Invertebrate hydrobiont fauna transformation in the Dombrovskyi pit lake during the period of 2014-2018

The aquatic invertebrate community of Dombrovskyi pit lake was investigated during 2014–2018. 25 species were recorded there. The only permanent component of plankton in the community is rotifer Brachionus plicatilis Müller, 1786; of benthos and neuston is hemipteran Sigara lateralis (Leach, 1817), beetle Hydrobius fuscipes (Linnaeus, 1758), flies Aedes sp., Ochlerotatus lepidonotus (Edwards, 1920), Culicoides salinarius Kieffer, 1914, Ephydra glauca Meigen, 1830 are permanent components of benthos and neuston. The main diversity of invertebrates is concentrated in littoral zone up to 2 m of depth. This is primarily due to the desalination of these areas by surface runoff. Over the last decade the mineralization of the surface water layer has decreased from 120–138 to 25–28 g/l. Combined with the desalination of water, the increase of species diversity of aquatic invertebrates was noticed in spring seasons (from 7 to 17 taxa). The freshwater taxa, which are not tolerant even to low water salinity, appeared in the community in 2018 (7–25 % of the species diversity), among them Hydrometra stagnorum (Linnaeus, 1758), Rhyacophila tristis Pictet, 1834, Coelambus impressopunctatus (Schaller, 1783), Enochrus coarctatus (Gredler, 1863), Hydrophilus caraboides (Linnaeus, 1758). The role of freshwater taxa, which are tolerant to low water salinity (up to 5 g/l), increased from 0–10 % of the species diversity in 2014–2015 to 22–35 % in 2018. Among them Eucyclops serrulatus (Fischer, 1851), Candona sp., Cypris pubera O. F. Müller, 1776, Cloeon dipterum (Linnaeus, 1760), Libellula depressa Linnaeus, 1758, Sympecta fusca Vander Linden, 1820, Paracorixa concinna (Fieber, 1848) was noted in 2018 for the first time. On the contrary, the number of saltwater taxa decreased from 20–29 % (2014–2015) to 5–14 % (2018). The saltwater aquatic invertebrate community of this pit lake has transformed into brakish-freshwater one over time. 83 % taxa of aquatic invertebrate communities from freshwater puddles near reservoir coast are noted in the Dombrovska reservoir. The introduction of species from protective canals of the reservoir is unlikely, as only 5 % of taxa from there are marked in it. The main way of the forming communities of invertebrates in this reservoir is the periodical flooding of freshwater coastal puddles.