Stability and changes of biomass of emerging insects and their possible causes

Summary

In Breitenbach/Germany there is a downstream increase of daily as well as annual temperature amplitudes on the 2 km stretch between a large spring entering the stream and its mouth into the Fulda River. Importance of this thermal gradient for longitudinal zonation of Ephemeroptera, Plecoptera and Trichoptera is re-evaluated. Less species than previously suggested are actually affected. A regular upstream increase of abundance is observed only in 3 species, but these include 2 of the most important secondary producers of the stream community, which is dominated by oligo-ste-nothermal species. More species exhibit a downstream increase of abundance, but none of these more polyeurythermal ones is among the numerically important species in the stream. Distribution and abundance of most species in the stream are irregular and apparently determined by other factors, e. g., substrate composition at the stream bottom.