Tributaries, sediment sources, and the longitudinal organisation of macroinvertebrate fauna along river systems

Tributary confluences are sites along a main channel where, because of the introduction of water and (or) sediment, the water volume, bed sediment character, and water quality of the mainstream can change abruptly. These shifts ensure that abiotic gradients seldom vary smoothly or continuously for distances of more than 100–102 km along any river system. The ways in which tributaries and related sediment recruitment points structure longitudinal changes in physical habitat are examined. Variables of importance to stream biota are affected and, in turn, it is suggested that the arrangement of tributaries and related features is an important control on the longitudinal organisation of macroinvertebrate benthos at moderate spatial scales. A new model is presented that stresses the importance of hydrological and sedimentological networks for organising lotic fauna. The link discontinuity concept emphasises the discontinuous nature of lotic ecosystem gradients, addresses the importance of tributaries in unregulated as well as regulated rivers, and extends, to its logical conclusion, the limited recognition of tributary influence in the river continuum concept. A case study from British Columbia, Canada, illustrates the general merit of the new model.