Ocean zones from sunlight to midnight

Imagine diving down into the sea.

At the top, the water is bright and blue. As you go down, it gets darker and colder. Far down, it is dark all the time. Each level is called a zone.

The ocean is not the same all the way down. Scientists at Woods Hole Oceanographic Institution (WHOI), an ocean research institution, divide it into five zones stacked like layers: the sunlight zone, the twilight zone, the midnight zone, the abyssal zone and the hadal zone.

Three things change as you go deeper. Light fades, because water soaks up sunlight. The water gets colder. And the weight of all the water above pushes harder and harder: this push is called pressure. Each zone has its own living things, suited to its light, cold and pressure.

Oceanographers classify the water column into depth zones. WHOI uses five: sunlight (epipelagic), twilight (mesopelagic), midnight (bathypelagic), abyssal (abyssopelagic) and hadal, reaching down to roughly 11,000 meters. NOAA's National Ocean Service sorts the same column first by light: the euphotic zone (top 200 m), the dysphotic zone (200 to 1,000 m) and the aphotic zone below 1,000 m, which it subdivides into the bathypelagic, abyssopelagic and hadopelagic zones.

The boundaries are conventions, and sources draw them a little differently; WHOI, for example, places the abyssal zone at 3,000 to 6,500 m, while NOAA gives 4,000 to 6,000 m. What every scheme shares is the trend: with depth, sunlight fades, temperature falls toward about 4 °C, and pressure rises all the way to the seafloor.

The top of the sea is the sunlight zone.

Sunshine reaches down here. Tiny floating plants use it to grow. Fish, sea turtles and whales live here too.

The sunlight zone is the top layer, from the surface down to about 200 meters. WHOI explains that water soaks up light so strongly that sunlight can power photosynthesis, the way plants and phytoplankton make food, only down to about this depth.

This thin layer is full of life. Tiny drifting plant-like living things called phytoplankton use sunlight to make food, and WHOI says they make about half of the oxygen we breathe. Tiny animals called zooplankton eat them, and they in turn feed animals from small fish to huge blue whales. NOAA notes that this zone holds the vast majority of commercial fisheries, along with many protected sea mammals and sea turtles.

The sunlight, epipelagic or euphotic zone extends to about 200 m (656 ft), the depth to which sunlight is strong enough to drive photosynthesis. Over the continental shelf it reaches all the way to the seafloor; in the open ocean, where depths can reach 4,000 m or more, it is a thin surface layer. WHOI notes that it makes up only 2 to 3 percent of the ocean by volume, yet it hosts an enormous share of marine life.

Phytoplankton such as diatoms and photosynthetic bacteria form the base of the food web, produce about half of the oxygen we breathe and draw carbon dioxide from the atmosphere. Zooplankton graze on them and feed larger animals, from small fish to blue whales. NOAA adds that this zone holds the vast majority of commercial fisheries and many protected marine mammals and sea turtles.

Below the sunlight zone, the water is dim, like evening.

There is too little light for plants. Many animals here make their own light! At night, lots of them swim up to the top to eat.

From 200 to 1,000 meters down is the twilight zone. A little faint light still reaches it, but not enough for plants or phytoplankton to grow. The water is cold.

Many twilight zone animals, from tiny microbes to jellies, glow with their own light. This is called bioluminescence. Some small fish have glowing spots on their bellies that match the faint light from above, so hunters looking up from below can hardly see them. And every night, huge numbers of fish, squid and plankton swim up toward the surface to feed in the dark, then sink back down at daybreak. WHOI calls it the largest animal migration on the planet, and it happens every 24 hours.

The twilight or mesopelagic zone lies between 200 and 1,000 m (about 650 to 3,300 ft), beyond the reach of most sunlight; NOAA notes that so little light gets below 200 m that photosynthesis is no longer possible. The water is cold, and pressure can reach 1,500 pounds per square inch.

Many residents use bioluminescence to hide or hunt. Some prey fish practice counter-illumination, with light organs called photophores along their bellies that match the surface light above, making them nearly invisible from below; some predators dangle a glowing lure. Every 24 hours many animals rise hundreds or thousands of feet to feed at night and descend at daybreak: the largest animal migration on Earth, according to WHOI. In World War II, U.S. Navy sonar showed what looked like a seafloor rising toward the surface after dark; it was this migrating layer of animals. Recent studies suggest the zone's fish biomass may exceed that of all the rest of the ocean combined, and the small bristlemouth may be the most abundant vertebrate on Earth.

