Resistance of riverine macroinvertebrate assemblages to hydrological extremes

Macroinvertebrates were sampled in the lowland Drzewiczka River downstream from a dam reservoir and just below a whitewater slalom canoeing track. For over 20 years, pulse flow fluctuations of moderate intensity, an effect of two-three hour long releases of water per day to enable training of canoeists, induced a patchy mosaic in the tailwater riverbed compared to a natural site. After these regular disturbances, three accidental events of increased discharge of different magnitudes (three, five and sixteen times higher compared to a long-term median) occurred in two following years and we were able to investigate their impact on the habitat-specific processes.

Two of the three events (in September 2000 and March 2001) had a minor effect on abiotic and biotic variables, while the third one (in February 2002, over 40 m3 s−1 discharge) destabilized the bed habitat, washing away the flood-sensitive macroinvertebrates of Ephemeroptera and Trichoptera. In the dominant benthic group, i.e. Chironomidae, varied resistance patterns were observed, depending on their mode of life and patch occupancy. In conclusion, biota in the Drzewiczka River have adapted their life history to long-term moderate flow disturbance, but the largest flood mobilized bed sediments together with most of their dwellers.