Abstract
The Aral Sea has undergone one of the most dramatic anthropogenic hydrological transformations, resulting in fragmentation of the former lake into several isolated basins with rapidly evolving physical and ecological regimes. This study examines recent changes in the South (Large) Aral, the deepest hypersaline remnant of the former Aral Sea, based on field observations data from the period 2017–2022, combined with autonomous temperature and dissolved‑oxygen measurements and remote sensing. During this period, the basin completed its transition to a fully isolated system without influence of inter-basin water exchange, with water salinity reaching 161 g/kg. The first estimates of the seasonal mixing cycle in the South Aral Sea reveals a seasonally stratified system, characterized by strong summer thermal stratification and winter ventilation. An exceptionally large seasonal range of surface temperature (> 33 °C) together with hypersaline conditions exerts a strong control on vertical stability and surface heat fluxes. Salt crystallization during winter cooling forms a thin near-bottom salinity-stratified layer that can suppress convection and prolong deep anoxia. The observed mixing regime determines a relatively stable biogeochemical cycle in which prolonged stratification from spring to winter leads to substantial near-bottom methane (CH₄) production, whereas complete winter mixing releases the accumulated CH₄. Biological surveys indicate further ecosystem simplification, characterized by the lowest recorded taxonomic diversity of pelagic microphytes in the South Aral Sea (eight taxa). This community is dominated by diatoms, alongside a strengthened yet likely food-limited monospecific zooplankton population of Artemia parthenogenetica .