Limnetica 30

Habitats are the places where organisms live and are defined by a set of abiotic and biotic characteristics that affect individual fitness and community structure. Therefore, the habitat of river organisms can be linked to a long list of relevant variables, including hydraulics, channel form, substrate composition, water quality, irradiance, and abundance of organic matter. Habitat preferences are well known for some species, particularly salmonids, but large uncertainties remain for many taxa.

River habitats are not composed of static features; instead the features fluctuate as the result of a large set of processes acting from the basin to the local scale. Local processes of importance in shaping river habitats include (depending on river type) channel migration, growth and mortality of riparian trees, formation of wood jams, storage of organic matter, macrophyte development, and travertine deposition. Sometimes it is possible to assess the positive or negative effect of a given process for a particular species, but most often researchers rely on the patterns of physical habitat rather than on the processes that shape these habitats.

As an example of the adjustment of river habitats to local processes, we show data from an experiment in which large wood was reintroduced to a small mountain stream. Wood jams produced large effects on hydraulics and substrate, large increases in the storage of organic matter, increases in invertebrate and fish densities, and changes in ecosystem functioning. This example emphasises the tight relationship between riparian dynamics and in-stream habitat, and the importance of local processes, in this case inputs of large wood to streams. Therefore, the assessment and management of river habitats must take into account not only the habitat itself but also key local processes that are essential for the long-term continuity of this habitat.