Terminology Tuesday: Zones Continued

The ocean is divided into two basic regions: the pelagic zone, or water column, and the benthic zone, or seafloor.

Where does the word benthic come from?
1. the animals and plants living at the bottom of a sea or lake
2. the bottom of a sea or lake
[ from Greek: depth; related to bathus deep]

Benthic region:
Supralittoral, or splash zone – rarely covered by water
Intertidal zone – regularly submerged and exposed as the tides rise and fall
Sublittoral, or subtidal zone – extends to the edge of the continental shelf
Abyssal zone – pertains to the ocean depths between 13,000 and 16,500 feet
Hadal Zone – encompasses the very deepest areas, in the trenches and anywhere below 19,800 feet (6,000m)

And where does the word “littoral” come from:
1650–60; < Latin littorālis, variant of lītorālis of the shore, equivalent to lītor- (stem of lītus ) shore + -ālis -al1

Get ready for more terminology.

In the ocean, there are two distinct zones.
The photic, or sunlit zone, is that part of the ocean that receives light.

The aphotic, or sunless zone.

Just as on land, many organisms need light to make energy.

There is another division in the ocean – which reflects water chemistry and density. Water that is warmer or less salty is less dense than cooler or saltier water, and water that is less dense tends to stay at the surface. Between the surface and deep zones is the pycnocline, or zone of rapidly changing density.

(The word cline came into use around 1935–40; from the Greek klī́nein to lean.)

A pycnocline is a sharp change in density with depth within a body of water. In freshwater environments such as lakes this density change is primarily caused by water temperature (thermocline), while in seawater environments such as oceans and estuaries, the rapid density change in the water column is often caused by a combination of decreasing water temperature and increasing salinity (halocline).

Areas where freshwater and saltwater environments meet, such as bays and near river mouths, often have strong, well defined pycnoclines. A large amount of runoff from land of warmer freshwater can float upon colder saltwater entering an estuary forming a salt wedge. The amount of mixing from top to bottom will determine the stratification or strength of the pycnocline. With little mixing from the friction of currents, the density differences between the fresh and salt water dominate.

Pycnoclines tend to disappear in the open ocean at around 50 or 60 degrees North or South Latitude. This is due to lowered salinity and temperature change near the poles.

Thermocline - A cline based on difference in water temperature,
Chemocline - A cline based on difference in water chemistry,
Halocline - A cline based on difference in water salinity.