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.

The zooplankton community of a flooded opencast sulphur mine was investigated during two years. The complicated physical and chemical relationships in this water-filled pit caused an atypical plankton assemblage. At the time of the investigation 20 species of rotifers, 6 of Cladocera, and 8 species of Copepoda were found. A single Ostracod and Chaoborus flavicans were also noted in the samples. Usually the epilimnion was monopolised by rotifers whereas the hypolimnion had a more diversified assemblage of rotifers, cladocerans and copepods. The result of clustering analysis indicated that each predator was associated with its own group of prey. Discrimination analyses showed weak seasonal differences in the zooplankton and three zones in the vertical profile -epilimnetic, upper hypolimnetic and anaerobic. Principal component analysis reduced the chemical parameters to the main contributors of conductivity (the main anions and cations) plus toxic sulphide and hydrogen sulphide. The animals were classified by the first two components into two groups: cold stenotherms and medium-sized filtrators, such as Bosmina longirostris. A specific analysis of vertical profiles generated five variants of diel vertical migrations. When time -space distributions of the plankters were analysed the highest numbers of different species were usually in different times and depths. The epilimnion was inhabited by small fishes and was dominated by rotifers but was free from C. flavicans which inhabited the cold hypolimnion, together with the remaining invertebrate predators (copepods and Asplanchna).Chaoborus and the other invertebrate predators participated more evenly in the contributions of species to the total density. In the presence of invertebrate predators the structure of the rotifer population shifted from soft-bodied species to spined or fast-moving species. When the predation pressure was too strong, as in the hypolimnion, the soft-bodied species were, in practice, absent. When the pit was in an oligotrophic state the hypolimnion population was composed of up to 60 -90% rotifers. When the trophic state shifted to mesotrophy, the proportion of cladocerans increased.

General relationships of algae with chemical, physical and biotic parameters were analysed in a meromictic sunken sulphur mine (50°33.08¢ N, 21°36.01¢ E). Both zooplankton and phytoplankton were in low densities and number of species. Among the zooplankton there were 20 rotifers, six cladocerans and eight copepods. Algal species belonged to the cyanoprokaryotes, chrysophytes, diatoms, euglenophytes, dinophytes, cryptophytes and chlorophytes. Some diatoms (e.g. Navicula rhynchocephala) were able to live in darkness and in an anoxic hypolimnion and the monimolimnion in presence of sulphide. However, only single individuals of zooplankton were able to live in this layer. Discriminant analysis showed that the annual phytoplankton period was stable, excluding November and December. The same method showed that in vertical profile layer -the fifth meter of depth was different than the others. Multidimensional scaling divided numerous biotic and abiotic features into several important groups: (1) large invertebrate predators, (2) a majority of rotifers, (3) small invertebrate predators with their potential prey, (4) factors responsible for non-carbonate hardness and importance for life, (5) large planktonic filtrators, (6) a majority of algae, (7) sulphide with its derivatives, and (8) electrolytic elements.