To understand the Benthic Biome, an understanding of the world’s largest biome, the Ocean Biome and its various zones. In the Ocean biome, in order, there are the intertidal, pelagic, benthic, abyssal and hadal zones. Thus the benthic zone is the third deepest zone in the ocean. There are other zones, but most of the oceans zoning is based on depth, the increasing pressure, decreasing sunlight, decreasing oxygen, and decreasing ability to sustain life as we know it.
On the basis of decreasing light penetration, the zones are the Epipepelagic (sunlight), Mesopelagic (twilight), Bathypelagic (midnight), Abyssopelagic (the abyss) and the Hadal, or the trenches that go to the greatest known depths.
Where does the Benthic zone fit in, then? Some definitions have the Benthic Zone at all levels, from the surface to the abyss. Others have the Benthic zone as between the pelagic and the abyss. Others do not even mention the Benthic Zone. Others have the Abyss as lying between the bathypelagic and the hadal zone, at around 2,000 feet deep.
It is a mystery, indeed, this “Benthic Zone”!
Wikepedia has the explantaion: The Benthic zone is the bottom of a body of water, either at the bottom or an inch or two beneath the bottom or substrate, where burrowing lifeforms go. In this definition, the benthic would include the superficial layers of soil, silt, sand bottoms, coral, bay mud, and outcrops that line the bottom of a body of water. Thus, the Benthic zone can extend along a shelf, such as a Continental shelf, which in relative form, gradually slopes from the intertidal zone to the hadal zone. In other words, If there is a straight drop to the hadal, or lowest possible depths, then there is no benthic zone at all until we get to the bottom of the hadal!
The lifeforms that habitate the benthic zone can then range from those which survive on the ocean or lake bottom, in environments which range from warm temperatures, little pressure and the fullest sunlight; to the coldest temperatures, maximum pressure and no light at all. In addition, bottom dwelling lifeforms that exist around oceanic vents could be considered benethic zone lifeforms, too.
In summary, the Benethic zone does not exist as a function of depth, it exists as a function of being at the bottom of a body of water! As a biome, the benthic zone supports any life that can sustain itself at any known depth by living on the bottom. Some lifeforms, living at abyssal depths, can exist on “sea snow” or the fallen debris from dead plant and animal life that has fallen from any higher level. Other lifeforms exist by chemical reactions that they produce to create biomass.
Most Benthic lifeforms are considered to be scavengers or detritivores, because they live off of the column of falling and dead biomass. When a Benthic life zone live on the surface of the floor, they are called epifauna. When they burrow and live an inch or two beneath the surface of the floor, they are called infauna.
Finally, Benthic lifeforms can live in a historically unmolested environment, as the greatest depths are unattainable, except through limited means, such as submarines that are built to withstand incredible pressure. They then are assumed to be relatively unevolved or living in conditions that may have been unchanged for long epochs of time. In addition, they live in a more oxygen rich environment as the depth increases, allowing for some to reach larger sizes than their higher living cousins!