Ecology of rivers

The ecology of a river changes as you move downstream. The river continuum concept describes this. Rivers ecosystems were once seen as discrete entities. An ecosystem in the middle reaches was different and seperate from ecosystems up and downstream. Now the river is seen as a continuum with gradual changes as you move downstream. All ecosystems within a river are linked to those above and below. This could be through the spiralling of nutrients as they move with the current and cycle through the ecosystem or through the movement of energy and nutrients as fish and other taxa migrate upstream.

Whilst this shift in ecosystem type is seen as a continuum if we chose an ecosystem from the lower reaches of a system and compared it to one from the upland headwater sections they would appear as discrete and unconnected. For example the number and type of collectors would be completely different with the lower reaches being composed of species that can tolerate low dissolved oxygen levels. Shredders and grazers would be absent from the lower reaches but very apparent in the upland river system.

The movement of matter from the upland system to the lower system connects the two. And salmon or trout may connect matter whenthey smolt to migrate coastal zones and headwater streams. These long range migratory species transport nutrients between different sections of a river. Observing these changes is fascinating, trying to understand them as part of a continuum is complex. But rivers are the result of numerous intercating processes that link apaprently discrete sections and draw nutrients in from land based ecosystems. Conversely rivers deposit nutrients onto the land whenever they flood and indirectly transfer nutrents through species such as bats, herons and kingfishers. A river is never isolated, it is a response to processes on the land and is connected along its course and to the floodplain both in time and space. Rivers connect land, freshwater and sea.