River continuum concept
Encyclopedia
The River Continuum Concept (RCC) is a model for classifying and describing flowing water, in addition to the classification of individual sections of waters after the occurrence of indicator organisms. The theory is based on the concept of dynamic equilibrium
Dynamic equilibrium
A dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants/products, but substances move between the chemicals at an equal rate, meaning there is no net change. It is a particular example of a system in a steady state...

 in which streamforms balance between physical parameters, such as width, depth, velocity, and sediment load, also taking into account biological factors. It offers the introduction to map out pure living communities, but also an explanation for their sequence in individual sections of water. This allows the structure of the river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 to be more predictable as to the biological properties of the water. The concept was first developed in 1980 by Robin L. Vannote, with fellow researchers at the Stroud Water Research Center.

Background

The River Continuum Concept is based on the idea that a watercourse
Watercourse
A watercourse is any flowing body of water. These include rivers, streams, anabranches, and so forth.-See also:* physical geography* Environmental flow* Waterway* Hydrology* Wadi-External links:...

 is an open ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

 that is in constant interaction with the bank, and moving from source to mouth, constantly changing. Basis for this change in the overall system is due to the gradual change of physical environmental conditions such as the width, depth, water, flow characteristics, temperature, and the complexity of the water. According to the hypothesis, which is based on the physical geomorphological theory, Vannote came up with the hypothesis that structural and functional characteristics of stream communities are selected to conform to the most probable position or mean state of the physical system. As a river changes from headwaters
Source (river or stream)
The source or headwaters of a river or stream is the place from which the water in the river or stream originates.-Definition:There is no universally agreed upon definition for determining a stream's source...

 to the lower reaches, there will be a change in the relationship between the production and consumption (respiration) of the material (P/R ratio).

Living communities and food types

The continuous differences of properties within the river are dependent primarily on the specific composition of the organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

s in different sections of the water. Throughout the continuum of the river, the proportion of the four major food types; shredders, collectors, grazers (scrapers) and predators
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...

 change. With the exception of the predators, all these organisms feed directly from plant material (saprobes).

Shredders

Shredders are organisms that feed off of coarse particulate organic material(CPOM) such as small sections of leaves. They ingest the organic matter along with volunteer organisms (fungi, microorganisms) attached to the source. The preferred size of the CPOM is about one millimeter, therefore shredders must break it up into a finer particulate. In the process of shredding, much of the now finer organic matter is left in the system, making its way further downstream. Some common shredders of North American waters include the Mayfly
Mayfly
Mayflies are insects which belong to the Order Ephemeroptera . They have been placed into an ancient group of insects termed the Palaeoptera, which also contains dragonflies and damselflies...

 and stone fly (Plecoptera
Plecoptera
Plecoptera are an order of insects, commonly known as stoneflies. There are some 3,500 described species worldwide, with new species still being discovered. Stoneflies are found worldwide, except Antarctica...

) larvae, whereas decapods
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...

 (particularly Atyid shrimp
Atyidae
Atyidae is a family of shrimp, present in all tropical and most temperate waters of the world. Adults of this family are almost always confined to fresh water...

) fulfill the same role in tropical environments.

Collectors

Collector organisms are designated by their use of traps or other adaptive features to filter and catch organic matter. The preferred particle size for collectors lies between 0.5 and 50 micrometers (UPOM = Ultrafine particulate organic matter and FPOM = fine particulate organic matter). This group includes fly
Fly
True flies are insects of the order Diptera . They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax...

 larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

e, nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

s, and many other animal groups.

Grazers

The grazers (scrapers) feed off of periphyton
Periphyton
Periphyton are a complex mixture of algae, cyanobacteria, heterotrophic microbes, and detritus that are attached to submerged surfaces in most aquatic ecosystems. It serves as an important food source for invertebrates, tadpoles, and some fish. It can also absorb contaminants; removing them from...

 that accumulates on larger structures such as stones, wood or large aquatic plants. These include snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s, caddisflies (Glossosoma genus), and other organisms.

