Effects of thermal stability on microcrustacean assemblages in spring fens

Springs are considered relatively stable aquatic environments and possible thermal refugia for cold-adapted taxa under climate change. However, permanent and pristine spring fens in the Western Carpathians show between-site variation in thermal stability with significant effects on macroinvertebrate assemblages. In this study, we investigated the impact of the thermal stability on microcrustaceans (Harpacticoida, Ostracoda). We disentangled various parameters of thermal stability, such as mean summer and winter temperatures, annual amplitude, and daily fluctuations, and related these parameters to mesoclimate, vegetation cover, and water table. We found that the relative abundance of cold-stenothermal species decreased significantly with increasing mean water temperature in summer, which had a significant effect on species composition. Surprisingly, ostracods were completely indifferent to thermal stability despite including cold stenotherms and crenobionts, suggesting that these species might have broader thermal tolerance. By contrast, harpacticoids significantly responded to both summer and winter mean temperatures, showing upper and lower limits of thermal tolerance. While vegetation cover significantly suppressed daily fluctuations in summer, no effect of daily fluctuations on microcrustaceans was found. The effect of water table was also significant but independent of thermal stability. We assume that the less thermally stable sites will not support the occurrence of cold-stenothermic harpacticoids as air temperature rises. However, an increase in winter temperatures may result in higher overall abundance of harpacticoids. We discuss how spring fens are probably most threatened by the combination of drought and increasing temperature.