Community structure and vertical distribution of planktonic ciliates in the saline meromictic lake Shira during breakdown of meromixis

Abstract The study deals with the vertical distribution and seasonal dynamics of planktonic ciliates in the pelagic and littoral zones of a saline meromictic Lake Shira. Fourteen species of free-living ciliates have been found in the Lake, seven of them inhabiting the pelagic zone. The richness of ciliates is higher both in terms of the number of species and biomass in the littoral zone compared to the pelagic zone. Although the ciliate species diversity is low in the pelagic zone, in certain seasons, the biomass of some of the species may reach considerable values, up to 3.4 g m−2 in the water column. The biomass of ciliates in Lake Shira is generally comparable to the average values for other mesotrophic lakes. The abundance and composition of ciliate populations in Lake Shira vary considerably with depth. Changes in the Lake ecosystem caused by meromixis breakdown in 2015-2016, which induced considerable variations in many Lake components, affected the vertical distribution of planktonic ciliates. However, the annual average biomass of the species that inhabited the Lake before meromixis breakdown remained unchanged. The total ciliate biomass increased due to the presence of the new species.