Spatial structure of the caddisfly (Insecta, Trichoptera) communities in a river basin in NW Spain affected by coal mining

Abstract

The changes in caddisfly community structure and composition in the Sil River basin (NW Spain), associated with a gradient in natural conditions and a succession of alterations induced by human activity (mainly urban wastes and coal mining), were studied. Twenty-nine Trichoptera taxa belonging to 11 families were collected. The community diversity analysis showed important spatial differences from source to mouth. Towns and, above all, coal mines produced an increase in sulphate and chloride concentrations and conductivity, which adversely affected the trichopteran assemblages. A stepwise regression model, which introduced sulphate and chloride as variables, explained the changes in diversity produced in the basin (r=0.837, p< 0.01). The lowest diversity and richness values were recorded in the rivers suffering the greatest impact from coal mining or urban wastes. The heterogeneity values of the communities for the main rivers were clearly different. The greatest community heterogeneity corresponded to the main axis of the basin and was explained by spatial variability in water chemical composition. No relation was detected between structural changes at spatial level and geographical distance, such that pollution processes basically controlled the dynamics of the Trichoptera communities. Partial-CCA using distance from the source as a covariable revealed alkalinity and urban and mining-derived pollution as the main factors influencing caddisfly distribution. However, an ordination of sites in relation to the corresponding vectors did not fully agree with the chemical characteristics of the water. The species composition of the Trichoptera communities, therefore, did not seem to be good indicators of the chemical features of the rivers. The structure of the community (as expressed by richness and the Shannon index) appeared to be the better indicator of pollution.