In
biology, 'detritus' is non-living particulate
organic material (as opposed to dissolved organic material). It typically includes the bodies of dead
organisms or fragments of organisms or
faecal material. Detritus is normally colonized by communities of
microorganisms which act to
decompose (or
remineralize) the material.
★ In terrestrial
ecosystems, the term refers to litter on the soil surface as well as organic matter intermixed with soil.
★ In aquatic ecosystems, the term refers to organic material found suspended in water. Together with
plankton, detritus is an important constituent of
seston (materials in suspension), and may also accumulate at the base of a
water column.
General theory
Dead plants or animals, material derived from body tissues such as skin cast off during moulting, and matter derived from organisms in the form of excreta all gradually lose their form, due to both physical processes and the action of
decomposers, such as
bacteria and
fungi.
Decomposition, the process through which organic matter is decomposed, takes place in many stages. Materials like
proteins,
lipids and
sugars with low molecular weight are rapidly consumed and absorbed by micro-organisms and organisms that feed on dead matter. Other compounds, such as
complex carbohydrates are broken down more slowly. In addition, the purpose of the various micro-organisms involved is not to break down these materials but to use them to gain the resources they require for their own survival and proliferation, and they are merely breaking them down as part of that process. Accordingly, at the same time that the materials of plants and animals are being broken down, the materials (
biomass) making up the bodies of the micro-organisms are built up by a process of
assimilation. When micro-organisms die, fine organic particles are produced, and if these are eaten by small animals which feed on micro-organisms, they will collect inside their intestines, and change shape into large pellets of dung. As a result of this process, most of the materials from dead organisms disappears from view and is not obviously present in any recognisable form, but is in fact present in the form of a combination of fine organic particles and the organisms using them as nutrients. This combination is detritus.
In
ecosystems on land, detritus is deposited on the surface of the ground, taking forms such as the humic soil beneath a layer of fallen leaves. In aquatic ecosystems, most detritus is suspended in water, and gradually settles. In particular, many different types of material are collected together by currents, and much material settles in slowly-flowing areas.
Much detritus is used as a source of nutrition for
animals. In particular, many bottom-dwelling animals (
benthos) living in
mud flats feed in this way. In particular, since excreta are materials which other animals do not need, whatever energy value they might have, they are often unbalanced as a source of nutrients, and are not suitable as a source of nutrition on their own. However, there are many
micro-organisms which multiply in natural environments. These micro-organisms do not simply absorb nutrients from these particles, but also shape their own bodies so that they can take the resources they lack from the area around them, and this allows them to make good use of excreta as a source of nutrients. In practical terms, the most important constituents of detritus are complex carbohydrates, which are persistent (difficult to break down), and the micro-organisms which multiply using these absorb carbon from the detritus, and materials such as nitrogen and phosphorus from the water in their environment to synthesise the components of their own cells.
A characteristic type of
food chain called the
detritus cycle takes place involving detritus feeders (
detritivores), detritus and the micro-organisms that multiply on it. For example, mud flats are inhabited by many univalves which are detritus feeders, such as
moon shells. When these detritus feeders take in detritus with micro-organisms multiplying on it, they mainly break down and absorb the micro-organisms, which are rich in proteins, and excrete the detritus, which is mostly complex carbohydrates, having hardly broken it down at all. At first this dung is a poor source of nutrition, and so univalves pay no attention to it, but after several days, micro-organisms begin to multiply on it again, its nutritional balance improves, and so they eat it again. Through this process of eating the detritus many times over and harvesting the micro-organisms from it, the detritus thins out, becomes fractured and becomes easier for the micro-organisms to use, and so the complex carbohydrates are also steadily broken down and disappear over time.
What is left behind by the detritivores is then further broken down and recycled by
decomposers, such as
bacteria and
fungi.
This detritus cycle plays a large part in the so-called purification process, whereby organic materials carried in by rivers is broken down and disappears, and an extremely important part in the breeding and growth of
marine resources. In ecosystems on land, far more essential material is broken down as dead material passing through the detritus chain than is broken down by being eaten by animals in a living state. In both land and aquatic ecosystems, the role played by detritus is too large to ignore.
