Influence of Disturbance on Habitats and Biological Communities in Lowland Streams

We studied 68 small lowland streams in Denmark of which the majority were affected by physical and chemical stress or a combination of both. Using DCA analyses, we analysed macrophyte and macroinvertebrate communities along a combined disturbance gradient. Both macrophytes and macroinvertebrate communities responded to the combined pressure gradient. We used a rigorous classification of the 68 sites, into 5 disturbance groups, with respect to physical and chemical disturbance and studied the effects of disturbance on physical habitat structure and density and diversity of macrophytes, macroinvertebrates and fish. Physical habitat structure in the disturbed streams was similar, except for variations in width which was lowest, and coverage of mud, which was highest in heavily disturbed streams. Macrophyte communities were impacted by disturbance. Average species richness and diversity were significantly lower in disturbed streams (8.6 and 2.8) than in relatively undisturbed streams (15.3 and 5.6). The total number of Ephemeroptera, Plecoptera and Trichoptera taxa (EPT) was significantly lower in disturbed streams (4.1) compared to streams experiencing intermediate disturbance (6.0-7.6) and undisturbed streams (7.0). Taxa associated with stable substrata, such as Leuctra sp. and Baetis sp., were reduced in abundance by approximately 50 % on disturbed sites. Density of trout (Salmo trutta L.) was markedly lower in disturbed streams (14 per 100 m²) than in undisturbed streams (55-204 per 100 m²). The results indicate that disturbance cascades through the stream ecosystem, primarily meditated by changes in macrophyte communities that are essential providers of habitat in unshaded lowland streams in which other structural elements, as coarse inorganic substrates and woody debris, are scarce. The analyses also show that the community variable responses to the combined stressors are not linear, which is an important issue implementing the ecological classification in the Water Framework Directive.