Spatial variations in physical structure and macroinvertebrates in lowland stream riffles

M.L. Pedersen and N. Friberg. Department of Freshwater Ecology, National Environmental Research Institute, Silkeborg, Denmark

The physical structure of two riffles in a lowland Danish stream was studied and its importance for the composition and density of the macroinvertebrate communities was evaluated. The two riffles were visually assessed to be very similar, but measurements revealed differences in hydraulic conditions, substrate and consolidation, which affected macroinvertebrate communities. The unconsolidated riffle had higher total macroinvertebrate abundance (4137 m^-2 vs. 1698 m^-2) and total species richness (31.7 vs. 28.8) and lower evenness (0.77 vs. 0.83) than the compact riffle. Macroinvertebrate communities were related to differences in mean substratum particle size on the unconsolidated riffle. Here a linear log log relationship existed between macroinvertebrate abundance and EPT taxa abundance and the median particle size (R²_TOTAL = 0.46, p=0.002; R²_EPT = 0.73, p< 0.001). No similar relationships were evident on the compact riffle. Moreover, macroinvertebrate communities on the unconsolidated riffle were dominated by species with a high colonising potential. Despite being assessed to the same scale inter riffle variation was high as the riffles differed substantially with respect to consolidation and hydraulic structure. This resulted in different macroinvertebrate community structure from the same species pool. The findings address the question, if macroinvertebrate communities can be assessed at the scale of the geomorphologic unit.