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The increase in anthropogenic impact on aquatic ecosystems causes significant alterations in the composition and structure of hydrobiont groups. These processes are characteristic of the hydrobiocenoses of the Uday River, the valley of which lies within the National Nature Park "Pyryatynsky", established in 2009 in the Pyryatyn district of Poltava region. Within this protected area it is convenient to carry out continuous monitoring of changes in anthropogenic load on shore ecosystems of the river. One of the most convenient monitoring groups is zooplankton, which is one of the essential components of the trophic networks of aquatic ecosystems and which is the feeding basis of planktonophagous and young fish. The object of this research was the major groups of zooplankton: rotifers (class Eurotatoria), cladocerans (class Branchiopoda, order Cladocera), different age stages of copepods (class Copepoda), ostracods (class Ostracoda). Zooplankton were collected in the daytime in spring (mid April), summer (end of July) and autumn (late September) 2016 within eight research stations. As a result of the conducted research, 69 species of zooplankton were registered within the littoral water area of the Uday River. Monogonont rotifers numbered 19 (27.5%) species, bdelloid rotifers (subclass Bdelloidea), cladocerans – 33 (47.9%) species, copepods – 17 (24.6%). According to the faunal spectrum in the zooplankton groups, representatives of the cladoceran complex predominated. This is due to the favourable conditions for the development of filtrators, of which a considerable part is cladocerans. Three species of rotifers were identified in Poltava region for the first time: Beauchampiella eudactylota (Gosse, 1886), Dipleuchlanis propatula (Gosse, 1886), Mytilina acanthophora (Hauer, 1938). During the spring, 33 species of zooplankton were collected: rotifers – 10 species, cladocerans – 14, copepods – 9 species. In summer 41 species were registered, including rotifers – 10, cladocerans – 18, and copepods – 13. In autumn 37 species were found: 6 species of rotifers, cladocerans – 21 and copepods – 10. The species composition of the zooplankton in different seasons had a low similarity, as evidenced by Jaccard index: spring and autumn (J = 25.4), spring and autumn (J = 34.5), and summer and autumn (J = 34.6). In the spring, pelagic (36.3%) and phytophilic (45.6%) groups prevailed, and phytophilic dominated in summer (41.4%) and autumn (48.7%). Of the various groups classified according to feeding type, representatives of the peaceful group dominated in all the studied seasons, in spring comprising 69.7%, in summer – 65.8%, and in autumn – 73%. The density and biomass of zooplankton of the Uday River were “low” in spring and summer, and "very low" – in autumn.

In late July 2018, we studied the chemical composition of the water and benthic communities in two hydrogen sulfide springs in the catchment of the Iska-Shor Creek (lefthand tributary of the Usa River), as well as zoobenthos and plankton in the Usa River around the creek’s mouth. Compared to the reference site, water in the springs contained manyfold higher concentrations of chlorine, sodium, calcium, sulfate ions, and differed in the content of organic matter, heavy metals, and several micro elements. Water temperature did not exceed +6 °С. Zoobenthos abundance and biomass was low, comprising31 taxa of invertebrates. Chironomid larvae and harpacticoids prevailed. Chironomids were the most diverse group in the creek – 20 taxa, of which 17 were found directly in the springs. Tanytarsus verralli Goetghebuer, 1928 was the most numerous species. Apart from chironomids, we found nematodes, ostracods, cladocerans, first instar larvae of mayflies, stoneflies and caddisflies, and others Dipteran families. The diversityof planktic and benthic invertebrates, and the set of dominant species showed little variation among the sites surveyed in the Usa River. The dominants in terms of abundance in the river’s benthic fauna were chironomid species from the subfamily Chironomini: Tanytarsus verralli, Cladotanytarsus (Cladotanytarsus) mancus (Walker, 1856), and Polypedilum (Pentapedilum) exsectum (Kieffer 1916). The share of roundworms decreased and the share of cyclops and water mites increased in the zoobenthos of the river downstream from the mouth of the creek. Among zooplankton dominants, the abundance of Euchlanis dilatata Ehrenberg, 1832 decreased and the share of Cladocera from the genus Bosmina increased downstream the river. Inside the area directly affected bythe mineral water springs, we found a high abundance of euplanktic rotifers Asplanchna priodonta Gosse, 1850 and copepods Mesocyclops leuckarti (Claus, 1857), as well asan increased abundance of juvenile Cyclopoida. The studies have expanded our knowledge of the adaptive capabilities of invertebrates, and the ecology of aquatic communities forming in this extreme environment.

Aquatic food webs are supported by primary production from within the system (autochthony) as well as organic matter produced outside of and transported into the system (allochthony). Zooplankton use allochthonous resources, especially in systems with high terrestrial loading and moderate to low internal primary production. We hypothesized that due to high terrestrial loads and remnant submerged terrestrial material, allochthonous resource use by zooplankton would be significant in all reservoirs and would decline along an increasing reservoir age gradient. Using hydrogen stable isotopes and a Bayesian mixing model, we estimated the contribution of allochthonous sources to organic matter pools and crustaceous zooplankton biomass for ten reservoirs. Dissolved organic matter (DOM) in all systems was dominated by allochthonous sources (posterior distribution median >92% allochthonous), while particulate organic matter (POM) composition varied (2-68% allochthonous) and had a lower allochthonous fraction in older reservoirs. There was no relationship between zooplankton allochthony and reservoir age. Crustaceous zooplankton allochthony varied among systems from 26 to 94%, and Chaoborus allochthony, measured in four reservoirs, was similarly variable (33-94%). Consumer allochthony was higher than POM allochthony in some reservoirs, potentially due to terrestrial DOM pathways being important and/or algal resources being inedible (e.g., cyanobacteria). As with many lakes, in the reservoirs we studied, allochthonous inputs account for a significant fraction of the organic matter of basal consumers. [ABSTRACT FROM AUTHOR]

Phytoplankton is widely recognized as being regulated mainly by resources (nutrients and light) and predation by higher trophic levels. In reservoirs, these controls also can be modulated by hydrology, for example through the influence of flow pulses generated by the operation of the dam. In this study, we tested the influence of light, nutrients, and zooplankton grazing pressure, and also hydrology (as water residence time) on the phytoplankton biomass in eight tropical hydroelectric reservoirs, which differ in size, morphometry, location, trophic state, and water residence time. Our hypothesis was that, as these reservoirs are used for hydroelectric purposes, the control that would otherwise be exerted on phytoplankton biomass primarily by resource availability and grazing will also be modulated by hydrology. Low phytoplankton biomass (range of system medians = 12-299 μg C l) occurred in most systems, except for one highly eutrophic reservoir (median = 1331 μg C l). Our data showed that phosphorus was more often likely to be the limiting nutrient in these systems, as assessed through nutrient limitation indexes (nitrogen and phosphorus), based on concentrations and ratios. For most reservoirs, excluding the eutrophic system with high cyanobacteria biomass, seasonal water residence time was the variable that best explained phytoplankton variation among the several environmental variables analyzed in this study ( P < 0.0001; adjusted r = 0.38). Hydrology was an important and additional factor modulating phytoplankton in these tropical reservoirs, directly removing phytoplankton populations and their potential zooplankton grazers by washout, and also affecting nutrient availability. [ABSTRACT FROM AUTHOR]