The aphotic zone (aphotic from
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
prefix + "without light") is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1 percent of sunlight penetrates. Above the aphotic zone is the
photic zone
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
, which consists of the
euphotic zone
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
and the disphotic zone. The euphotic zone is the layer of water in which there is enough light for net photosynthesis to occur. The disphotic zone, also known as the twilight zone, is the layer of water with enough light for predators to see but not enough for the rate of photosynthesis to be greater than the rate of respiration.
The depth at which less than one percent of sunlight reaches begins the aphotic zone. While most of the ocean’s biomass lives in the photic zone, the majority of the ocean’s water lies in the aphotic zone.
Bioluminescence
Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...
is more abundant than sunlight in this zone. Most food in this zone comes from
dead organisms sinking to the bottom of the lake or ocean from overlying waters.
The depth of the aphotic zone can be greatly affected by such things as
turbidity
Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality.
Fluids can ...
and the season of the year. The aphotic zone underlies the
photic zone
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
, which is that portion of a lake or ocean directly affected by sunlight.
The Dark Ocean
![Light penetration zones in the water column](https://upload.wikimedia.org/wikipedia/commons/b/b4/Light_penetration_zones_in_the_water_column.png)
In the ocean, the aphotic zone is sometimes referred to as the dark ocean. Depending on how it is defined, the aphotic zone of the ocean begins between depths of about to and extends to the ocean floor. The majority of the ocean is aphotic, with the average depth of the sea being 4267 m deep with the deepest part of the sea, being the Challenger Deep in the
Mariana Trench
The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maximum know ...
, is about 11,000 m deep. The depth at which the aphotic zone begins in the ocean depends on many factors. In clear, tropical water sunlight can penetrate deeper and so the aphotic zone starts at greater depths. Around the poles, the angle of the sunlight means it does not penetrate as deeply so the aphotic zone is shallower. If the water is turbid, suspended material can block light from penetrating resulting in a shallower aphotic zone. Temperatures can range from roughly to .
The aphotic zone is further divided into the
mesopelagic
The mesopelagic zone (Greek μέσον, middle), also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins ...
zone, the
bathyal
The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the open ocean that extends from a depth of below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The bathypelagi ...
zone, the
abyssal
The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. "Abyss" derives from the Greek word , meaning bottomless. At depths of , this zone remains in perpetual darkness. It covers 83% of the total area of the ocean an ...
zone, and the
hadal
The hadal zone, also known as the hadopelagic zone, is the deepest region of the ocean, lying within oceanic trenches. The hadal zone ranges from around below sea level, and exists in long, narrow, topographic V-shaped depressions.
The cumula ...
zone. The mesopelagic zone extends from to . The bathyal zone extends from to . The abyssal zone extends from to or , depending on the authority. The hadal zone refers to the greatest depths, deeper than the abyssal zone. Some twilight occurs in the mesopelagic zone, but creatures below the mesopelagic must be able to live in complete darkness.
Life in the aphotic zone
Though photosynthesis cannot occur in the aphotic zone, it is not unusual to find an abundance of
phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'.
Ph ...
there. Convective mixing due to cooling surface water sinking can increase the concentration of phytoplankton in the aphotic zone and lead to under-estimations of
primary production
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...
in the euphotic zone during convective mixing events.
Unusual and unique creatures dwell in this expanse of pitch black water, such as the
gulper eel
The saccopharyngiformes are a derived lineage of unusual eels within the order Anguilliformes, and includes families Cyematidae, Monognathidae, Eurypharyngidae, Saccopharyngidae, and the proposed family Neocyematidae. Most of the fish in this gro ...
, the
giant squid
The giant squid (''Architeuthis dux'') is a species of deep-ocean dwelling squid in the family Architeuthidae. It can grow to a tremendous size, offering an example of abyssal gigantism: recent estimates put the maximum size at around Trace ...
, the
anglerfish
The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...
, and the
vampire squid
The vampire squid (''Vampyroteuthis infernalis'', lit. 'vampire squid from hell') is a small cephalopod found throughout temperate and tropical oceans in extreme deep sea conditions. The vampire squid uses its bioluminescent organs and its uniq ...
. Some life in the aphotic zone does not rely on sunlight at all. Benthic communities around
methane seeps rely on methane-oxidizing microorganisms to supply energy to other microorganisms.
In some rare cases, bacteria use chemical energy sources such as sulfides and methane. Many of the animals in the aphotic zone are bioluminescent, meaning they can produce their light. Bioluminescence can be used both for navigation and luring small animals into their jaws. An excellent example of this is the angler fish, as it has a light lure protruding in front of its mouth from a unique appendage on its head which provides navigation and as bait for smaller animals.
Some animals can cross between the photic and aphotic zones in search of food. For example, the
sperm whale
The sperm whale or cachalot (''Physeter macrocephalus'') is the largest of the toothed whales and the largest toothed predator. It is the only living member of the genus ''Physeter'' and one of three extant species in the sperm whale famil ...
and the
southern elephant seal
The southern elephant seal (''Mirounga leonina'') is one of two species of elephant seals. It is the largest member of the clade Pinnipedia and the order Carnivora, as well as the largest extant marine mammal that is not a cetacean. It gets it ...
occasionally hunt in the aphotic zone despite the water pressure squashing their bodies; however, not fatally.
Aphotic zone migration
After sunset, millions of organisms swarm up from the depths to feed on the microorganisms floating in the warm epipelagic zone. Many
copepods
Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have p ...
and invertebrate larvae come up to shallower waters to eat the
phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'.
Ph ...
, which attracts many predators like
squid
True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting t ...
,
hatchet fish, and
lantern fish
Lanternfishes (or myctophids, from the Greek μυκτήρ ''myktḗr'', "nose" and ''ophis'', "serpent") are small mesopelagic fish of the large family Myctophidae. One of two families in the order Myctophiformes, the Myctophidae are represente ...
. The migration of the many bioluminescent animals is visible to the naked eye. This nightly vertical migration is the largest (in terms of the number of animals) on our planet.
See also
*
Abyssal zone
The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. "Abyss" derives from the Greek word , meaning bottomless. At depths of , this zone remains in perpetual darkness. It covers 83% of the total area of the ocean a ...
*
Benthic zone
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
*
Hadal zone
The hadal zone, also known as the hadopelagic zone, is the deepest region of the ocean, lying within oceanic trenches. The hadal zone ranges from around below sea level, and exists in long, narrow, topographic V-shaped depressions.
The cumula ...
*
Pelagic zone
The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...
*
Photic zone
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
References
Which Animals Live in the Aphotic Zone of the Ocean? (infobloom.com)askabiologist.asu.edu/animals-open-ocean.
Aquatic ecology
Oceanography
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