Because of the differences in the structure of organic matter at different sections in a river, the make up and frequency of these groups in a community vary. In the upper reaches of a river, shredders and collectors make up a large percentage of total macroinvertebrates
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

 due to the excess presence of coarse plant matter. In the midreaches of a stream or river, where more light is available, there is an increase in the proportion of grazers due to the presence of periphyton. Shredders only make up a small percentage of the total invertebrates due to the lack of coarse organic matter making its way downstream. In the lower reaches, organic matter has been shredded completely to the level of FPOM or UPOM (Ultra-fine Particulate Organic Matter). Due to the increase in fine particulate organic matter, collectors are the most abundant in the lower reaches, feeding off organic matter and surface films. The proportion of predators in all sections remains largely constant and only changes in species composition. The reason for the even distribution is that predators are not dependent on the size of the organic matter but on the availability of prey animals in the area. Atypical changes in the composition of these groups of organisms within a watercourse, such as an increased number of choppers in a major river area (mid to lower reach) or a lack of these organisms in the upper reaches, suggest a possible disturbance.

The division of the riverine

The river continuum concept assigns different sections of a river into three rough classifications. These classifications apply to all river waters, from small streams to medium-sized and large rivers.

Headwaters (Stream order 1 to 3)

The creek area in the upper reaches or headwaters of a water system is usually very narrow and lined by thick shore vegetation. This prevents the penetration of sunlight, in turn decreasing the production of organic material through photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 in the water. The majority of the organic matter that does make its way into the system is in the form allochthon
Allochthon
thumb|right|250px|Schematic overview of a thrust system. The hanging wall block is called a [[nappe]]. If an [[erosion]]al hole is created in the nappe that is called a [[window |window]]...

ous plant material that falls into the river, such as leaves and sticks. In this section, respiration (consumption) out paces production (P/R<1). Here shredders play a major role in breaking down coarse plant material. In this area, the largest diversity of organic material can be expected.

Midreaches (Stream order 4-6)

In the midreaches of a river, river structures such as rocks and trees play an important role as a supplier of organic material such as periphyton
Periphyton
Periphyton are a complex mixture of algae, cyanobacteria, heterotrophic microbes, and detritus that are attached to submerged surfaces in most aquatic ecosystems. It serves as an important food source for invertebrates, tadpoles, and some fish. It can also absorb contaminants; removing them from...

 and other autochthonous
Allochthon
thumb|right|250px|Schematic overview of a thrust system. The hanging wall block is called a [[nappe]]. If an [[erosion]]al hole is created in the nappe that is called a [[window |window]]...

 (see limnology section of link) organic materials. The production to respiration ratio is larger in this section and amounts to P: R> 1. The percentage of shredders in this area is less than that of the headwaters, due to lack of coarse plant particulate. Collectors and grazers make up a majority of the macro invertebrate structure in this area, with the predator's share remaining unchanged.

Lower reaches (Stream order >6)

In the lower reaches, there is a large flux in particulate material and also a decrease in production through photosynthesis, due to an increase in water cloudiness and surface film from suspended FPOM. Here, like the headwaters, respiration outpaces production, making the ratio again less than 1 (P: R <1). The living community in these areas are made up of almost exclusively collectors, as well as a small share of predators.

The continuum

The continuous changes down the water route are due to various factors. As described above, at its beginning, the river is very strongly influenced by material from outside the system, especially organic material which is consumed by various macroinvertebrates (mainly shredders). As you go further down the system there is an increase in autochthonous (i.e., within the system) production of organic material such as periphyton
Periphyton
Periphyton are a complex mixture of algae, cyanobacteria, heterotrophic microbes, and detritus that are attached to submerged surfaces in most aquatic ecosystems. It serves as an important food source for invertebrates, tadpoles, and some fish. It can also absorb contaminants; removing them from...

. The extent of this production varies depending on the amount of sunlight present. The last area is less dependent on the outside but still very much influenced by the degradation processes. In a continuous system without interference, such as by inflows, this development is possible in all river systems, with some variations occurring due to seasonal changes and other environmental factors (especially temperature).

