Seasonal variations in the benthos of a chalk trout stream, the River Samson, Belgium

Seasonal variations in the benthos of a Belgian chalk stream were investigated. Total biomass varies extensively over the year (30 to 100 g wet wt. m−2). In summer, however, it is smaller than fish biomass. Densities range from 30 000 to 50 000 individuals m−2, but the actual number of taxa is always near 60. Diversity indices (Shannon-Weaver and Pielou) exhibit slight fluctuations. Numerical indices are minimal in winter when the stream is in flood. Gravimetric indices are minimal in spring when biomass is maximal. Species-abundance curves almost always indicate a fauna with many rare species, with, also, some indication of a second maximum of moderately abundant species. Six taxa dominate: Oligochaeta, Gammarus fossarum, Ephemeroptera, Coleoptera, Trichoptera and Diptera. There are great fluctuations in their relative importance over a year. Thus, there seems to be a succession of peaks in densities of individual taxa permitting more efficient utilization of resources. However, biomass variations depend on the species involved. As a general rule, maximum biomass precedes maximum density.