Longitudinal patterns in fish and macrozoobenthos assemblages reflect degradation of water quality and physical habitat in the Bílina river basin

The aim of this study was to provide the first account of fish and macroinvertebrate communities in a heavily degraded river basin in the Czech Republic. Fish and macrozoobenthos were surveyed at 18 sites in the Bílina River and 11 sites in tributary streams during June-July 2007. Fish were sampled by electrofishing and macrozoobenthos were collected by kick-sampling using a sweep net. The composition of macroinvertebrate assemblages in headwater and reference sites in the upper Bílina River indicated clean water with saprobic index (SI) 1.31-1.43 followed by a transitional stretch downstream the Kyjická reservoir (SI 2.05-2.32) and dramatic decline of water quality to SI 3.18 in the river stretch downstream of industrial and municipal pollution at Litvínov-Záluží. Despite several minor pollution sources on the subsequent downstream river stretch until its mouth into the Elbe River, the water quality indicators fluctuated in the range of lower betamesosaprobity (SI 2.06-2.58). Species richness and biodiversity indices followed a similar pattern as river saprobity. Twenty-three fish species were documented in the Bílina River basin. Chub (Leuciscus cephalus), gudgeon (Gobio gobio) and roach (Rutilus rutilus) were the most frequent species at the Bílina sites. Chub was the most numerous fish in the tributaries. Fish species richness in the longitudinal profile did not increase downstream in the Bílina mainstem, most likely because the presence of reservoirs and water pollution interrupted the river continuum pattern. Qualitative data on fish assemblages corresponded to the course of environmental stress. A sustainable fish community was documented only in the lowermost site in Ústí nad Labem near the confluence with the Elbe River. The Bílina River tributaries constitute potential refuges for fish in this basin.