Water conductivity mediates differences in clitellate assemblages in spring fens and adjacent streams

Groundwater-fed helocrene springs constitute hydrologically heterogeneous environment, vulnerable to human and climate-induced changes. Using quantitative samples of clitellate assemblages, we investigated whether hydrologically stable nearby streams can serve as refugia for species inhabiting helocrenes, prone to seasonal desiccation. As water conductivity constitutes the main environmental gradient of helocrene springs, we categorized them as low or high-conductivity sites and compared their assemblage diversity. We hypothesized that the spring–stream association can change along this gradient, expecting assemblage homogenization is promoted by high tufa precipitation, creating differences with tufa-free nearby streams. Contrary to this prediction, the assemblages of low-conductivity helocrenes were more homogeneous, being also significantly different from those in the streams. This result is related to the apparently more favorable tufa-free substrate at low-conductivity fens, as shown by the high taxa richness and the number of indicator species. Contrary to the other invertebrates, the clitellates differed between spring fens and streams only under acidic conditions. It seems that small adjacent streams can only serve as potential refugia for spring fen biota at sites with high conductivity, while at sites with low conductivity, clitellate assemblages differ more from those in adjacent streams and thus are more susceptible to disturbance.