Defining the Deep Sea

How is the deep sea defined? Many assign an exact depth limit to this zone: beginning at 1000 fathoms (1828 m). However, conditions at exact depths vary from place to place, so deep-water ecologist Tony Koslow defines it as the area below which photosynthesis can function. In general, that means that anything below about 600 feet (200m) is considered the deep sea. In this short biome video we discuss the deep sea while going to 2,000 feet in a homemade submarine. We hope it is informative and useful as you’re learning about this unique habitat.

The Zones of the Deep Sea

The ocean is a vast body of water that stretches from tropical waters to frigid polar regions, from shallow seas to deep ocean trenches. The oceans cover about 71 percent of Earth’s surface, and about 90 percent of that is considered deep water (below about 600 feet). This vast, pelagic area is further divided into Deep Sea Zones. Each zone has characteristic life and conditions. Below is a short description of each.

Mesopelagic Zone: Also called the “Twilight Zone,” the Mesopelagic zone still receives some light, but it is not enough to fuel photosynthesis. Organisms here have adaptations to see and keep from being seen. Eyes of animals in this zone are often quite large, to collect any available light. Bioluminescence is prevalent here, helping creatures disguise their silhouettes from animals below them . Most sources give the depth range of this zone as 200 meters down to 700 or 1000 meters, but the exact depth is dependent on the clarity of the water, determining where photosynthesis can no longer take place.

Bathypelagic Zone: The Mesopelagic Zone becomes the Bathypelagic Zone when light no longer penetrates the water. It is a depth that in most open ocean systems is about 1000 meters, but can be much shallower along coastlines. For instance, in Honduras (where Karl Stanley flies his submarine) the depth is closer to 550 meters, due to the proximity to land. This zone is also marked by very low temperatures (5 or 6 degrees Celsius) and having a very low organismal biomass, a trend that will continue until reaching the ocean floor.

The boundary between the mesopelagic zone and the bathypelagic zone contains The Deep Scattering layer – a layer of fish, squid, crustaceans etc, that migrate each day from the deep ocean to the shallows at night. Scientists noticed a huge, scattered sonar signal that was deep during the day and rose to shallower water as night fell. It was perplexing at first. Instead of one, giant creature that moved up and down the water column, it turned out to be countless small creatures in their daily migration from deep water protection to feeding in shallower water at night.

Abyssopelagic Zone: This zone is also called the Abyssal Zone or the Abyss – derived from the Greek word meaning “bottomless sea.” It refers to waters directly above the continental rise (at about 3000 meters) down to about 6000m. The exact line between the Bathypelagic Zone and the Abyssopelagic Zone is difficult to define, but it is often defined as the area where the water hits 4 degrees Celsius. It is also marked by darkness and uniform environmental conditions.

The average depth of the ocean is 4000m (about 2.5 miles), making the Abyssal Zone the largest living environment on Earth. It covers over 300,000,000 square km (115,000,000 miles)–about 83% of the ocean’s total area and 60% of Earth’s surface.

The conditions of the Abyssal Zone are almost constant. It is dark and cold at all times (averaging 2 degrees Celcius at 4000 meters). It is calm and unaffected by sunlight and turbulent seas, far above. The water is very still, and the oxygen in the water derives solely on the amount of dissolved oxygen it had when it left the polar regions, from which all Abyssal water originates. No photosynthesis takes place to replenish oxygen, but the creatures here do consume more than is provided. The nutrient salt concentration (nitrogen, phosphorus and silica) is uniform and higher than in the waters above because the Abyssal and Hadal Zones are the areas where dead biological materials settle to the floor.

The composition of the sea floor varies with depth. In areas where the sea floor is shallower than 4000 meters, the floor is mainly composed of calcareous shells of foraminiferan zooplankton and phytoplankton, like coccolithophores. If the sea floor is deeper than 4000 meters, the shells tend to dissolve, and the sediment is mainly composed of brown clays, siliceous remains of radiolarian zooplankton and phytoplankton, such as diatoms.

In spite of the fact that water near the sea floor is devoid of oxygen, there is a spike in biomass here. This is because the collection of sediment gives a food source. All major marine invertebrate phyla and many fish are represented at this depth, although there are few species and low numbers of each. Anything that lives here with a stalk must grow above that oxygen-poor layer, and anything with legs usually has long legs to walk above it. Fish at these depths often have huge, gaping, underslung jaws to sift through the sand and catch food.

