Impact of pH and conductivity on species richness and community structure of dragonflies (Odonata) in small mining lakes

Although acidification in freshwaters reduces the richness of aquatic species in general, dragonflies are less affected. However, detailed knowledge regarding the effects of very acidic (pH < 4.0) and highly conductive (>700 mu S cm(-1)) water on dragonfly species richness and composition is still scarce. To assess this, 19 anthropogenically influenced waters with a wide range of pH (2.64-6.81) and conductivity (113-2620 mu S cm-(1)) were investigated in the Muskau Arch area (western Poland, eastern Germany). Of the 41 dragonfly species found, 31 were autochthonous. Both total (S-t) and autochthonous (S-a) species richness correlated positively with pH and negatively with conductivity. However, the correlations for autochthonous species were strongly influenced by the samples from the extremely acidic (pH 2.64-2.86) and most ion-rich (conductivity > 1200 mu S cm(-1)) waters, where no species developed. The S-a values from acidic waters with slightly higher pH values (between 3.0 and 4.0) did not differ significantly from those found in neutral waters. Nevertheless, species preferring acidic or neutral conditions, respectively, were clearly separated, showing a direct or indirect effect of pH on the community structure in the area. We thus conclude that only pH values below 3.0 and conductivity above 1200 mu S cm(-1) have a detrimental effect on dragonflies. Other acidic waters are suitable habitats for specialists, which do not develop in neutral waters. Thus, moderate acidification enhances the dragonfly species richness of a region like the Muskau Arch area.