The ocean is a place of mystery and legend, of discovery and tragedy, and it doesn’t give up its watery secrets. Mankind has attempted to understand the ocean for thousands of years. Today, thanks to scientific advancements, we’ve divided the ocean into 5 distinct zones. These zones have different environmental conditions that support diverse life forms.
In this article, we’ll explore the 5 zones of the ocean. Each zone has its own characteristics as well as the characteristics of the nearby shore. The life forms living in these zones are well adapted to the demands of high pressure, little or no light, and sparse prey.
The ocean is one of the last unexplored frontiers on Earth. Many advancements have been made in the last 100 years, but we still have much more to go. Scientists estimate that only 1/3 of species have been discovered and only 20% of the ocean floor has been mapped. Marine biologists hope that technological advancements will allow people to journey deeper into the ocean’s depths than ever before.
Let’s embark now on a journey through the levels of the ocean. We’ll examine the depth, temperature profile, and organisms that call each level home.
The 5 Zones of the Ocean
Before we examine the five main zones of the ocean, let’s discuss the difference between pelagic and benthic zones. These zones aren’t part of the five main zones. Instead, they act as umbrella labels to identify the difference between the first four zones and the last zone.
Pelagic zones refer to ocean waters that are not constrained by the ocean floor’s topography. These waters have strong currents that move freely.
In fact, the word originates from the Greek word pelagos, which means ‘open sea.’ The pelagic zones are the epipelagic, mesopelagic, bathypelagic, and abyssopelagic zones.
The benthic zones are the zones on the ocean floor. They are at the very base of the oceanic water column. Only one of the 5 primary zones, the hadopelagic zone, is a benthic zone.
Now, let’s examine each of the five zones of the ocean. Remember that you might see them named differently in textbooks or scientific literature. We’ll provide all of the alternative names so you can easily recognize the right ocean zone.
1. Epipelagic
The epipelagic zone of the ocean is the uppermost level that is closest to the surface. It’s also the most shallow, extending downwards only up to 200 meters (650 ft). Conditions in the epipelagic region support a vast quantity of aquatic life.
This zone is also known as the ‘sunlight’ zone because it is the only layer of the ocean sunlight is capable of penetrating. Water’s chemical properties allow it to easily absorb sunlight. This means that the further down you descend, the more dim it gets.
This measure of how much sunlight the water absorbs is called visibility. Ocean water with high levels of dissolved sediment absorbs more sunlight than seawater without particulate matter. Since the epipelagic zone contacts the coastlines of the continents and island chains, there are many opportunities for sand and sediment from the coasts and beaches to cloud the water.
Temperatures
Temperature fluctuates heavily in the epipelagic zone. Sunlight, which heats up the surfaces it contacts, warms the water temperature depending on how strong the rays are.
Surface water around the Earth’s equator is warmer than surface water at the poles, for example. The hottest water temperature found on the surface of the ocean is 97 degrees Fahrenheit, which was measured in the Persian Gulf.
Wind constantly changes the temperature of the water in the epipelagic zone. If you’ve ever seen a video of towering waves in the midst of a storm, you’ll notice that tens or even hundreds of feet of water builds up and crashes in waves.
Wind is a primary cause of wave movement in storms. Such movement mixes up the temperature gradient that forms in a relatively still ocean layer and also increases the dissolved oxygen available for life forms.
Life
Speaking of life forms, there are millions in the epipelagic layer of the ocean. Biologists label the epipelagic zone the ‘photic zone’ because of the amount of photosynthesis that occurs here.
Phytoplankton that live in this sunlight zone do 50% of all photosynthesis on earth. When microscopic and miniscule organisms use photosynthesis to create oxygen and sugar from sunlight, it is called primary production. They are the first link in the ocean’s complex food chains.
All kinds of animals call this zone home. In fact, about 90% of all marine life lives in the epipelagic zone. Their home ranges depend on what they eat, what kinds of habitats they prefer, and the temperature of water they have adapted to.
Marine mammals, fish, sharks, and mollusks that live in the epipelagic zone include well-known and rare animals. Most of the ocean’s food chains all take place in this zone. Bony fish, such as tuna, salmon, and herring, live in the open ocean.
Bottlenose dolphins hunt for fish in this zone. Great blue whales swim through underwater clouds of krill. Sharks hunt for schools of fish and scent blood from miles away.
Bottom-feeding mollusks, fish, and rays forage for food and plant matter along the shallow edges of the epipelagic zone, which is part of the continental shelf. Most marine mammals, such as otters, seals, and manatees, spend their lives close to the surface and the coast.
Coral reefs
Coral reefs are at the edge of the epipelagic. Biodiversity, which is very high in these zones, supports a fantastic array of plant and animal life.
Invertebrate anemones thrive among stands of coral and clownfish hide among their tentacles. Parrotfish, which eat sponges, dart around the reefs. They are prey to the lemon shark, which roams through the coastal epipelagic in search of food.
Sea turtles and other marine reptiles live here. All of the marine reptiles that can’t breathe underwater must remain close to the surface in order to get enough air. Sea turtles are more efficient than many other
2. Mesopelagic
The mesopelagic zone is also called the ‘dysphotic’ zone or the ‘twilight’ zone. Sunlight doesn’t penetrate fully into the water here, so it is much dimmer than the zones above. The depth of this zone goes from 200 meters (650 ft) to 1,000 meters (3,300 ft) under the surface of the ocean. It is distinctly less hospitable than the euphotic zone.
