Marine Ecosystems Description Page
Marine ecosystems are a type of aquatic ecosystems. They are the largest ecosystems on the Earth. In fact, marine
ecosystems cover more than two-thirds of Earth’s surface. Marine ecosystems have water with higher salt content than
freshwater. This water is called seawater. Seawater is salty because salts and other minerals are deposited in it. Rivers and
streams carry salts and minerals that end up in the ocean or other marine ecosystem. Surface runoff deposits salts and
minerals into marine ecosystems as well. Over long periods of time, the accumulation of salt makes oceans and other marine
ecosystem salty.
Types of Marine Ecosystems
Oceans, saltwater marshes and lagoons are typical marine ecosystems. Oceans are the largest marine ecosystems. They are
enormous, open bodies of water. Saltwater marshes are a type of wetland. They are found along the coastline and form a
transition from ocean to land. They are areas of land that are covered with seawater. Lagoons are shallow bodies of water
that are found along the coast of continents and islands. They are separated from the ocean by a barrier such as a coral reef
or sandbar. Coral reefs are a special type of marine ecosystem. They are underwater structures in the ocean. Organisms
called corals make them. Corals are simple animals that secrete calcium carbonate. Calcium carbonate is the same substance
that makes up eggshells. Coral reefs are often called the “rainforests of the sea” because there is great biodiversity in the
reef. They provide a home for more than 25% of marine species. Estuaries are a different type of marine ecosystem.
Estuaries connect freshwater rivers or streams to the ocean. They are partially enclosed. Unlike other marine ecosystems, the
water in estuaries is brackish. Brackish water has more salt than freshwater but not as much as seawater. The water is
brackish because saltwater from the ocean and freshwater from the river or stream mix in the estuary.
Marine Zonation
We divide marine ecosystems into zones. This is called marine zonation. Each zone has distinct characteristics. The
characteristics determine the environment and organisms that live in each zone.
Intertidal Zone
The area along the coast is called the intertidal zone. During high tide, the intertidal zone is covered by seawater. During low
tide, the zone is exposed. Ocean waves crash onto the shore in the intertidal zone. Sunlight penetrates to the ocean floor in
the intertidal zone. The sun warms the water and so the water in this zone is warmer than other parts of the ocean. Many
animals live in the intertidal zone. Starfish, sea urchins, crabs, shrimp and snails are just some of the organisms that live here.
There is a lot of vegetation and seaweed here as well. Remember seaweed is made of algae, it is not a plant.
Neritic Zone
The neritic zone is a relatively shallow part of the ocean. The depth is no more than 200 meters. Sunlight can penetrate down
to the ocean floor in the neritic zone. Like the intertidal zone, the water in this zone is warmer than other parts of the ocean.
The neritic zone is located between the intertidal zone and the edge of the continental shelf. The continental shelf is an
extension of a continent. It forms an underwater perimeter around a continent. At some point, there is an abrupt end to the
continental shelf. This is called the continental margin. The depth of the ocean dramatically increases at the continental
margin. The neritic zone is where most marine organisms are found. Algae and phytoplankton are found here. They are
photosynthetic and form the base of the food chain in the neritic zone. Many fish and crustaceans live in the neritic zone.
Coral reefs are commonly found in the neritic zone as well.
Benthic Zone
The bottom of the ocean is called the benthic zone. It extends from the shoreline down to the abyss. The environment of the
benthic zone dramatically changes from the shallow waters to deep waters of the abyss. In shallow waters, the ocean floor
can be covered with filter feeders like sponges and sea stars and snails. Marine plants and algae can be found in the benthic
zone of shallow waters as well. Organisms that live on the ocean floor in the abyss are adapted to low oxygen and
MARINE ECOSYSTEMS DESCRIPTION PAGE (Page 2)
temperature conditions. Most of these organisms are scavengers that feed on detritus and dead organisms that float to the
bottom of the ocean.
Oceanic Zone
The oceanic zone is offshore water where the depth of the ocean is 200 meters or greater. The oceanic zone includes all the
water from the surface to the ocean floor. The zone begins at the end of the neritic zone at the continental margin. The
depth of the ocean varies greatly in the oceanic zone. Different organisms live in different depths of the oceanic zone. The
oceanic zone is divided into 5 regions: the sunlight zone (epipelagic), the twilight zone (mesopelagic), the midnight zone
(bathypelagic), the abyss (abyssopelagic) and the trenches (hadalpelagic). We will use the common names for the regions of
the oceanic zone. The scientific names are listed in parentheses after the common names.
Sunlight Zone (Epipelagic)
The sunlight zone receives enough sunlight to support photosynthesis. This region is also called the photic zone. The zone
includes the entire intertidal and neritic zone where sunlight penetrates to the ocean floor. It also includes water in the
oceanic zone. The sunlight zone extends to 200 meters below the surface of oceanic waters. Only very little sunlight can
penetrate deeper than 200 meters. Plants and algae are found in sunlight zone. They use energy from the sun to make food
and oxygen. This process is called photosynthesis. Plankton, jellyfish, tuna, sharks and dolphins live in this zone as well.
Twilight Zone (Mesopelagic)
The twilight zone receives very little sunlight. Photosynthesis cannot take place in this zone. Because no photosynthesis takes
place, there is very little oxygen in the water of this zone. The twilight zone extends 200 meters to 1,000 meters below the
surface. Organisms that live in the twilight zone are adapted to live in water with very little oxygen. Fish that live in this zone,
such as swordfish, have gills that are very efficient at extracting oxygen out of water. Squid and cuttlefish are found in this
region. They minimize their movement in the water and so require less oxygen to survive. Some organisms are
bioluminescent. This means they can produce their own light. This helps the organism see in the dark waters of the twilight
zone.
Midnight Zone (Bathypelagic)
The midnight zone extends from 1,000 to 4,000 meters below the ocean surface. The water is pitch-black dark in this region.
The water pressure is also very great in the midnight zone. There are very few organisms that live here. Most organisms that
survive in this region are bioluminescent. These organisms feed on detritus, or waste, that is produced by organisms in upper
layers of the ocean. The detritus falls into this zone. Scavengers such as the giant squid and dumbo octopus can survive in this
zone. The sperm whale can dive deep into this zone to feed on the giant squid.
The Abyss (Abyssopelagic)
The abyss is the second lowest region of the ocean. It extends from 4,000 meters to the ocean floor. The ocean floor is
around 6,000 meters below the ocean surface. It is very dark and very cold in this region. However, some species can survive
here. Some squids, sea cucumbers and sea spiders live here. Many of these species do not have eyes because there is total
lack of light in the abyss.
The Trenches (Hadalpelagic)
There are some very deep regions of the ocean called trenches. Trenches extend 6,000 to more than 10,000 meters below
the ocean surface. The Mariana Trench is the deepest trench in the world. It is located in the western Pacific Ocean. This
trench is 10,911 meters deep – almost 7 miles! You may be very surprised to learn that organism can live in trenches. Many
bacteria live near thermal vents that release heated water in the trenches. The bacteria convert chemicals in the heated
water into food. Tubeworms, sea cucumbers and jellyfish that live here feed on the bacteria. Many scientists believe the first
forms of life on Earth may resemble those creatures that live in the ocean trenches.