Spatial distribution of planktonic ciliates in autumn at the Priplotinny Reach of the Kuibyshev Reservoir, the Volga River, Russia

Abstract

Spatial distribution of planktonic ciliates at the Priplotinny Reach of the Kuibyshev Reservoir (The Volga River, Russia), including the Usinsky Bay, was studied in detail at a vast station grid in the early-autumn period. The species diversity, species richness, abundance, biomass, and production of ciliates at the Priplotinny Reach were quite low, in contrast to those at the Usinsky Bay. At the Priplotinny Reach, the number of species per sample ranged as 3-23 species, abundance, as 18-356 cells L−1, and biomass, as 0.13-17.94 μg L−1. In the Usinsky Bay, these parameters were 17-38 species, 360-2284 cells L−1, and 5.2-61.4 μg L−1, respectively. Despite relative homogeneity of the hydrophysical conditions at the reach, a high variability in the quantitative characteristics of ciliate community was observed, probably due to local hydrological conditions. On the one hand, the peaks in the ciliates abundance coincided with the period of decrease of the water inflow, i.e., they were associated with the operating mode of hydroelectric power station, and on the other hand, they were caused by the direct influence of water masses of a large tributary. The structural organization of the ciliate community, in contrast to the general quantitative characteristics, was determined by local environment. An increasing role of bacteriovorous and histophagous ciliates was recorded in that areas characterized by high level of organic matter, particularly, in the area of conditional wastewater discharge, bridge construction, and tributary confluence. In general, the distribution of plankton ciliates at the Priplotinny Reach of the Kuibyshev Reservoir was largely influenced by the Usa River, which affected the redistribution of quantitative characteristics of the ciliate communities between the deep-water and shallow-water zones of the reach.