Resources and stability of the system

At any point in the system when organic material is added, it is used or stored, with a small proportion making its way further downstream. The existing energy is the limiting growth factor of the system, therefore the system is striving to be as efficient as possible. Free resources will enable new types of life in the community to establish, so that the excess resources are quickly exploited. This principle is not exclusively for river ecosystems but applies to most systems. Here, however, it plays a larger role, because resources are not spent in one place but are being constantly transported downstream.
The temporal aspect of this continuity can be seen by its daily and seasonal changes. In the course of a day there are many changes in the structure of living communities, mainly due to increased resource pressure during the day (better rate of detection) and abiotic factors such as changes of temperature and light. The midreaches are the most affected by daily periodic changes, because here there is the greatest biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

, each with different ideal conditions.
Because there is a uniform use of resources and high stability, disturbances and fluctuations are usually corrected relatively quickly. Inequalities in the use of resources will be quickly compensated for, creating a new equilibrium
Equilibrium
Equilibrium is the condition of a system in which competing influences are balanced. The word may refer to:-Biology:* Equilibrioception, the sense of a balance present in human beings and other animals...

. Also, there is no ecological development of the system (succession) and changes in the system are a result of outside geological changes, such as a change in the level of water making its way into the system, change of organic inputs or earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

s. Even after these changes, however, it returns to a steady and modified equilibrium. This ensures that the ecosystem stays as an optimal functioning river system.

Development and application of the concept

The first comprehensive presentation of the 1980 concept was part of a two-day conference at Stroud Water Research Center, whose head director was Robin Vannote. It was the result of a multi-year study conducted by the Rockefeller Foundation
Rockefeller Foundation
The Rockefeller Foundation is a prominent philanthropic organization and private foundation based at 420 Fifth Avenue, New York City. The preeminent institution established by the six-generation Rockefeller family, it was founded by John D. Rockefeller , along with his son John D. Rockefeller, Jr...

. The publication of the hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...

 was released later that same year under the title "The River Continuum Concept" in the Canadian Journal of Fisheries and Aquatic Sciences. The concept built on the work of other American limnologists such as Ruth Patrick, from which the modern riverine ecosystem model has emerged, and Luna Leopold, which deals with the physical changes of water. The essential goal of the concept was to further assess and explain the various communities in the system. Vannote himself described the current situation as follows, "in those days, most people studied a square meter of water to death ”. Meaning that previous research was always only on small pieces of water and only rarely was the entire river system considered, allowing for the creation of a general model.

After its publication, the River Continuum Concept was adopted as the accepted model in the limnology
Limnology
Limnology , also called freshwater science, is the study of inland waters. It is often regarded as a division of ecology or environmental science. It covers the biological, chemical, physical, geological, and other attributes of all inland waters...

 community, becoming a favorite means for describing the communities living in flowing water. Here it broke the classic idea of riverine structure. Previous approaches had their disadvantages because they only described small zones of water and had no consideration for the system in its entirety.

In practice, the River Continuum Concept is used today mainly for environmental assessment of rivers. River studies that assess riverine biological communities and have determined the species composition of an area can then be compared with the ideal species composition from the River Continuum Concept. From there, any variations in species composition may shed light on disturbances that might be occurring to offset the system.

Problems, limitations and modifications

Although the River Continuum Concept is a broadly accepted theory, it is limited in its applicability. It describes a perfect and even model without taking into account changing riverine disturbances and irregularities. Disturbances such as congestion by dams or natural events such as shore flooding are not included in the model.

Various researchers have since expanded the River Continuum Concept to account for such irregularities. For example, J.V. Ward and J.A. Stanford came up with the Serial Discontinuity Concept in 1983, which addresses the impact of geomorphologic
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...

 disorders such as congestion and integrated inflows. The same authors presented the Hyporheic Corridor concept in 1993, in which the vertical (in depth) and lateral (from shore to shore) structural complexity of the river were connected. The flood pulse concept
Flood pulse concept
In contrast to previous ecological theories which considered floods to be catastrophic events, the river flood pulse concept argues that the annual flood pulse is the most important aspect and the most biologically productive feature of a river's ecosystem. It is a geomorphological concept that...

, developed by W.J. Junk in 1989, further modified by P.B. Bayley in 1990 and K. Tockner in 2000, takes into account the large amount of nutrients and organic material that makes its way into a river from the sediment of surrounding flooded land.
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