Light is an important energy source for shallow benthic systems. However, because light isabsorbed 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. Thisdead and decaying matter sustains the benthicfood chain; most organisms in the benthic zone arescavengers ordetritivores.
Many organisms adapted to deep-water pressure cannot survive in the upper parts of thewater column. The pressure difference can be significant (approximately oneatmosphere for every 10 metres of water depth).[2]
The termbenthos, coined byHaeckel in 1891,[3] comes from theGreek nounβένθος 'depth of the sea'.[1][4]Benthos is used infreshwater biology to refer to organisms at the bottom of freshwaterbodies of water, such as lakes, rivers, and streams.[5] There is also a redundant synonym,benthon.[6]
Compared to the relatively featurelesspelagic 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 ofintertidal immersion. The seafloor varies widely in the types ofsediment it offers.Burrowing animals can find protection and food in soft, loose sediments such asmud,clay andsand.Sessile species such asoysters andbarnacles 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).[7]
Microbenthos, prefix from the Greekmikrós 'small', comprises microscopic benthic organisms that are less than about 0.1 mm in size. Some examples arebacteria,diatoms,ciliates,amoeba,flagellates.
Marine microbenthos are microorganisms that live in thebenthic zone of the ocean – that live 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. Inphotic zones benthic diatoms dominate as photosynthetic organisms. Inintertidal zones changingtides strongly control opportunities for microbenthos.
Elphidium a widespread abundant genus of benthic forams
Both foraminifera and diatoms haveplanktonic andbenthic forms, that is, they can drift in thewater 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 asclimate 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 stableoxygen 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.[9]
Benthic diatom
The suddenextinction 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.[10][11]
Someendoliths 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.[12] These are slowly metabolizing and not in a dormant state. SomeActinomycetota found inSiberia are estimated to be half a million years old.[13][14][15]
Endobenthos (or endobenthic), prefix from Ancient Greek éndon'inner, internal', lives buried, or burrowing in the sediment, often in theoxygenated top layer, e.g., asea pen or asand dollar.
The main food sources for the benthos arephytoplankton and organic detrital matter.[16][17] In coastal locations, organic run off from land provides an additional food source.[18] Meiofauna and bacteria consume and recycle organic matter in the sediments, playing an important role in returningnitrate andphosphate to the pelagic.[19]
The depth of water, temperature and salinity, and type of local substrate all affect what benthos is present. In coastal waters and other places where light reaches the bottom, benthicphotosynthesizingdiatoms can proliferate.Filter feeders, such assponges andbivalves, dominate hard, sandy bottoms. Deposit feeders, such aspolychaetes, populate softer bottoms. Fish, such asdragonets, as well assea stars,snails,cephalopods, andcrustaceans are important predators and scavengers.
Benthic macro-invertebrates play a critical role inaquatic ecosystems. These organisms can be used to indicate the presence,concentration, and effect of water pollutants in the aquatic environment. Some water contaminants—such as nutrients, chemicals fromsurface runoff, and metals[20]—settle in thesediment of river beds, where many benthos reside. Benthos are highly sensitive to contamination, so their close proximity to high pollutant concentrations make these organisms ideal for studying water contamination.[21]
Benthos can be used asbioindicators ofwater pollution through ecological population assessments or through analyzingbiomarkers. In ecological population assessments, a relative value of water pollution can be detected. Observing the number and diversity of macro-invertebrates in a waterbody can indicate the pollution level. In highly contaminated waters, a reduced number of organisms and only pollution-tolerant species will be found.[22] In biomarker assessments,quantitative data can be collected on the amount of and direct effect of specific pollutants in a waterbody. Thebiochemical response of macro-invertebrates' internal tissues can be studied extensively in the laboratory. The concentration of a chemical can cause many changes, including changing feeding behaviors,[23]inflammation, and genetic damage,[24] effects that can be detected outside of the stream environment. Biomarker analysis is important for mitigating the negative impacts of water pollution because it can detect water pollution before it has a noticeable ecological effect on benthos populations.[25]
Organic matter produced in the sunlit layer of the ocean and delivered to the sediments is either consumed by organisms or buried. The organic matter consumed by organisms is used to synthesizebiomass (i.e. growth) converted to carbon dioxide throughrespiration, or returned to the sediment as faeces. This cycle can occur many times before either all organic matter is used up or eventually buried. This process is known as thebiological pump.[26][27]
In the long-term or at steady-state, i.e., the biomass of benthic organisms does not change, the benthic community can be considered a black box diverting organic matter into either metabolites or the geosphere (burial).[27] The macrobenthos also indirectly impacts carbon cycling on the seafloor throughbioturbation.[28]
Benthos (organisms that live at the ocean floor) can be contrasted withneuston (organisms that live at the ocean surface)plankton (organisms that drift with water currents) andnekton (organisms that can swim against water currents)
^Sokolova, M. N. (2000).Feeding and trophic structure of the deep-sea macrobenthos. Enfield, NH: Science Publishers.ISBN978-1-57808-090-8.OCLC46724477.
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