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Groundwater chemistry is a major determinant of assemblages of various taxonomic groups in spring fens, but its effect on insect assemblages has not been proved yet. We investigated dipteran assemblages at 17 isolated spring fens, which encompass the whole mineral richness gradient from rich (calcareous) to poor (highly acidic) sites, and analyzed faunal patterns at two contrasting mesohabitats: flowing water and standing water. The effect of water chemistry, substratum features, discharge, and temperature on the dipteran assemblages were assessed using PERMANOVA and GAM. Highly diverse dipteran assemblages (156 taxa) were closely related to the mineral richness gradient at both mesohabitats, showing a continual and nearly complete species exchange along the gradient, while their total abundance and taxa density did not change significantly. The assemblages included both habitat generalists and taxa specifically associated with acidic, moderate, or calcareous conditions. The mineral richness gradient was also reflected by changes in substratum properties, thus creating a complex environmental gradient that we suggest is the main environmental gradient structuring aquatic assemblages in spring fens.

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Predation may significantly control number and density of coexisting species. The effects of predation on species diversity have traditionally been tested in experiments and theoretical models of simple trophic systems. In complex natural ecosystems, however, disentangling multiple sources of variation is difficult. In groundwater‐fed environments, a significant effect of predation can be expected due to the relatively stable environmental conditions; however, it has never been properly examined. We analysed species diversity and total abundance of macroinvertebrate assemblages in 48 Western Carpathian spring fens, separately for whole sites and mesohabitat/season, and partitioned the effects of predation intensity from those of environmental variables in robust models using a bootstrapping technique. We verified our results by accounting for taxa resistant to predation. The assumption that predation‐mediated coexistence of species is the main mechanism responsible for the relatively species‐rich assemblages in the Western Carpathian spring fens was not supported. However, predation may significantly influence abundance of non‐predatory species and, under some conditions, it may contribute to explaining patterns in species diversity. The effect of predation did not differ between the mesohabitats with different stability. However, we found higher environmental control in spring and a stronger effect of predators in autumn, which suggests that different mechanisms influence fen assemblages in different seasons. Our study provides a new robust approach how to test the effect of predation on natural macroinvertebrate assemblages. The importance of predation was lower than expected in equilibrium assemblages but it may vary in time.

The metacommunity concept incorporates spatial dynamics into community ecology, shedding light on how local and regional processes interact in structuring ecological communities, and to which measure they are deterministic or stochastic. We reviewed metacommunity studies on freshwater meiobenthos published since 2004, when the main principles of metacommunity theory were conceptualized. The studies (together 19) were observational, focused mainly on ostracods, and rarely on rotifers and nematodes. In accordance with general expectations, the prevalent structuring force was species sorting.Ostracods showed more dispersal limitations than nematodes and rotifers, and there was very little support for dispersal surplus. We discussed the role of body size, dispersal mode, and attachment to sediment for the meiofauna dispersal. Effects of metacommunity context (habitat connectivity, spatial extent, and environmental heterogeneity), study design (e.g., sample size), and statistical approach could not be sufficiently disentangled due to the low number of studies. Local stochasticity, consistent with neutral theory and patch dynamics, was indicated for taxa with weak specialization and metacommunities in small habitats. Our understanding of meiofaunal metacommunities is only fragmentary and it would highly benefit from direct comparisons of taxa with different species traits and between different spatial scales, and studies incorporating temporal dynamics and hypothesis-driven experiments.

a b s t r a c tHarpacticoids are an important component of meiofaunal assemblages in springs. No information so far has been available on harpacticoid assemblages of the Western Carpathian spring fens, unique biotopes of high conservation value which cover a very long gradient of mineral content of groundwater, due to the variable geological background setting. Spring fens are isolated habitats of different age which can be assessed by radiocarbon dating of their basal sediment layers. This enables to test a possible effect of habitat age on species composition and species richness. In this study, we examined harpacticoid assemblages in 50 permanent tree-less spring fens (helocrenes) in the Western Carpathians (Slovakia and Czech Republic) in terms of species composition, total abundance, species density, and species richness. We tested mainly the effect of 12 explanatory variables describing water chemistry and temperature, climatic conditions, amount of nutrients, organic carbon, sediment structure, habitat age and size, using Canonical Correspondence Analyses (CCA) with stepwise forward selection. For the computation of species richness rarefaction was used. In total, 20 harpacticoid species were recorded with the total median density of 950 individuals in m −2 . Three significant explanatory variables, Ellenberg Indicator Values of plant community for nutrients, in situ measured pH, and average January temperature, explained together 19.0% (adj. 13.7%) of the total variance in the species composition data. The relationships of harpacticoids to these three explanatory variables were species specific and no uniform response of the total assemblage to the environmental variables was found (in terms of total abundance and number of species). The only exception was the influence of overall unfavourable conditions in the mineral-poor acidic Sphagnum-fens. Pilocamptus pilosus was significantly associated with a higher amount of nutrients and warmer climate. Nutrient enrichment was clearly indicated by a decrease or absence of crenophile Bryocamptus cuspidatus, and accompanied by an increase in ubiquitous Attheyella crassa. Moraria brevipes was confined to low pH, B. cuspidatus showed a high tolerance for low pH, whereas Bryocamptus echinatus preferred alkaline conditions. Despite a significant correlation between habitat age and species density we found no clear evidence that any colonisation driven process could influence the number of harpacticoid species within the tested time scale. We hypothesize that rather other habitat characteristics connected with age, i.e. habitat heterogeneity and stability, may be determinant for species richness. The occurrence of some species (e.g. P. pilosus, B. cuspidatus) was clearly geographically limited, but due to the spatial structuring of significant environmental variables no conclusion on dispersal limitations could be made.

Our understanding of functional roles of aquatic invertebrate taxa is still limited even for common species, although being crucial for explanations of patterns observed in natural communities. As only recently shown, the common native European amphipod Gammarus fossarum, traditionally treated as a shredder of leaf litter, shows predatory behaviour which may influence the composition of invertebrate assemblages. However, the evidence for the predation effect of G. fossarum on natural assemblages at the within-site scale is still lacking. Therefore, we collected 50 quantitative samples of macroinvertebrates along with the important environmental variables within a heterogeneous calcareous spring fen. Using linear regression, we explored the relationships between the abundance of G. fossarum (separately adult and juvenile) and the abundance and number of taxa for two groups of invertebrates differing in their susceptibility to predation, (a) hard-bodied taxa with protective body structures, such as shells and calcified cuticles, and (b) soft-bodied taxa without those protections. We separated the effect of G. fossarum from that of environmental conditions using variation partitioning. Our results showed that only the abundance of soft-bodied invertebrates was negatively correlated with the abundance of adult G. fossarum. However, the proportion of variation explained purely by predation (5.5%) was much lower than the one explained by the environment (33.8%). Both G. fossarum and soft-bodied invertebrates were positively associated with organic matter. Although hard-bodied invertebrates consisted of only a few taxa, they were more numerous than soft-bodied invertebrates, and only environmental control was confirmed for them. Despite the limitations of the used correlative approach, we conclude that G. fossarum can significantly control the abundance of vulnerable taxa in natural assemblages. Its predatory effects, however, may be relatively low and easily confounded by the effect of environmental control.