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The River Continuum Concept

An Overview
The River Continuum Concept 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 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 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 is an open ecosystem 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 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 changes of properties within the river are dependent primarily on the specific composition of the organisms 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 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 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 and stone fly larvae. 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 larvae, nematodes, and many other animal groups. Grazers The grazers (scrapers) feed off of periphyton that accumulates on larger structures such as stones, wood, or large aquatic plants. These include snails, 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 due to the excess presence of coarse plant matter. In the midreaches of a stream or river, 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 film. 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 in the water. The majority of the organic matter that does makes its way into the system is in the form allochthonous 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 roll as a supplier of organic material such as periphyton and other autochthonous 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 predators 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 is due to various factors. As described above, the river begins very strongly influenced from outside the system, especially in the form of organic material which is consumed by various macroinvertebrates (mainly choppers). As you go further down the system there is an increase in autochthonous production of organic material such as periphyton. 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 pretty much all 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 most biodiversity, each with different ideal conditions. Because there is a uniform use of resources and high stability, disturbances and fluctuations are usually corrected relatively quick. Inequalities in the use of resources will be quickly compensated for, creating a new equilibrium. 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 earthquakes. 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. The publication of the hypothesis 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 quickly the accepted model in the limnology 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, unlike the River Continuum Concept.

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 any disturbances that might be occurring to offset the system.

Problems, Limitations, and Modifications
Although the River Continuum Concept is a broadly accepted thoery, 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 in the model. For example, J.V. Ward and J.A. Stanford came up with the Serial Discontinuity Concept in 1983, which addresses the impact of geomorphologic 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, 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.