Depth Zones

Fact: Only 5% of the sea floor has been mapped in as much detail as the surface of Mars.

In water as depth increases so does pressure, whilst at the same time the temperature, light, food supply and fish abundance decreases. These changes have effect on which types of organisms can live and grow at different depths. As the depth increases the environmental conditions change gradually. There are 5 main depth zones, Sunlit Zone, Twilight Zone, Dark Zone, Abyssal Zone, Hadal Zone. The different zones can be identified by different physical and biological factors.

Sunlit Zone 0-200m. Huge amounts of biodiversity. On the sea bed… Plenty of different types of seaweed, corals, sessile animals (animals unable to move). In the water… Copious plankton, fish and cetaceans (sea mammals such as whales)

Twilight Zone 200-1000m. Less biodiversity than in the Sunlit zone. On the sea bed… crinoids, sponges, sea fans, sea pens, sea cucumbers, Greenland Shark. In the water… zooplankton, squid, praws, silver fish such as hatchet fish and lanternfish and Sperm Whale.

Dark Zone 1000-4000m. Similar to the Twilight zone. On the sea bed… crinoids, sponges, sea fans, sea pens, sea cucumbers. In the Water… mostly small, dark coloured fish with large mouths and stomachs, gulper eels, rattails, anglerfish, red shrimps, dep sea jellies.

Abyssal Zone 4000-6000m. Very little life here. On the sea bed… few large animals, rattails, hagfish and sea cucumbers, very diverse protists, nematode worms, bacteria. In the water… some deep sea fish for example Black Swallower.

Hadal Zone 6000m and deeper. This is a little known region to humans, but some large organisms are found in the deepest depths, the depest fish ever caught was at 8000m

As a recreational diver, however, you may not ever dive any deeper than 100m, and you will probably average your maximum depth at 30 or 40m. So you are probably wondering how do all of these zones affect me? Why do I need to know the different zones exist?

Knowing about the different zones comes in handy at night when certain predators do something called a ‘Vertical Migration’ . In many temperate and tropical regions during the daylight hours animals will spend the day at depth, but by night they will come up to shallower waters to hunt. In a single day this vertical movement may range from 400-1000m, depending on the size and type of animal. In Polar regions, where darkness lasts for several months, animals will migrate to the shallower waters during the summer and remain at depth in winter.

Courtesy of The main reason for the Vertical Migration is due to the zooplankton. It is thought that zooplankton rises to the surface to feed on the phytoplankton that lives in the surface waters, but then they retreat to the depth for two possible reasons; safety, or possibly because zooplankton expends less energy in cooler water. During the day when many animals remain at depth to remain out of sight of predators, the phytoplankton uses the sun’s energy to produce food. As the zooplankton come up to feed off the pytoplankton the zooplankton are, in turn eaten by larger predators.  This regular movement of animals up and down the water column is the greatest mass migration on earth.

UPDATE!! 13/11/13 Today I found a cut out from the New Scientist titled ‘Not Dead! The abyss is a bacteria zoo’ Ronnie Glud from the university of Denmark found that at 11,000 m deep, (technically not the Abyssal zone) the ocean trenches are particularly good at capturing food. Their steep sides funnels food to the bottom where it fertilises the bacteria zoo.

3 responses to “Depth Zones

  1. Pingback: More Whales, Healthier Ocean? | Divers Who Want To Learn More·

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