The benthic zone, , is the ecological region at the lowest level of a body of water, such as a stream, river, lake, or ocean, including the sediment surface and some sub-surface layers.
Organisms living in this zone are called benthos or, more informally, bottom dwellers. They include microorganisms such as bacteria and fungi, as well as larger invertebrates such as crustaceans and polychaetes. Benthos generally live in close relationship with the substrate and many are permanently attached to the bottom. The benthic boundary layer, which includes the bottom layer of water and the uppermost layer of sediment directly influenced by the overlying water, is an integral part of the benthic zone, as it greatly influences the biological activity that takes place there. Examples of contact soil layers include sand bottoms, rocky outcrops, coral, and bay mud.
Physical description
Oceans The benthic region of the ocean begins at the shore line (intertidal or littoral zone) and extends downward along the surface of the continental shelf out to sea. Thus, the region incorporates a great variety of physical conditions differing in depth, light penetration and pressure. The benthic zone includes all areas of bottom that are below the water.
The continental shelf is a benthic region of a tectonic plate that extends away from the shoreline of a land mass. At the continental shelf edge, usually about deep, the gradient greatly increases and is known as the continental slope. The continental slope drops down to the deep sea floor. The generally flat part of the deep-sea floor is called the abyssal plain and is usually about deep. The ocean floor is not all flat but has submarine ridges, seamounts and deep ocean trenches known as the hadal zone. For comparison, the pelagic zone is the ecological region above the benthos, comprising the water column up to the surface. The benthos of the deep ocean includes the bottom levels of the oceanic abyssal zone.Nichols, Williams (2009): "abyssal zone"
The deeper areas of the oceans beyond the penetration of daylight are the aphotic zone. Generally, this region is inhabited by life forms that tolerate cool temperatures and low oxygen levels, depending on the depth of the water.Nichols, Williams (2009): "aphotic zone"
Lakes As with oceans, the benthic zone is the floor of the lake, which may be covered by accumulated sunken organic matter and mineral sediments, and the organisms that live in and on it. The littoral zone is the zone bordering the shore; light penetrates easily and aquatic plants thrive. The pelagic zone is the water between the surface and the bottom. the photic zone is the water column down to the depth to which no light penetrates. This depth varies depending on clarity of the water.
Benthos
Benthos is the community of organisms that live in the benthic zone, that is, on, in, or near the bottom of a stream, river, lake, or ocean.Benthos from the Census of Antarctic Marine Life website This community lives in or near marine or freshwater sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.
The term benthos, coined by Haeckel in 1891, 18: 232–336. BHL. comes from the Greek noun 'depth of the ocean'.. Additionally to oceans, benthos is also used in freshwater biology to refer to organisms at the bottom of freshwater bodies of water, such as streams, rivers, and lakes. There is also a redundant, occasionally used synonym, benthon.Nehring, S. & Albrecht, U. (1997). Benthos und das redundant Benton: Neologismen in der deutschsprachigen Limnologie. Lauterbornia 31: 17-30, . Benthos is also referred to more loosely and informally as bottom dwellers or bottom feeders.
Light is an important energy source for shallow benthic systems. However, because light is absorbed before it can reach deep ocean water, the energy source for deep benthic ecosystems is often organic matter from higher up in the water column that drifts down to the depths. This dead and decaying matter sustains the benthic food chain; most organisms in the benthic zone are scavengers or detritivores.
Many organisms adapted to deep-water pressure cannot survive in the upper parts of the water column. The pressure difference can be significant (approximately one atmosphere for every 10 metres of water depth).
Compared to the relatively featureless pelagic zone, the benthic zone offers physically diverse habitats. There is a huge range in how much light and warmth is available, and in the depth of water or extent of intertidal immersion. The seafloor varies widely in the types of sediment it offers. Burrowing animals can find protection and food in soft, loose sediments such as mud, clay and sand. Sessile species such as oysters and barnacles can attach themselves securely to hard, rocky substrates. As adults they can remain at the same site, shaping depressions and crevices where mobile animals find refuge. This greater diversity in benthic habitats has resulted in a higher diversity of benthic species. The number of benthic animal species exceeds one million. This far exceeds the number of pelagic animal species (about 5000 larger zooplankton species, 22,000 pelagic fish species and 110 marine mammal species).
Benthos are the organisms that live in the benthic zone, and are different from those elsewhere in the water column; even within the benthic zone variations in such factors as substrate, light penetration, temperature and salinity give rise to distinct differences, delineated vertically, in the groups of organisms supported.Walag (2022) p.2 Many organisms adapted to deep-water pressure cannot survive in the upper parts of the water column: the pressure difference can be very significant (approximately one atmosphere for each 10 meters of water depth). Many have adapted to live on the substrate (bottom) or within the upper layers of the bottom. In their habitats they can be considered as dominant creatures, but they are often a source of prey for Carcharhinidae such as the lemon shark.