The primary microorganisms that break down matter are called
mesophilic (microorganisms thriving at medium temperatures). They cause a lot of heat which is why compost becomes warm after a while.
Aquatic ecosystems
In contrast to land ecosystems, dead materials and excreta in aquatic ecosystems do not settle immediately, and the finer the particles involved are, the longer they tend to take.
Consumers
There are an extremely large number of detritus feeders in water. After all, a large quantity of material is carried in by water currents. Even if an organism stays in a fixed position, as long as it has a system for filtering water, it will be able to obtain enough food to get by. Many
rooted organisms survive in this way, using developed
gills or
tentacles to filter the water to take in food, a process known as
filter feeding.
Another more widely used method of feeding, which also incorporates filter feeding, is a system where an organism secretes mucus to catch the detritus in lumps, and then carries these to its mouth using an area of
cilia. This is called
mucus feeding.
Many organisms, including sea slugs and serpent's starfish, scoop up the detritus which has settled on the water bed. Bivalves which live inside the water bed do not simply suck in water through their tubes, but also extend them to fish for detritus on the surface of the bed.
Producers
In contrast, from the point of view of organisms using photosynthesis, such as
plants and
plankton, detritus reduces the transparency of the water and gets in the way of their photosynthesis. However, given that they also require a supply of
nutrient salts, in other words
fertilizer for photosynthesis, their relationship with detritus is a complex one.
In land ecosystems, the waste products of plants and animals collect mainly on the ground (or on the surfaces of trees), and as decomposition proceeds, plants are supplied with fertiliser in the form of inorganic salts. However, in water, relatively little waste collects on the water bed, and so the progress of decomposition in water takes a more important role. However, investigating the level of inorganic salts in sea ecosystems shows that, unless there is an especially large supply, the quantity increases from winter to spring but is normally extremely low in summer. In line with this, the quantity of seaweed present reaches a peak in early summer, and then decreases. This is thought to be because organisms like plants grow quickly in warm periods and the quantity of inorganic salts is not enough to keep up with the demand. In other words, during winter, plant-like organisms are inactive and collect fertiliser, but if the temperature rises to some extent, they use this up in a very short period.
However, it is not the case that their productivity falls during the warmest periods. Organisms such as
dinoflagellate have mobility, the ability to take in solid food, and the ability to photosynthetise. This type of micro-organism can take in substances such as detritus to grow, without waiting for it to be broken down into fertiliser.
Aquariums
In recent years, the word detritus has also come to be used in relation to
aquariums (the word "aquarium" is a general term for any installation for keeping aquatic animals).
When animals such as fish are kept in an aquarium, substances such as excreta, mucus and dead skin cast off during moulting are produced by the animals and, naturally, generate detritus, and are continually broken down by micro-organisms.
If detritus is left unattended, it dirties the inside of the water tank, and harms the health of the animals inside. Sea-dwelling animals, in particular, have little resistance to the toxins that are produced by the decomposition of detritus. Modern sealife aquariums often use the
Berlin system, which employs a piece of equipment called a
protein skimmer, which produces air bubbles which the detritus adheres to, and forces it outside the tank before it decomposes, and also a highly porous type of natural rock called
live rock where many bentos and bacteria live (hermatype which has been dead for some time is often used), which causes the detritus-feeding bentos and micro-organisms to undergo a detritus cycle. The
Monaco system, where an anaerobic layer is created in the tank, to
denitrify the organic compounds in the tank, and also the other nitrogen compounds, so that the decomposition process continues until the stage where water, carbon dioxide and nitrogen are produced, has also been implemented.
Initially, the filtration systems in water tanks often worked as the name suggests, using a physical filter to remove foreign substances in the water. Following this, the standard method for maintaining the water quality was to convert
ammonium or
nitrates in excreta, which have a high degree of neurotoxicity, but the combination of detritus feeders, detritus and micro-organisms has now brought aquarium technology to a still higher level.
Sources
★ Much of this article was translated from the , as it was on
September 1,
2006.
See also
★
Organic material
★
Soil food web
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