Some creatures, like the anglerfish use a combination of a huge mouth and bioluminescence to catch prey, though carnivores and scavengers are much less abundant here than animals that feed on sea-floor mud and suspended matter.

Animals here must withstand pressures of up to 11,000 psi. They tend to be grey or black (for camouflage) and unstreamlined (for energy conservation). Many are blind, and they are thought to reproduce very slowly. Some examples of deep sea life here are the tripod fish, anglerfish and giant squid.

Hadalpelagic Zone: The Hadal Zone or the Hadalpelagic zone is the layer of the deep sea below 6000 meters. Its found almost exclusively in deep ocean trenches. Here the pressures are tremendous, making it humanity’s least explored place. More people have been to the moon than to the Marianas Trench – the deepest part of the ocean at approximately 11,021 meters (36,160 feet). (Ref. NODC Frequently Asked Questions )

An example of a Deep Sea Organism: Blunt-nosed Sixgill Shark

Challenges to Life and Adaptations to the Deep Sea

Intense Pressure: The deeper you go, the more intense the pressure. At the surface, there is one atmosphere of pressure; for every ten meters you go underwater, the pressure increases by one atmosphere. When we were down in Karl’s sub at 2000 feet, the pressure was 66 times the pressure at the surface; the pressure on the sub is 905 Pounds/Square Inch. Humans would be crushed at this depth (imagine what it would feel like to put a a bucket of water on your head. Then picture the pressure of thousands of bucket of water pressing down on you). One way some animals have adapted to this pressure is that they have no air spaces.

Cold: The deep sea has extremely low temperatures. In fact, the beginning of the Abyssal Zone is conveniently defined as the area where water plummets to 4 degrees Celsius. At the ocean’s average depth off 4000m, the average temperature is 2 degrees Celsius. Brrr! Fish in this cold environment tend to move and reproduce very slowly.

Darkness: Below the Epipelagic Zone, there is not enough sunlight for photosynthesis, and below the Mesopelagic Zone no sunlight penetrates. Animals in these areas of low to no light have many adaptations. Some have very large eyes to catch any small amount of light. Other emit their own light with bioluminescence, disguising their silhouette, attracting food or attracting a mate. It is thought that 90 percent of all deep sea animals have bioluminescence. Many of the animals in very low light are transparent, red or black in color. In the deep sea, red and black look the same, hiding the animal in the darkness.

Low Biomass: There is large biomass at the surface where the variety of ocean creatures are typically observed. Descending through the water column, the biomass decreases to a very small amount. The small biomass stays relatively constant until reaching the ocean floor, where the number of organisms increases again. This occurs because the deep sea food web is fueled by dying plants and animals that sink through the water column. As the dead biological material sinks, it becomes food for bacteria and animals, but it is only a transient source of food, coming and going quickly. The final remains of the falling biological material settles on the sea floor, giving nourishment to the depths. This accumulation of dead organisms is greatly responsible for the spike in biomass here.

Constant conditions: While constancy may not seem like a challenge, it is a unique characteristic of the deep sea that has shaped the evolution of many deep sea animals. There are no diurnal or seasonal changes; day is night and summer is winter in the deep sea. The icy water of the very deep sea (about 3000 feet) originates at the poles and moves slowly to the sea floor. The conditions (including temperature, salinity and amount of oxygen) of the water at the poles are the conditions it maintains in the deep sea. Animals at these depths tend to move very slowly, have bulky and unstreamlined bodies, and require little oxygen. In fact, the sea floor is completely devoid of oxygen for the first few inches. The Abyssal Zone retains several cubic centimeters of oxygen per liter because animals here require much less oxygen than is available.

Bioluminescence

Bioluminescence refers to the production of light via a chemical reaction. This is not to be confused with phosphorescence or florescence. In the deep sea, scientists estimate that about 90% of organisms have the ability to produce bioluminescence.

Organisms that bioluminesce in the deep sea include marine hatchetfish, anglerfish, flashlight fish, pineconefish, gulper eels, many rattails, many sea pens, certain nudibranchs, the colossal squid and the Sparkling Enope Squid.

Deep Sea Conservation

We thought it was really important to wrap up our discussion of the deep sea by reminding you that the deep sea is still prone to some destruction. In fact, deep sea trawlers are causing huge problems right now all around the world. They scrape the bottom of the ocean and pull up all the corals and bottom fauna. To get a better feel for the deep sea conservation, watch this short video we made.

Links to other good deep sea information and great books