Detritus

In biology, detritus is dead organic material, as opposed to living organisms or inorganic matter. However, what specifically is included as detritus varies according to different researchers (Day et al. 1989). Some consider detritus to only include non-living particulate organic matter (POM) (Paerl 1974). Other researchers include both POM and dissolved organic matter (DOM) as together comprising detritus (O'Reilly and Dow 2006; Wetzel 2006). Some also include as part of detritus the associated microbial decomposers, or detritivores, whether as part of the POM (Odum and de la Cruz 1967) or the total organic matter, POM and DOM (Mann 1972).

It typically includes the bodies of dead organisms or fragments of organisms or fecal material. Detritus is normally colonized by communities of microorganisms which act to decompose (or remineralize) the material.  

• In terrestrial ecosystems, detritus refers to leaf litter and other organic matter intermixed with soil, here known as humus.
• In aquatic ecosystems, detritus refers to organic material suspended in water, here known as marine snow.

A food chain in which the primary consumer feeds on living plants is called a grazing pathway; that in which the primary consumer feeds on dead plant matter is known as a detritus pathway. Both pathways are important in accounting for the energy budget of the ecosystem.

Because light does not penetrate very deep ocean water, the energy source for the benthic ecosystem 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 deep water benthic zones are scavengers or detritivores.

Unlike detritivory, herbivory involves the consumption of living plants and their products rather than the consumption of dead organic material (detritus).

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 photosynthesise. 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.

References

ISBN links support NWE through referral fees

O'Reilly, J. E., and D. D. Dow. 2006. http://www.nefsc.noaa.gov/publications/crd/crd0615/19.pdf Detritus: Particulate organic carbon (POC) and dissolved organic carbon (DOC). 107-109 in J. S. Link, C. A. Griswold, E. T. Methratta, and J. Gunnard, eds., Documentation for the Energy Modeling and Analysis eXercise (EMAX). US Dep. Commer., Northeast Fish. Sci. Cent. Ref. Doc. 06-15; 166 p.

Paerl, H. W. 1974. Bacterial uptake of dissolved organic matter in relation to detrital aggregation in marine and freshwater systems. Limnology and Oceanography 19(6): 966-972.

Death, detritus, and energy flow in aquatic ecosystems ROBERT G. WETZEL Freshwater Biology Volume 33 Issue 1, Pages 83 - 89

Published Online: 30 May 2006

variously defined by different researchers. Estuarine ecology

By John W. Day, Charles A. S. Hall, W. Michael Kemp, Alejandro Yáñez-Arancibia

0471062634 1989 John Wiley and Sons p. 282

Odum, E. and A. de la Cruz. 1967. Particulate or- ganic detritus in a Georgia salt marsh-estuarine ecosystem. p.383-388. In: G. Lauff (ed.). Estu- aries. Am. Assoc. Ad. Sci. Pub. 83. Washington, DC.

Mann, K. H. 1972. Macrophyte production and detritus food chains in coastal waters. - Mem. Ist. Ital. Idrobiol. 29 Suppl.: 353-383

See also

• Organic material
• Soil food web
• Marine debris

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