Effects of hydrological disturbance of different magnitude on riverine habitats and benthic invertebrates

Spates of different magnitude in a four order section of the lowland Drzewiczka River (central Poland) downstream from a dam reservoir and a wild-water slalom canoeing track (abbr. WCT) located just below the dam were studied. For over 20 years daily fluctuations enabling the training of canoeists induced a patchy mosaic of the riverbed, with five dominant habitats: pool, stagnant with emergent plants, submersed macrophyte, bank and riffle. Artificial floods, in September 2000, March 2001 and February 2002, were other flow events of this reach, thus the main aim of this paper is to assess the resistance of macroinvertebrates, measured as the relative lack of loss in density after floods. The three- (September) and five-fold (March) increased discharge in relation to the median affected the riverine environmental variables and caused the entrance into the water column and/or washing away of Ephemeroptera, Trichoptera and Simuliidae. In turn the highest-flow event (February, 16-fold flow increase) induced the instability of all bed patches. Oligochaeta, one of the dominant, inbenthic groups of passive drifters, were dragged along the bottom and then stopped by macrophytes. Meanwhile, chironomids, the second dominant benthic group, showed two kinds of behaviour patterns. Orthocladiinae, organisms prone to drift, were washed away after each flood; consequently the riffle and submersed macrophytes became partly depleted of them. Other midges, inbenthic Chironomini (Chironominae) with worse propensities to drift, were also washed out, except from deeper sediment layers, between the roots of macrophytes and in the bank habitat. Thus the macroinvertebrate assemblages in the Drzewiczka River have adapted to the moderate pulse disturbances, but their response to high flow events depends on the species' traits.