Under-ice Thermal and Oxygen Dynamics in Saline Lakes from the Tibetan Plateau

Dissolved oxygen (DO) is crucial for aquatic ecological and biogeochemical processes in lakes, yet under-ice thermo- and DO dynamics, particularly in saline alpine lakes, remain poorly understood. This study examines DO, temperature, and salinity in three brackish lakes (Selin Co, Nam Co, Bamu Co) on the central Tibetan Plateau. The results reveal that solutes redistribution after ice-on strongly shaped under-ice thermal structures and DO regimes. Early ice-on period, all lakes exhibited unusual hypolimnetic DO ventilation, which was triggered by benthic solutes accumulation in snowy winter and penetrative heating in snow-free conditions. In more salt lakes (Selin Co and Bamu Co with average salinity of 11.42 ± 0.04 and 12.16 ± 0.05 g L^-1, respectively), as high salinity lowered the temperature of maximum density (T_max, 1.20 and 1.35 °C for Bamu Co and Selin Co) and enhanced solute gradients, the atypical under-ice warm stratification formed approximately two weeks before ice-off. In Bamu Co, along with the warm stratification, the dissolved oxygen showed a abrupt increased to supersaturated from the surface to ~23.2 m below the surface, suggesting abundant biological productions. Subsequently, combined warm thermal and chemical stratification inhibited DO ventilation after ice break-up, except during some instantaneous mixing events. These findings highlight the critical role of salinity gradients in shaping thermal dynamics and oxygen transport in ice-covered saline lakes, offering mechanistic insights into global limnological responses to warming and brine rejection.