Functional feeding groups--
In order to understand the fate of organic matter in rivers, we must know how animals use this material; specifically, how they feed on it. While details vary among species, it is possible to simplify matters by using a functional characterisation of animals according to how they feed. This scheme was originally developed for insects because they dominate the benthos of streams and rivers. However, it is applicable, in principle, to other aquatic animals such as crabs, shrimps, snails and fishes.

There are several widely recognised functional feeding groups (Cummins and Klug 1979).

• The grazer-scraper category comprises herbivores that feed on periphyton and biofilms.

• Shredders are detritivores feeding on CPOM, especially leaf litter derived from the riparian zone.

• Collectors eat FPOM and UFPOM, and can be subdivided according to whether the food particles they collect are suspended in the water (as in the case of filtering-collectors or filter-feeders) or have been deposited on the substratum (in the case of collector-gatherers).

• Predators are species that eat other animals.

There is an additional category of small herbivores that pierce plant tissues or cells and suck out fluids but, typically, they are rather scarce. Deposit-feeders could be added to this classification to take account of species (such as annelid worms) ingesting fine bottom sediments and the organic material that they contain. Alternatively, these animals may be considered as specialised collector-gatherers.

One difficulty with this functional classification is that there may be overlaps between functional groups if animals have a mixed diet. Furthermore, certain animals may change category as they grow. Despite these drawbacks, functional classification of animals (especially invertebrates) has the advantage of reducing the difficulty of dealing with families that are not well known taxonomically, since closely-related species usually belong to the same functional group. In addition, it simplifies data on community composition in rivers, and this is helpful because it can facilitate recognition of patterns in ecosystems. The importance of this point will become apparent when we discuss the River Continuum Concept.

Organic matter processing--

Bacteria in biofilms covering submerged stones and wood take up much of the DOM. An important source of DOM is soluble material leached from CPOM during the first few hours or days after they enter the river. Subsequent processing of the CPOM involves microorganisms and invertebrate feeding groups and is characterised by changes (usually reduction) in the size of particles (for details, see Cummins & Klug, 1979; Webster & Benfield, 1986). Fine particles may also be produced by physical fragmentation of CPOM. CPOM eaten by shredders is mostly plant structural material and is rather indigestible; consequently, it is not a valuable food. However, the surface of the CPOM is colonised by fungi and bacteria that use exoenzymes to break down plant tissue and produce microbial biomass (see Section 6).

Additional microbial production may be fuelled by DOM uptake. The microorganisms enhance the food value of the CPOM to shredders because they increase its protein content. The resulting detritus can be seen as equivalent to a dry biscuit spread with peanut butter; the biscuit representing the CPOM, and the peanut butter – a more valuable food – representing the microbes. Shredders eat detritus that has been colonised or 'conditioned' by microbes in preference to unconditioned CPOM.

By the action of chewing, swallowing and defaecating, shredders break up the CPOM to produce FPOM that can be gathered or filtered by collectors. Because the CPOM itself is difficult for most shredders to digest, they need to eat a lot of it and thus produce an ample supply of FPOM. The faeces that collectors produce are generally of similar size or smaller than the FPOM that they ingest. Grazer-scrapers that eat periphyton produce faeces in the FPOM size range also. Microbial colonisation of these faecal particles (mainly by bacteria) enhances their food value and so collectors eat them. This results in conversion of some of the energy in the detritus to animal protein or carbon dioxide. The microorganisms are digested during passage through the gut, but the detritus is recolonised when it returns to the aquatic environment in the form of faeces. At each cycle of microbial colonisation, some of the detritus is converted to microbial biomass or released as carbon dioxide during microbial respiration, and this process combines with repeated consumption of particles by animals. Eventually, the detritus is converted to animal/microbial tissue or has been respired as carbon dioxide; any remaining FPOM or UFPOM is carried out to sea.

Figure: The links between shredders and CPOM, fungi and bacteria modeled for a small stream within a temperate deciduous forest. Physical abrasion, microbial activity (especially by fungi) and invertebrate shredders reduce much of the CPOM to smaller particles. Chemical leaching and microbial excretion and respiration release DOM and CO2, but much of the original carbon is converted to FPOM

A reduction in particle size may not occur at every step of detritus processing, because filter-feeding animals (such as larvae of Simuliidae: ) can collect UFPOM and, after eating it, convert it into faeces in the FPOM size range. That FPOM is available to other animals that are not able to collect UFPOM but can eat larger particles.

Breakdown of aquatic macrophyte tissue – i.e., autochthonous detritus – is more rapid than that of allochthonous plants (Webster and Benfield 1986). Aquatic plants get more support from the surrounding medium (water) than plants growing in air, and hence do not need much structural support tissue. This makes them easier for detritivores to digest. Autochthonous detritus is contributed also by the death and decay of phytoplankton in large rivers or on floodplains. Significantly, most of the organic material exploited by detritivorous fish appears to originate from phytoplankton rather than from decay of CPOM.

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