river continuum concept A holistic view of rivers, first proposed by Robin L. Vannote and others in 1980, which permits a broad zonation of river systems based on the utilization of energy through the orderly processing of organic matter by the resident biota. Upstream, the river receives allochthonous material from adjacent and overhanging vegetation, supplying coarse particulate organic matter. This is broken down by ‘shredder’ organisms in a system that is largely heterotrophic (P/R < 1), because it operates in deep shade. The shredders produce fine particulate organic matter. This is carried downstream, where it is processed by ‘collectors’. As the stream widens, primary productivity increases and the shredders are replaced by grazers, living alongside collectors. Collectors predominate once more under mainly heterotrophic conditions still further downstream, where the river widens and becomes too deep for benthic plants. Predators occur throughout. The concept cannot be applied to all rivers (e.g. to those that originate above the tree-line) and breaks down when a river is blocked or passes through a lake.

river continuum concept The concept of a river as an ecosystem whose character changes continually between its source and its mouth according to the nature of its energy inputs. The headwaters are likely to be narrow, fast flowing, and shaded by trees and other vegetation, so that virtually all the energy enters in the form of leaves, twigs, and other debris from the surroundings. The fauna is dominated by detritivores and filter feeders. Further downstream, as the river broadens and becomes less shaded, it is colonized by algae and plants, which contribute significant energy to the community and are exploited by grazers. Towards the mouth, increased sediment loading reduces light penetration and in situ photosynthesis may decline. Energy, in the form of biomass and detritus, is constantly flowing downstream, hence the energetics of any particular section of the river are influenced by events upstream. The result is a longitudinal continuum of ecosystem structure, with certain predictable properties.