Influence of dam removal on trichopteran assemblages in the lowland Drzewiczka River, Poland

Abstract

The aim of this study is to assess the response of Trichoptera in a patchy tailwater stretch of the Drzewiczka River (Poland) to dam removal as compared to the previous pulse discharge disturbance. The study was carried out in the river at the end of a whitewater slalom canoeing track located downstream of the dam reservoir called Drzewieckie Lake (area 0.84 km²). Between February and April 2002, the dam reservoir was gradually emptied before its dredging, and the Drzewiczka River recovered its natural discharge (1.7–4.5 m³ s⁻¹) for several years (temporary renaturisation). Altogether, 120 monthly samples of macroinvertebrates and their environments were collected in two sampling cycles, S1 in 2000–2001 during high discharge fluctuations (2.1–12.0 m³ s⁻¹ daily) and S2 in 2002–2003 during renaturisation after emptying the reservoir, from the following five habitats: H_P—pool habitat, H_S—stagnant habitat, H_M—macrophyte habitat, H_B—bank habitat and H_R—riffle habitat. On the basis of trichopteran abundance, the patterns in their assemblages were recognised with use of a self-organising map that was a Kohonen artificial neural network. The obtained classification of trichopteran samples was found to be based in general on the spatial criterion, i.e. dependent on habitats, irrespective of which sampling cycle the samples originated from, which clearly showed that the habitat mosaic of the river bed was observed both in S1 and S2. In addition, a very important function was noted for riparian and land plants, which developed intensively at the bottom of the Drzewieckie Reservoir immediately after it was emptied. They restricted extensive transport of reservoir sediments to the downstream river reach, thus reducing differences between S1 and S2 in the tailwater. It is also worth noting that in S1, in comparison to S2, Cyrnus trimaculatus, Mystacides azurea and Lype reducta flowing downstream from the reservoir were observed more frequently in the river, which confirmed that impoundments can be conducive to the presence of certain species downstream of dams. Summing up, although artificial short-term flow fluctuations usually diminish the quality and quantity of benthos, the article presents a case in which they were small and short enough to allow the formation of a mosaic of bed patches and positively affected certain parameters of macrobenthic communities according to the intermediate disturbance hypothesis.