Relative importance of space and time in determining the biotic structure in the upper part of spring-fed streams

An increasing number of recent investigations of a wide range of species have considerably changed perspectives in spring ecology. However, relatively little is known about the differences between the eucrenon and assemblages in downstream habitats. We studied changes in environmental conditions and in 4 taxonomic groups along a downstream continuum. Assemblages of diatoms, other benthic macroalgae including cyanobacteria, macroinvertebrates, and bryophytes were compared among eucrenal, hypocrenal, and epirhithral stations in 2 spring-fed streams of the southeastern Italian Alps. Environmental factors were less variable at the eucrenal station than at downstream stations, but this pattern was not reflected by an equivalent response of biotic assemblages. The eucrenon clearly differed from assemblages at downstream stations in ways that varied between taxonomic groups and time. Diatom and macroinvertebrate assemblage compositions were more variable at the eucrenal station than at downstream stations. Space was a more important factor than time for diatoms and benthic macroalgae, whereas space and time were almost equally important for macroinvertebrates. Downstream changes were inconsistent between taxonomic groups, and a larger number of spring-fed streams must be studied to detect more generally valid patterns in downstream variation of different taxonomic groups.