Drivers of Small-Scale Diptera Distribution in Aquatic-Terrestrial Transition Zones of Spring Fens

Little is known about macroinvertebrate assemblages inhabiting aquatic-terrestrial transition zones, particularly at groundwater-fed wetlands. We studied diversity and vertical distribution of Dipteran assemblages in 27 spring fens characterised by variable groundwater chemistry ranging from acidic to extremely calcium rich. We sampled semiaquatic habitats of aquatic-terrestrial zones and compared their dipteran assemblages with those of truly aquatic habitats in spring patches. Our study showed that semiaquatic habitats create an important part of spring fens, harbouring about one half of dipteran taxa inhabiting spring fens, with a similar proportion of spring specialists as in aquatic habitats. Dipteran abundance decreased gradually with sediment depth, being the highest in the uppermost layer. However, vertical distribution of Diptera differed among fens of different mineral richness, with the average depth of dipteran counts decreasing from calcareous to mineral-poor Sphagnum fens. Calcium carbonate incrustations in calcareous fens might block migration to deeper horizons and therefore may cause high sensitivity of dipteran assemblages to water level fluctuations induced by various factors, such as climate changes and water abstraction. Semiaquatic habitats contribute significantly to the species richness of dipteran assemblages by providing additional microhabitats in the peripheral parts of spring fens, which are rare habitats of high conservation value.