Salinity represents a major structuring factor in aquatic habitats which strongly affects species richness. We studied the relationships among species richness, density and phylogenetic diversity of zooplankton communities along a natural salinity gradient in astatic soda pans in the Carpathian Basin (Hungary, Austria and Serbia). Diversity and density showed opposing trends along the salinity gradient. The most saline habitats had communities of one or two species only, with maximum densities well above 1000 ind l−1. Similarity of communities increased with salinity, with most of the highly saline communities being dominated by one highly tolerant calanoid copepod, Arctodiaptomus spinosus, which was at the same time the only soda‐water specialist. Salinity obviously constrained species composition and resulted in communities of low complexity, where few tolerant species ensure high biomass production in the absence of antagonistic interactions. The pattern suggests that environmental stress may result in highly constrained systems which exhibit high rates of functioning due to these key species, in spite of the very limited species pool.

Considering the ongoing loss of aquatic habitats, anthropogenic ponds are gaining importance as substitute habitats. It is therefore important to assess their functioning in comparison to their natural precursors. Here we assess the biodiversity value of sodic bomb crater ponds by comparing their gamma diversity to that of natural reference habitats, astatic soda pans, and assess their importance on the landscape level by studying alpha and beta diversity. We studied aquatic organisms ranging from algae to vertebrates in a dense cluster of 54 sodic bomb crater ponds in Central Europe. Despite the overall small area of the pond cluster, gamma diversity was comparable to that found in surveys of natural habitats that encompassed much wider spatial and temporal scales. We also found a considerable number of species shared with reference habitats, indicating that these anthropogenic habitats function as important refuge sites for several species that are associated with the endangered soda pans. Moreover, we found a number of regionally or worldwide rare species. Among the components of beta diversity, species replacement dominated community assembly. Individual ponds contributed similarly to beta diversity in terms of * Corresponding author, csaba.vad@wcl.ac.at (C.F. Vad). Europe PMC Funders Group Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts replacement, being equally important for maintaining high gamma diversity and emphasising the role of the pond network rather than individual ponds. This pattern was seen in all studied groups. Bomb crater ponds therefore acted as important contributors to aquatic biodiversity. Considering the tremendous losses of ponds throughout Europe, anthropogenic ponds should be taken into consideration in nature conservation, especially when occurring in pond networks.

Despite the high conservation value of intermittent soda pans, these habitats were mostly targeted by small‐scale studies and little is known about their microscopic fauna. We sampled all the remaining 110 representatives of this habitat type (including recently restored ones) in Europe in an area covering approximately 125 000 km2. We analyzed spring and summer zooplankton species composition including rotifers, copepods, and cladocerans. We found 105 Rotifera, 26 Cladocera, and 26 Copepoda taxa, with local species richness ranging between 1 and 38 taxa. Alpha diversity was comparable in the two main groups (Rotifera, Crustacea), while beta and gamma diversity was higher in rotifers. The most frequently occurring zooplankters were crustaceans, while rotifer taxa were rarer as the most frequent species inhabited less than half of the pans. Dominance of the natronophilic Arctodiaptomus spinosus proved to be a reliable indicator for high quality soda pans. We concluded that the most typical soda pan zooplankton communities developed by early summer, in the concentrated pans, thus we would suggest considering this period as a reference for ecological state monitoring, and especially for evaluating the success of soda pan rehabilitations or reconstructions with the help of zooplankton. Based on these criteria, restorations could be considered only moderately successful.