Summary Ursu Lake is located in the Middle Miocene salt deposit of Central Romania. It is stratified, and the water column has three distinct water masses: an upper freshwater‐to‐moderately saline stratum (0–3 m), an intermediate stratum exhibiting a steep halocline (3–3.5 m), and a lower hypersaline stratum (4 m and below) that is euxinic (i.e. anoxic and sulphidic). Recent studies have characterized the lake's microbial taxonomy and given rise to intriguing ecological questions. Here, we explore whether the communities are dynamic or stable in relation to taxonomic composition, geochemistry, biophysics, and ecophysiological functions during the annual cycle. We found: (i) seasonally fluctuating, light‐dependent communities in the upper layer (≥0.987–0.990 water‐activity), a stable but phylogenetically diverse population of heterotrophs in the hypersaline stratum (water activities down to 0.762) and a persistent plate of green sulphur bacteria that connects these two (0.958–0.956 water activity) at 3–3.5 to 4 m; (ii) communities that might be involved in carbon‐ and sulphur‐cycling between and within the lake's three main water masses; (iii) uncultured lineages including Acetothermia (OP1), Cloacimonetes (WWE1), Marinimicrobia (SAR406), Omnitrophicaeota (OP3), Parcubacteria (OD1) and other Candidate Phyla Radiation bacteria, and SR1 in the hypersaline stratum (likely involved in the anaerobic steps of carbon‐ and sulphur‐cycling); and (iv) that species richness and habitat stability are associated with high redox‐potentials. Ursu Lake has a unique and complex ecology, at the same time exhibiting dynamic fluctuations and stability, and can be used as a modern analogue for ancient euxinic water bodies and comparator system for other stratified hypersaline systems.

Geography, babeş-bolyai university, 5-7 clinicilor str., 400006 cluj-napoca, romania; 2 national institute of research and development for biological sciences (nirdbs), institute of biological research, 48 republicii str., 400015 cluj-napoca, romania; 3 department of Aquatic Microbial ecology, institute of Hydrobiology, biology center of the Academy of sciences of the czech republic, na sádkách 7, 370 05 České budějovice, czech republic; 4 department of Molecular biology and biotechnology, Faculty of biology and Geology, babeş-bolyai university, 5-7 clinicilor str., 400006 cluj-napoca, romania; 5 5department of taxonomy and ecology, Faculty of biology and Geology, babeş-bolyai university, 5-7 clinicilor str., 400006 cluj-napoca, romania; 6 electron Microscopy center, babeș-bolyai university, 5-7 clinicilor street, clujnapoca, romania; 7 center for systems biology, biodiversity, and bioresources, babes-bolyai university, 5-7 clinicilor str., 400006 cluj-napoca, romania *corresponding author: gheorghe.serban@ubbcluj.ro ABSTRACTin the present work, we review the current knowledge on genesis, limnology and biodiversity of salt lakes distributed around the inner contour of eastern carpathian arc (transylvanian basin, central romania). transylvanian salt lakes formed on ancient halite (nacl) deposits following natural processes or quarrying activities. Most of these lakes are located in eastern (sovata area), southern (ocna sibiului), and western (turda-cojocna) parts of the transylvanian basin, have small surfaces (0.1-4 ha), variable depths (2-100 m), are hypersaline (>10%, w/v, total salts, mainly nacl) and permanently stratified. As consequence of steady salinity/density gradient, heat entrapment below surface layer (i.e., heliothermy) develops in several transylvanian lakes. the physical and chemical water stratification is mirrored in the partition of plankton diversity. lakes with less saline (2-10% salinity) water layers appear to harbor halotolerant representatives of phyto-(e.g., marine native Picochlorum spp. and Synechococcus spp.), zoo-(e.g., Moina salina), and bacterioplankton (e.g., Actinobacteria, Verrucomicobia), whereas halophilic plankton communities (e.g., green algae Dunaliella sp., brine shrimp Artemia sp., and members of Halobacteria class) dominate in the oxic surface of hypersaline (>10% salinity) lakes. Molecular approaches (e.g., Pcr-dGGe, 16s rrnA gene-based clone libraries, and dnA metabarcoding) showed that the o 2 -depleted bottom brines of deep meromictic transylvanian lakes are inhabited by known extremely halophilic anaerobes (e.g. sulfate-reducing delta-Proteobacteria, fermenting clostridia, methanogenic and polymer-degrading archaea) in addition to representatives of uncultured/unclassified prokaryotic lineages. overall, the plankton communities thriving in saline transylvanian lakes seem to drive full biogeochemical cycling of main elements. However, the trophic interactions (i.e., food web structure and energy flow) as well as impact of human activities and predicted climate chang...

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Background: Examining the effects of pesticides is difficult in regions such as Western Europe because of the relatively ubiquitous use of agrochemicals and the lack of unaffected areas. To study a wide gradient of agrochemical stress, we conducted a study in Central Romania, where traditional agriculture, which is assumed to use less agrochemicals, exists adjacent to intensive agriculture. We investigated potential effects of land use related stressors including pesticides on aquatic-terrestrial predator-prey relationships using stable isotope analysis. Therefore, we sampled spiders, as well as their aquatic and terrestrial prey along streams with a pesticide pollution gradient and determined spider and prey stable carbon and nitrogen signals. Results: Aquatic prey contributed 40.8-55.4% to the diet of the orb web weaving spider Tetragnatha sp. and 34.0-53.0% to the diet of the ground-hunting Pardosa sp. The biomass of potential aquatic prey increased along a gradient of increasing riparian habitat conditions and water quality and decreasing agriculture (e.g. arable land) in the catchment. The proportion of aquatic prey in the orb web weavers diet responded positively to the biomass of potential aquatic prey and negatively to this gradient. Increasing potential prey biomass resulted in an increasing proportion of aquatic prey for the orb web weaver. The proportion of aquatic prey in the ground hunters diet increased with instream pesticide toxicity and along a gradient of increasing pastoralism in the catchment as well as increasing riparian habitat and water quality. Conclusion: The diet of two riparian predators responded to complex gradients of catchment land use, stream and riparian habitat quality as well as to in-stream pesticide toxicity. The responses to the different environmental variables can be attributed to direct (e.g. change in spider community) and indirect (e.g. changes in aquatic prey and terrestrial prey) effects. Future manipulative field studies or experiments on aquatic-terrestrial food webs are required to examine the causality of our findings and should also consider the quality of prey organisms to foster mechanistic understanding of cross-ecosystem effects.