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

Abstract

Although acidifi cation in freshwaters reduces the richness of aquatic species in general, dragonfl ies are less affected. However, detailed knowledge regarding the effects of very acidic (pH 700 µS cm -1 ) water on dragonfl y species richness and composition is still scarce. To assess this, 19 anthropogeni- cally infl uenced waters with a wide range of pH (2.64 - 6.81) and conductivity (113 - 2620 µS cm -1 ) were investi- gated in the Muskau Arch area (western Poland, eastern Germany). Of the 41 dragonfl y species found, 31 were autochthonous. Both total (St) and autochthonous (Sa) species richness correlated positively with pH and negatively with conductivity. However, the correlations for autochthonous species were strongly infl uenced by the samples from the extremely acidic (pH 2.64 - 2.86) and most ion-rich (conductivity > 1200 µS cm -1 ) waters, where no spe- cies developed. The Sa values from acidic waters with slightly higher pH values (between 3.0 and 4.0) did not differ signifi cantly from those found in neutral waters. Nevertheless, species preferring acidic or neutral conditions, re- spectively, 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 µS cm -1 have a detrimental effect on dragonfl ies. Other acidic waters are suitable habitats for specialists, which do not develop in neutral waters. Thus, moderate acidifi cation enhances the dragonfl y species richness of a region like the Muskau Arch area.