taken on a red or black color so they
become almost invisible. Red light is not present in this
zone so red organisms don’t reflect any light back to their predators eyes. Some fish have larger eyes to help take in as
much light as possible to see any predators or prey, while others can emit their own light known as bioluminescence,
making light with special organs called photophores.
The final light zone is the Aphotic zone (midnight). No light at all reaches this zone as it has been dissipated in all the
water above. The aphotic zone extends from about 1,000m to depths of about 11,000m (36,500 ft) almost 7 miles! This
zone is very cold with the complete absence of light, but the temperature is relatively stable due to the layering of
different water masses. Challenges for life in this region are different due to the incredible amount of pressure
(upwards of 4000 psi) Due to the lack of light all together some organisms have evolved to not have any eyes at all.
Others possess bioluminescence like seen in the dysphotic zone. Organisms in this zone rely heavily on their sense of
smell, vibrations, and electrical impulses given off by other organisms to find food.
Zone Overlap - The different zones you read above all overlap. The parameters given in this overview are
approximations and determined by other features of the ocean that are not always exact. Because of this ocean zones
are cross referenced with each other to more accurately describe a particular habitat or ecosystem.
Epipelagic zone- The surface layer of the ocean and extends to approximately 200m deep. This zone is identical to the
euphotic zone.
Mesopelagic zone- This zone ranges from 200m to about 1,000m deep. This zone is identical to the dysphotic zone.
Bathypelagic zone- This zone ranges from 1,000m to approximately 4,000m
deep.
Abyssopelagic zone- Known as the abyss or without bottom extends from
4,000m to 6,000m deep.
Hadalpelagic zone- This zone is the deepest and extends from 6,000m to the
deepest points of the ocean around 11,000
Same as the Aphotic zone
Ocean Zones Activity
Purpose -
Identify the zones of the ocean.
Create a diagram of ocean zones and the organisms that live in each zone.
Draw the appropriate scale, 0-7000m, for each zone.
State how temperature and light differ in various zones of the ocean.
Materials -
Sheet of paper
Pencil(s)
Colored pencils
Ruler
Information sheet
Procedure 1. Read the Background Information Sheet “Ocean Zones”
2. Trace the basic ocean cross section on your paper. (Fig. 1)
3. Measure and label the meter scale on one side of the paper. (Use 2 cm = 1,000 m) 0 meters will be at
sea level going down to 7,000 meters at the bottom of the trench.
4. Label the Benthic division using a brown colored pencil. Use a brown dashed line to show where each
zone boundary exists.
a. Littoral
b. Sublittoral
c. Bathyal
d. Abyssal
e. Hadal
5. Label the Pelagic division using a black colored pencil. Use a black bracket to show where the boundary
for each division exists.
a. Neritic
b. Oceanic
6. Label the Oceanic division using a blue colored pencil. Use a blue dashed line to show where each zone
boundary exists.
a. Epipelagic
b. Mesopelagic
c. Bathypelagic
d. Abyssopelagic
e. Hadalpelagic
7. Label the Neritic division using a red colored pencil. Use a red dashed line to show where each zone
boundary exists.
a. Intertidal
b. Subtidal
8. Label the Photic division using a green colored pencil. Use a green dashed line to show where each zone
boundary exists.
a. Euphotic
b. Dysphotic
c. Aphotic
9. Draw the thermocline using the boundaries of the photic zones as guides. (See Fig. 2)
Fig.1
Fig. 2