We studied the physicochemical and biological characteristics of the Dombrovska pit lake in Ukraine. The lake formed in an abandoned opencast potassium salt mine and is one of the most saline inland water bodies in the world. It is 85 m deep (November 2015) and an annual inflow of about 2 Mm 3 of water. The lake has two distinct layers. The mesohaline surface (0-5 m) layer is well oxygenated and slightly alkaline (pH = 7.5-8.8). Its mineralization, expressed as dry mass, was 50-134 g dm − 3 , and its electrical conductivity (EC) was 58-134 mS cm − 1 . The underlying layer consists of hypersaline water with low amounts of dissolved oxygen, a neutral pH (6.7-7.4), high mineralization (179-420 g dm − 3 ), high EC (169-215 mS cm − 1 ), and higher concentrations of major anions and cations (except Ca 2+ ) and nutrients than the overlying water. The vertical relationship between major ions and metals and the future salinity of the lake are discussed. In terms of zooplankton, in July we found living specimens of the rotifer Brachionus plicatilis and the ciliates Paradileptus elephantinus and Tindinnidium sp. as well as dead rotifers, cladocerans, and copepods (in total, 19 species), but only live B. plicatilis and 9 dead species in November. In the littoral part of the pit lake, we found the diatoms Nitzschia pusilla and some Halamphora species (H. borealis, H. tenerrima, H. acutiuscula), which favour highly saline waters.
Global biodiversity decline is believed to be caused by high anthropopressure. Particularly vulnerable habitats are freshwater ecosystems, which are hotspots of biodiversity. The threat to these ecosystems are cyanobacterial blooms, which tend to proliferate in the face of climate changes. Cyanobacteria development and dominance affect the whole food web, especially the zooplankton community. We used three classical biodiversity indexes (species richness, Simpson's Diversity Index and Shannon Diversity Index) and three functional diversity indexes (functional richness, functional evenness and functional divergence) to study the impact of cyanobacterial bloom on the zooplankton community. The study was conducted in water bodies with a different period of bloom duration (short-lasting blooms vs. long-lasting blooms) in order to determine the impact of the proliferated blooms on the aquatic ecosystems. Use of functional diversity indexes allowed for identifying changes that can be overlooked by classical biodiversity indexes. We conclude that cyanobacterial bloom involves modifications of functional trait space of studied communities and, in consequence, functioning of aquatic ecosystems.
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