Mesopelagic zone

The middle ocean is a dim place.

It is like a permanent twilight. Very little sunlight reaches this deep.

Many animals live here. Some are giant squid. Others are small jellyfish. Some fish make their own light. This helps them in the dark.

Food falls from the top. It sinks down like snow. Small bits of food drift down. This helps the animals eat.

Some animals move a lot. They swim up at night. They go back down during the day. This helps them stay safe.

This part of the sea is huge. It covers much of our world. It is a very busy place.

The mesopelagic zone is a vast part of the ocean. Some people call it the twilight zone. It sits between the sunny surface and the dark deep.

Sunlight is very dim here. Only 1% of light reaches these depths. This zone covers 60% of the Earth's surface.

Many animals live in this dim world. You might find giant squid or blobfish here. Some creatures use bioluminescence. This means they make their own light to see or talk.

Many animals also use diel vertical migration. This is a way of moving up and down. They swim to the surface at night to eat. Then they hide in the dim water during the day.

Food travels through this zone in a way called the biological pump. Organic matter sinks from the surface like falling snow. Tiny microbes and fish eat this food. This helps recycle nutrients back into the water.

The zone also has a special sound layer. It is called the SOFAR channel. In this layer, sound travels very slowly.

The mesopelagic zone is a huge part of our ocean. Some people call it the twilight zone. It sits between the sunny surface and the pitch-black deep.

This zone starts where only 1% of sunlight can reach. It ends where there is no light at all. This area covers 60% of the Earth's surface. It also makes up about 20% of the ocean's total volume.

This layer is a very important part of the biosphere.

Water in this zone changes in many ways. There are layers called the thermocline, halocline, and pycnocline. These names describe sharp changes in heat, salt, and density.

These changes create different layers of water. These layers affect how nutrients and gases mix together. This makes the mesopelagic zone a very dynamic place. The temperature can change from warm at the top to cold at the bottom. The salt levels usually stay between 34.5 and 35 psu.

Scientists have studied this zone for a long time. During World War II, the US Navy used a special sound layer. This is called the SOFAR channel.

Sound travels very slowly in this specific layer. The Navy thought it could save lives during shipwrecks. Survivors could drop an explosive to show their location. In the 1950s, the Navy also used it to find submarines. They built a system of underwater microphones called SOSUS. Later, oceanographers used this to track deep ocean currents.

Many unique animals live in this dim world. You might see a giant squid or a blobfish.

Many creatures use bioluminescence to make their own light. This helps them talk, hunt, or hide. Some animals use diel vertical migration to survive. They swim to the sunny surface at night to eat. Then they hide in the dim water during the day.

This movement is so thick it can even confuse sonar. Some fish in this zone weigh as much as 10 gigatonnes.

This zone also helps move carbon through the ocean. This process is called the biological pump. Organic matter sinks from the surface like falling snow. Tiny microbes and fish eat this sinking food. This is called remineralization. It recycles nutrients back into the water. Some bacteria can even make energy without any oxygen. This is called chemosynthesis. This helps keep the ocean's cycles moving smoothly.

The mesopelagic zone is a massive region of the ocean. Many scientists call it the twilight zone. This name describes the dim light found there. It sits between the sunny epipelagic zone and the dark bathypelagic zone.

The zone begins where only 1% of sunlight can reach. It ends where no light exists at all. This area covers about 60% of the planet's surface. It also holds 20% of the ocean's total volume. This makes it a vital part of the global biosphere.

Physical conditions in this zone change very rapidly. These changes create distinct layers of water. A thermocline marks a sharp change in temperature. A halocline shows a change in salinity, or salt levels. A pycnocline marks a change in water density.

Temperature can drop from warm surface levels to much colder depths. Salinity usually stays between 34.5 and 35 psu. Water density ranges from 1023 to 1027 g/L. These layers prevent easy mixing of nutrients and gases. This stratification makes the mesopelagic a very dynamic environment.

This zone also has unique acoustic features. At its base, between 600 and 1,200 meters, lies the SOFAR channel. In this layer, sound travels at its slowest speed. This happens because of changes in salinity and temperature. The channel acts as a wave-guide for sound waves.

During World War II, the US Navy proposed using it for rescues. Survivors could drop explosives to signal their location. In the 1950s, the Navy used the Sound Surveillance System, or SOSUS. This array of hydrophones helped detect Soviet submarines. Oceanographers later used SOSUS to track deep ocean currents using SOFAR floats.

Life in the mesopelagic is highly specialized. Many creatures use bioluminescence to create their own light. This helps them communicate, attract prey, or deter predators.

Some animals use diel vertical migration to survive. They swim to the sunny surface at night to feed. During the day, they retreat to the safety of the dim mesopelagic. This movement involves a massive amount of biomass. Some estimates place the weight of mesopelagic fish at 7 to 10 gigatonnes. This layer of life is so thick it can even confuse sonar.

The mesopelagic zone is essential to the ocean's biological pump. This process moves carbon from the surface to the deep. Sunlight drives photosynthesis in the upper layers. This produces organic carbon. A portion of this carbon sinks into the mesopelagic. Particles sink at rates of 10 to 100 meters per day. Larger fecal pellets sink faster due to lower friction. In this zone, microbes perform remineralization. They break down organic matter and release nutrients. It is estimated that 90% of exported carbon is respired here.

Oxygen levels can be very low in this zone. This creates an oxygen minimum zone, or OMZ. This happens because the zone lacks direct contact with the atmosphere. Also, the sinking organic matter uses up oxygen during respiration.

The OMZ is most severe in the eastern tropical Pacific and tropical Indian Oceans. In some anoxic regions, life uses chemosynthesis. This means organisms make energy using compounds like sulfide or ammonia. This process is comparable in rate to other forms of production. It allows life to persist even when oxygen is scarce.

Microbes and viruses play a huge role in this ecosystem. Viruses are very abundant, with up to $10^{12}$ per cubic meter. They help drive the microbial loop by recycling organic matter.

Some microbes are chemoautotrophs. For example, certain Archaea can oxidize ammonium without any oxygen. This helps drive the nitrogen and carbon cycles. While microbial diversity declines with depth, their impact remains massive. The mesopelagic zone connects the surface world to the deep ocean through these complex cycles.