Because light does not penetrate very deep into ocean-water, the energy source for the benthic ecosystem is often marine snow. Marine snow is organic matter from higher up in the water column that drifts down to the depths. This dead and decaying matter sustains the benthic food chain; most organisms in the benthic zone are scavengers or detritivores. Some microorganisms use chemosynthesis to produce biomass.
Benthic organisms can be divided into two categories based on whether they make their home on the ocean floor or a few centimeters into the ocean floor. Those living on the surface of the ocean floor are known as epifauna. Those who live burrowed into the ocean floor are known as infauna. Extremophiles, including piezophiles, which thrive in high pressures, may also live there.
By taxon
By size
Macrobenthos
Macrobenthos, prefix , comprises the larger, visible to the naked eye, benthic organisms greater than about 1 mm in size. In shallow waters, seagrass meadows, coral reefs and kelp forests provide particularly rich habitats for macrobenthos. Some examples are polychaete worms, bivalves, echinoderms, sea anemones, corals, sponges, sea squirts, turbellarians and larger crustaceans such as crabs, lobsters and cumaceans.
Meiobenthos
Meiobenthos, prefix , comprises tiny benthic organisms that are less than about 1 mm but greater than about 0.1 mm in size. Some examples are nematodes, foraminiferans, tardigrades, gastrotriches and smaller crustaceans such as copepods and ostracodes.
Microbenthos
Microbenthos, prefix from the Greek mikrós 'small', comprises microscopic benthic organisms that are less than about 0.1 mm in size. Some examples are bacteria, diatoms, ciliates, amoeba, flagellates.
Marine microbenthos are microorganisms that live in the benthic zone of the ocean – that is, near or on the seafloor, or within or on surface seafloor sediments. Microbenthos are found everywhere on or about the seafloor of continental shelves, as well as in deeper waters, with greater diversity in or on seafloor sediments. In photic zones benthic diatoms dominate as photosynthetic organisms. In intertidal zones changing tides strongly control opportunities for microbenthos.
Both foraminifera and diatoms have planktonic and benthic forms, that is, they can drift in the water column or live on sediment at the bottom of the ocean. Regardless of form, their shells sink to the seafloor after they die. These shells are widely used as climate proxies. The chemical composition of the shells are a consequence of the chemical composition of the ocean at the time the shells were formed. Past water temperatures can be also be inferred from the ratios of stable oxygen isotopes in the shells, since lighter isotopes evaporate more readily in warmer water leaving the heavier isotopes in the shells. Information about past climates can be inferred further from the abundance of forams and diatoms, since they tend to be more abundant in warm water.Bruckner, Monica (2020) "Paleoclimatology: How Can We Infer Past Climates?" SERC, Carleton College. Modified 23 July 2020. Retrieved 10 September 2020.The sudden extinction event which killed the dinosaurs 66 million years ago also rendered extinct three-quarters of all other animal and plant species. However, deep-sea benthic forams flourished in the aftermath. In 2020 it was reported that researchers have examined the chemical composition of thousands of samples of these benthic forams and used their findings to build the most detailed climate record of Earth ever.Earth barreling toward 'Hothouse' state not seen in 50 million years, epic new climate record shows LiveScience, 10 September 2020.
Some endoliths have extremely long lives. In 2013 researchers reported evidence of endoliths in the ocean floor, perhaps millions of years old, with a generation time of 10,000 years.Bob Yirka 29 Aug 2013 These are slowly metabolizing and not in a dormant state. Some Actinomycetota found in Siberia are estimated to be half a million years old.Sussman: Oldest Plants, The Guardian, 2 May 2010
By trophic level
Zoobenthos Zoobenthos, prefix , animals belonging to the benthos. Examples include polychaete worms, starfish and anemones.
Phytobenthos Phytobenthos, prefix , plants belonging to the benthos, mainly benthic diatoms and macroalgae (seaweed).
By location Endobenthos Endobenthos (or endobenthic), prefix , lives buried, or burrowing in the sediment, often in the oxygenated top layer, e.g., a sea pen or a sand dollar.
Epibenthos Epibenthos (or epibenthic), prefix , lives on top of the sediments, e.g., sea cucumber or a sea snail.
Hyperbenthos Hyperbenthos (or hyperbenthic), prefix , lives just above the sediment, e.g., a rock cod.
By habitat