Deeper still is the midnight zone.

No sunlight gets here at all. It is dark and cold all the time. The only light comes from glowing animals.

Below 1,000 meters, the last sunlight is gone. This is the midnight zone, and it reaches down to about 4,000 meters. It is dark all the time, and WHOI says the water stays at about 4 °C (39 °F).

The midnight zone is enormous: it holds about 70 percent of all the seawater on Earth. Animals here use their own light to catch food and find mates, and many have good eyes and hearing. Some visitors come from above: NOAA says sperm whales dive to hunt giant squid at 7,000 feet (more than 2,000 meters) or deeper.

WHOI explains why the dark begins here: water absorbs red, orange and yellow light quickly, leaving blue, and by about 1,000 m even the faintest light is gone. The bathypelagic or midnight zone extends from there to about 4,000 m, which is the ocean floor in many places. Its temperature holds near 4 °C and pressure ranges from about 100 to 400 atmospheres.

It is by far the biggest of the zones, holding 70 percent of all seawater, yet it is poorly understood. Its animals rely on bioluminescence to lure prey and attract mates, and on keen vision and hearing. Because they do not detect red wavelengths, newer imaging systems on remotely operated vehicles use red-filtered light that the animals cannot see. Recent studies have also found abundant microbes here, recycling carbon that sinks from the surface.

At the bottom of the sea is the abyss. It is very dark and very cold.

In a few places, deep cracks called trenches go down even farther. They are the deepest places in the ocean.

Below the midnight zone is the abyssal zone, or the abyss. It reaches the wide, dark sea floor, thousands of meters down. Food is scarce: many animals here depend on the little bits of food that drift down from above. Scientists have found animals in the abyss that live nowhere else on Earth.

The deepest zone of all, the hadal zone, goes from 6,000 meters to about 11,000 meters. It is found only in deep trenches in the sea floor. WHOI says that, added together, the trenches cover an area about the size of Australia. The water there is just above freezing, and at the deepest points the pressure is more than 1,000 times what we feel at the surface.

WHOI defines the abyssal zone as the seafloor and water column from 3,000 to 6,500 m (NOAA's abyssopelagic zone is 4,000 to 6,000 m). Pressure there can reach 600 atmospheres. Researchers have sampled the abyssal seafloor since the 1960s and found surprisingly diverse communities, including animals found nowhere else, sustained by the limited carbon that sinks from above, by whale falls, or by chemicals from hydrothermal vents.

The hadal zone, from 6,000 to about 11,000 m, occurs only in trenches, most of which form where one tectonic plate is subducted beneath another. Combined, the trenches cover an area about the size of Australia. Temperatures hover just above freezing, and at the deepest points pressure exceeds 1,000 times that at the surface. To explore them, WHOI and NASA's Jet Propulsion Laboratory developed small autonomous underwater vehicles, built to withstand that pressure and work alone or in groups.

Water is heavy. The deeper you go, the more water is on top of you.

That water presses on you. Deep-sea animals are built for the squeeze.

Air presses on us all the time, but we don't feel it, because our bodies push back just as hard. Dive just a few feet underwater, though, and you can feel the water pressing on your ears.

NOAA explains that for every 10 meters (33 feet) you go down, the pressure grows by the same amount as the whole weight of the air pressing on us at the surface. Scientists call that amount one atmosphere. So at 1,000 meters, the water presses about 100 times harder than the air does. Many sea animals cope well: whales have bendable ribs, and their lungs can collapse safely under pressure.

At sea level the atmosphere presses on us with 14.7 pounds per square inch, which we don't notice because the fluids in our bodies push outward equally. Underwater, hydrostatic pressure, the force per unit area exerted by a liquid, adds one atmosphere for every 33 feet (10.06 m) of depth, NOAA explains. That gives roughly 100 atmospheres near 1,000 m, the 100 to 400 atmospheres WHOI gives for the midnight zone, and more than 1,000 times surface pressure in the deepest trenches.

Many marine animals have no trouble with this. Whales withstand dramatic pressure changes because loose, bendable cartilage binds their ribs, letting the rib cage collapse, and their lungs can collapse without rupturing. Explorers need strong vehicles instead: NOAA's example is Pisces V, a three-person submersible that can operate at depths up to 6,500 feet. The What If? lab on diving to the deepest trench lets you follow the squeeze all the way down.