Dramatic fluctuations in salinity and ecological systems of Aral Sea remnant bodies

The Aral Sea area, formerly the world fourth largest lake, has been encountering one of the world’s greatest ecological disasters and currently represented by a chain of remnant lakes (Aladin et al, 2008) with a significant increase in salinity up to 130-145 g/L in the parts of Southern (Large) Aral. In this work, we are focusing on results of our 2018-2019 years field expeditions along the former Aral Sea area from the most southern point of Large Aral (Uzbekistan) up to Northern Aral remnant lakes and Karakol dam area (Kazakhstan) where we sampled 10-25 sampling points across a salinity gradient. The Northern Aral lakes including Shevchenko Bay, Butakov Bay, and Kotyrnak Cape have salinity returned to 3.7-9.3 g/L whereas a northern part of Southern (Large) Aral Sea, namely Chernyshev Bay, unexpectedly revealed dramatic fluctuations in salinity depending on season/year from 15 g/L up to 115-130 g/L. The significant fluctuations in salinity and other hydrochemical parameters(conductivity, ion concentrations etc.) are confirmed by results of ecosystem studies including evaluation of phytoplankton and zooplankton diversity, and plankton biomass distribution in these remnant parts of former the Aral Sea. To analyze phytoplankton communities from picoplankton up to colonial size algae we applied a combination of light and fluorescent microscopy as well as imaging flow cytometry (IFC) approaches. The phytoplankton samples were analyzed fresh and/or fixed with glutaraldehyde or paraformaldehyde applying Flowcam (Fluid Technologies Inc., USA), 6-lasers Imagestream X Mark II (Amnis-Luminex, USA) imaging flow cytometers, fluorescent laser scanning microscopic system (Zeiss, Germany) and light microscope (Leica, Germany). To evaluate fresh phytoplankton samples and shorter a period between sampling and analysis some instruments were transported to Barsakelmes Nature Reserve and used for sample acquisition. According to our results in hypersaline sites (126-145 g/L salinity), plankton communities were mostly represented by small pennatae diatoms, Nitzschia cf. communis and Dunaliella cf. viridis. Though we expected that hypersaline systems occupied all parts of Southern Aral, dramatic fluctuations of salinity and phytoplankton communities were discovered in Chernyshev Bay (northern part of Southern Aral). Shortly, in May 2018 salinity at Chernyshev Bay was only 15 0/00 and phytoplankton communities at this season were represented by Monoraphidium circinale, green algae Oocystis parva and Oocystis cf. marssonii, diatoms Cyclotella sp. and Cylindrotheca closterium, as well as prokaryotic and eukaryotic picoplankton. However, by May 2019 salinity of surface water stratum reached levels similar to other parts of Southern Aral. As documented by data taken with a help of a drone, these drastic fluctuations in water salinity can be explained by getting water flow in Chernyshev Bay through channel system both from Northern Aral lakes and also by 20 m-wide channel from Large (Southern) Aral. Moreover, the Chernyshev Bay is stratified with a salinity reaching 200 g/L levels in a bottom water layer. We conclude that though the ecosystems of Northern Aral lakes are recovering, Southern Aral continues its retreat to further hypersalinization and drying out, and may form soon a chain of separate lakes, starting with the separation of Chernyshev Bay area.