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Summary The relevance of environmental and spatial processes for species distributions varies among environments and types of metacommunities. Here, for the first time, we use modern statistical approaches to test the contribution of these two processes in structuring ecologically unique and threatened biotas of insular spring fens. We applied two species categorisations, common/rare and generalists/specialists, to disentangle the roles of dispersal capacity and habitat specialisation. In accordance with current understanding of headwater ecosystems, we found that environmental processes played a major role in most of the spring fen taxonomic and functional groups. However, we observed significant spatial structure in passive dispersers (Clitellata, a class of annelid worms), common species and habitat specialists. Spatial processes played the leading role in structuring the metacommunity of passively dispersing specialists. In contrast, all analysed insect groups, even those known to be poor dispersers, were able to reach virtually all favourable sites. We conclude that dispersal mode (active versus passive) and, to a lesser extent, habitat specialisation are the main factors determining the mechanism of spring fen metacommunity structuring.

Species composition changes along the pH and calcium gradients within wetlands were frequently studied for different groups of organisms, but few data are available for algae. Here we list 188 diatom taxa collected as epibryon and epipelon at 13 spring fens in the Western Carpathians distributed along the gradient of mineral richness. Species richness decreased along the gradient from calcareous fens to mineral-poor Sphagnum-fens. In agreement with fen typology based on higher plants, bryophytes, and molluscs, the same four fen types were identified. For each spring-fen type indicator diatom species were suggested. Conductivity and pH appeared to be the most important environmental factors responsible for the variation in diatom species data.

Aims The aim was to identify the main drivers of aquatic macroinvertebrate species richness in spring‐fen habitats (i.e. groundwater seepage wetlands) because these habitats are among the most threatened temperate biodiversity hotspots. Location Isolated spring fens in the western Carpathian Mountains. Methods Assemblages of Tricladida, Clitellata, Mollusca, Ephemeroptera, Plecoptera, Trichoptera and Diptera were sampled at 48 fens distributed evenly along a gradient of water mineral richness and grouped according to habitat specialization and dispersal mode. Several physicochemical parameters that have been reported to be the main drivers of species distribution were recorded for each site, along with radiocarbon measurements of the absolute age of the sites. The numbers of species in the taxonomic groups analysed were modelled as a function of the predictors via multiple linear regressions. Results We achieved a notably higher sampling efficiency than had been used in previous spring aquatic macroinvertebrate studies. In total, we collected and identified 255 species within 331 taxa from > 235,000 individuals. The number of species in all taxonomic groups increased with water discharge but the highest predictive power was obtained with water redox potential, which explained as much as 55.7% of the specialist species richness variance. We found contrasting and systematic differences in the importance of predictors related mainly to the level of species habitat specialization. Species richness of spring‐fen specialists was strongly determined by the main environmental gradient of change in groundwater chemistry, while generalists primarily reflected habitat stability linked to higher water discharge, habitat size and absolute age. Main conclusion Isolated island‐like spring fens can harbour unusually species‐rich assemblages of aquatic macroinvertebrates, the species richness of which is shaped by contrasting mechanisms dependent mainly on habitat specialization and also partly on dispersal mode. The richness of habitat specialist species seen at calcareous fens indicates their conservation priority.

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Summary The response of planktonic (phytoplankton, ciliates, rotifers and crustaceans) and littoral (Ephemeroptera, Plecoptera, Trichoptera and Heteroptera: Nepomorpha) assemblages to chemical recovery was studied over a twelve‐year period (1999–2011) in eight glacial lakes in the Bohemian Forest (central Europe). The region suffered from high atmospheric pollution from the 1950s to the late 1980s, but has since been recovering from acidification due to 86% and 44% decrease in sulphur and nitrogen deposition, respectively, during the 1990s–2000s. Despite the rapid improvement in water chemistry of all the eight studied lakes, only four have partly recovered so far (low‐aluminium lakes), while the other four lakes still remain strongly acidic (high‐aluminium lakes). All present lake assemblages are dissimilar by 40–90% from those during the early phase of chemical recovery as a result of species (re)colonisation (ciliates, crustaceans and insects) in the high‐Al lakes, and considerable species replacement (zooplankton) and (re)colonisation (insects) in the low‐Al lakes. Phytoplankton remained very similar in the high‐Al lakes, but changed (loss and/or replacement of some acid‐tolerant species) in the low‐Al lakes. Aluminium (Al) concentrations were dominant in structuring the assemblages of phytoplankton, rotifers and Nepomorpha, but also affected crustaceans through the seston carbon to phosphorus (P) ratio. Both direct (toxicity) and indirect (P availability) effects of Al control biological recovery in the Bohemian Forest lakes. A concentration of 200 μg L−1 of total Al is the main barrier preventing the high‐Al lakes from recovery. In contrast, pH and total P rather than Al significantly influenced Ephemeroptera, Plecoptera and Trichoptera. Although biotic responses (especially in the low‐Al lakes) showed important signs of recovery, such as reappearance of some indigenous or acid‐sensitive species, decline in eurytopic acid‐tolerant species and colonisation by vagile species, the assemblages of all the lakes still suffer from acid stress. Our results also indicate an increasing role of biotic interactions between colonisers and residents leading to the reconstruction of aquatic food webs in the low‐Al lakes.

In order to assess and compare the ecological impacts of channelization and shallow lowland reservoirs, macroinvertebrate communities of a lowland metapotamal river below reservoirs with epilimnial release were studied. The study was carried out in the Dyje River (Czech Republic) at five sites located from 1.5 to 22.5 km downstream of the reservoir outfall. The five sites differed in the degree of channel modification from natural muddy banks to riprap regulation. Seven samples were collected during the years 1998 and 1999 at each site using a semiquantitative method. The data were processed using multivariate analyses and methods for assessing the ecological and functional structure of communities. Altogether, 261 species of benthic macroinvertebrates were recorded including several rare and threatened taxa. Based on the results of principal component analysis (PCA), most of the variability within the species data (the first PCA axis) was explained by the degree of channel modification, from natural muddy banks with aquatic vegetation to a man-made riprap. The second axis was strongly correlated with current velocity. The sites differed in species richness, total abundances, proportion of individual functional feeding groups, pattern of the distribution of the current preference groups, and values of several biotic indexes, all of which also corresponded to the degree of channel modification. Thus, the morphological man-made modifications of the river channel were found to be the main factor affecting lowland river macroinvertebrates and their biodiversity. Our results suggest that the biggest threat to benthic macroinvertebrate diversity of lowland rivers comes from channelization. The impact of reservoirs can be completely overwhelmed by the impact of channelization, especially when muddy banks with aquatic vegetation present a substantial part of habitat diversity and significantly contribute to the total species pool.

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