The big, open ocean is a home.
Many animals swim in the deep water. Some fish live near the sun. Others live where it is dark. It is a very big place.
Can you swim in the sea?
The open ocean is a huge home.
It has many layers of water. The top layer has bright sunlight. This light helps tiny plants grow. Most sea life lives near the top.
As you go deeper, it gets cold. It also gets very dark. The water pressure grows too.
Some animals live in the deep. They may have no eyes. Others use their own light.
Many birds fly over the waves. They find food in the sea. The ocean is a busy place.
The pelagic zone is the open part of the ocean.
It is the water far away from the shore. In this zone, animals can swim freely in any direction. The ocean is split into five layers by depth.
The top layer is the epipelagic zone. This is the sunlight zone. Most sea life lives here, like whales and sharks. Tiny plants called phytoplankton live here too. They use sunlight to make food.
Below that is the mesopelagic zone. This is the twilight zone. It has very little light. Some fish here use bioluminescence, which is light made by living things. Deep down is the bathypelagic zone. This is the midnight zone. It is pitch black.
The next layer is the abyssopelagic zone. It is very cold and dark. The last layer is the hadopelagic zone. This is the deepest part. It is found in deep trenches.
As you go deeper, things change. The water gets colder. The pressure gets much higher. There is also less food for animals to eat.
The pelagic zone is the vast, open part of the ocean.
It is the water column away from the shore. In this zone, marine life can swim freely in any direction. Unlike the benthic zone at the very bottom, the pelagic zone is the open water. It covers about 71 percent of the Earth's surface. The average depth of the ocean is 3,688 meters. Some parts reach a maximum depth of 10,924 meters.
Conditions change as you travel deeper into the water. Light and temperature both decrease as you go down. The water pressure gets much higher the deeper you dive. Salinity and oxygen levels also change with depth. The water column can be divided into five vertical layers. These layers depend on how deep the water is. This is similar to how the Earth's atmosphere has different layers.
Each layer has its own special environment. The epipelagic zone is the sunlight zone at the surface. Nearly 90 percent of marine life lives here. This includes whales, sharks, and jellyfish. Below that is the mesopelagic zone, also called the twilight zone. It has very little light. Some creatures here use bioluminescence, which is light made by living things. The bathypelagic zone is the midnight zone where it is pitch black.
Deep below are the abyssopelagic and hadopelagic zones. The abyssopelagic zone is near the ocean floor. It is very cold and dark. The hadopelagic zone is the deepest part of all. It is found in narrow trenches. These trenches can be 5 to 10 kilometers deep. In this zone, the pressure can reach 1,100 atm. Very few creatures can live in such extreme conditions.
Everything in the pelagic zone is linked together. The ecosystem starts with tiny phytoplankton. These tiny plants use photosynthesis to make food. They stay in the sunlit top layer. When they die, they fall as "marine snow" to deeper levels. Whales also help by moving nutrients. This is called the whale pump. Whales feed deep down and bring nutrients to the surface.
The pelagic zone is the vast, open water column of the ocean. It is defined as the area away from the shore where marine life can swim freely in any direction. This zone is not the seafloor, which is known as the benthic zone. Instead, the pelagic zone is the water itself, stretching from the surface down to the very bottom. It covers about 71 percent of the Earth's surface. The mean depth of the ocean is 3,688 meters. However, the maximum depth can reach 10,924 meters.
Conditions in the water column change significantly as you descend. As depth increases, the amount of light and the temperature both decrease. At the same time, the water pressure increases. Other factors like salinity, oxygen, and micronutrients such as iron, magnesium, and calcium also change. These changes create different environments. Scientists divide the water column into up to five vertical layers. This stratification is similar to how the Earth's atmosphere is divided into layers.
The first layer is the epipelagic zone, also called the sunlight zone. This is the only region with enough light for photosynthesis. Photosynthesis is the process where plants use sunlight to make food. Because of this light, nearly all primary production occurs here. About 90 percent of all marine life is concentrated in this top layer. It includes plankton, jellyfish, tuna, whales, sharks, and dolphins. This zone sits just above the continental shelf.
Below the sunlight zone is the mesopelagic zone, or the twilight zone. This layer is located right under the continental shelf. It contains only a trace amount of sunlight. The pressure here ranges from about 20 to 100 atm. A thermocline, which is a layer where temperature changes quickly, acts as the boundary from the warmer top zone. Many creatures here, such as squid and swordfish, have evolved bioluminescence. Bioluminescence is the ability to produce light through chemical reactions in the body.
As you go deeper, you enter the bathypelagic zone, known as the midnight zone. This environment is pitch black with no trace of sunlight. The temperature and salinity in this zone remain stable. Since no plants can live here, many creatures survive on detritus. Detritus is organic matter that falls from upper zones, often called "marine snow." Some animals, like the marine hatchetfish, survive by preying on other inhabitants. The anglerfish is a famous example of a creature that uses bioluminescence in this dark space.
The next layer is the abyssopelagic zone, or the abyssal zone. This zone is near the ocean floor, where tectonic plate movements form volcanoes and mountains. It is characterized by cold temperatures, high pressure, and complete darkness. Only a few specialized creatures live here. These include species of squid, sea spiders, and echinoderms like the sea pig. Many animals at these depths are transparent or have no eyes at all.
The deepest region is the hadopelagic zone, named after Hades. This zone is found in narrow ocean trenches and troughs. These depths can reach more than 10,000 meters. The deepest trenches stretch to 10,924 meters, while average trenches are 5 to 10 kilometers deep. The atmospheric pressure here can reach 1,100 atm. Interestingly, temperature can actually increase here due to adiabatic heating. Very few creatures live in this extreme environment, including some mollusks and amphipods.
The pelagic ecosystem is built upon phytoplankton. These tiny organisms manufacture food through photosynthesis in the sunlit zones. Interestingly, marine biomass pyramids are often inverted at the base. This means the biomass of consumers, like krill and forage fish, is larger than the biomass of primary producers. This happens because phytoplankton grow and reproduce very rapidly. They also sequester about 2 billion tons of carbon dioxide each year. This process helps the ocean act as a sink for carbon dioxide.
Whales play a vital role in moving nutrients through the water. This process is known as the whale pump. Whales feed on krill in deeper levels and return to the surface to breathe. When they defecate, they release liquid rich in nitrogen and iron. This liquid stays at the surface where phytoplankton can consume it. In the Gulf of Maine, the whale pump provides more nitrogen than rivers do.
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