Mesopelagic organisms have rare traits like having translucent skin or being bioluminescent. Invertebrates like squid and octopi are great examples of mesopelagic residents. Eels and jellyfish thrive here too.
Tiny yet numerous zooplankton also reside in the mesopelagic zone. All mesopelagic animals are carnivorous; there isn’t enough sunlight to support photosynthesis.
Marine animals
Mesopelagic marine animals don’t spend their entire lives in the mesopelagic zone. They migrate up to the epipelagic zone each evening. Once the sun sets, they swim upwards to eat phytoplankton on the surface of the ocean. Others prey on the fish that are busy eating phytoplankton.
Anglerfish are common inhabitants in the dysphotic zone. This fish has a bioluminescent lure on its forehead. When a fish swims close to investigate, it chomps down. There’s lunch!
The mesopelagic zone occasionally has visitors from above. Cetaceans, the group of marine mammals that includes dolphins, orcas, and whales, include some members that can dive to profound depths.
Sperm whales can dive up to 1,000 meters (3,280 ft) underwater. They do so when hunting squids, which prefer the bathypelagic, mesopelagic and lower epipelagic zones. This includes giant squids, which can reach over 5.5 meters (18 ft long)!
3. Bathypelagic
The bathypelagic zone is where the pressure is on! This lightless zone of the ocean is also called the ‘aphotic’ or ‘midnight’ zone because it has no light. It extends from 1,000 m (3,300 ft) to 4,000 m (13,000 ft). No organisms that use photosynthesis can survive here; all are carnivorous.
Unlike mesopelagic fish, bathypelagic fish rarely venture up into the higher zones. Instead, they subsist on scavenged material that falls from the upper layers.
Marine biologists call this organic material ‘marine snow’ since it has broken down into tiny pieces that appear like snowflakes. It is a primary way that nutrients are transferred from the productive surface down to the less productive bathypelagic zone.
Bathypelagic fish are less understood than fish in the upper two zones. Many specimens are hard to study because they are adapted to intense levels of pressure that would kill divers. Expeditions to retrieve specimens are limited to submarine expeditions and unmanned craft.
Catching a bathypelagic fish is a difficult feat. Maintaining the fish’s body structure once it’s caught is even harder.
When such a fish is reeled in, its body usually has difficulty maintaining structural integrity in the surface’s lower-pressure environment. Fish from this depth have no scales, are almost all water, and are slippery and slimy.
Anglerfish, viperfish, gulper eels, bristlemouth, and other fish with needle-sharp teeth live in the bathypelagic zone. What sets them apart from other ocean fishes is how they lie in wait to ambush prey instead of pursuing it directly. The ambush method works better at this depth because it requires less energy than an outright chase.
Traits in common
Animals in the abyssal zone have some interesting traits. Firstly, they are bigger than their shallow relatives. This trait is called ‘abyssal gigantism.’
It’s the tendency for deep-sea creatures to be larger on average than similar creatures in shallow depths. The giant squid is a great example of abyssal gigantism.
A second unique trait of deep sea animals is extreme sexual dimorphism. It is when the adult male and female of a species are different sizes, colors, or have different physical characteristics related to mating and raising young.
Anglerfish present an extreme example of sexual dimorphism. Males are a fraction of the size of females. When the two meet, the male bites the female and fuses to her body in an irreversible connection.
4. Abyssopelagic
This zone of the ocean isn’t well studied because it’s simply so hard to get to. Much of what we know about the zone is thanks to sonar and occasional submarine expeditions. Its boundaries are from 4,000 to 6,000 meters (13,000 to 20,000 ft).
Almost all of the ocean – 83% – is the abyssal zone. Water pressure here is hundreds of times that on the surface; almost nothing survives. The base of the abyssal zone is the ocean floor. Marine snow that isn’t consumed by animals in higher zones lands and accumulates at the bottom of the ocean.
Most of the animals in the abyssal zone subsist on the marine snow. Others, such as bacteria, live near underwater vents. It’s pitch black here, so photosynthesis is out of the question.
Shrimp, mollusks, crustaceans, and fish live in the abyssal zone. Most animals have little or no pigmentation since there isn’t any light. Some of the only animals that live in the abyssal zone are tripod fish, cusk eels, and the dumbo octopus.
5. Hadopelagic
The hadopelagic zone is the deepest level of the ocean. It’s also the only one of the 5 zones to be in the benthic category instead of the pelagic.
The hadopelagic is a benthic zone because it is technically below the ocean floor. The hadal zone, which was named after Hades, Greek mythology’s god of the underworld, is any part of the ocean deeper than 6,000 meters (20,000 ft).
We know about the topography of the ocean floor thanks to sonar scanning. In the mid-twentieth century, geologists scanned the oceans with sonar. To determine the depth of the water, they sent a sonar signal straight down into the water.
The signal ‘pinged’ off of the ocean floor and bounced back up. By measuring how long it took for the ping to return to the ship, they could measure the depth of the water.
It may come as a surprise to learn that the ocean floor is not a flat plain. The bottom of the sea is a varied landscape with distinct geological features. Underwater ridges and valleys dot the ocean floor in a topography similar to mountains, valleys, and plains on dry land.
Underwater valleys are called ‘trenches.’ Trenches contain some of the coldest water and rarest animal life on earth. The animal life down here in the hadal zone is rare. Pressure is over 700 times that on the surface. There is no light and little food.
Humans have yet to explore the trenches in the open ocean, so there is great potential for more species to be discovered. The Mariana snailfish and the Cusk eel are some of the only known animals that survive this deep in the ocean.
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