New profiling and mooring records help to assess variability of Lake Issyk-Kul and reveal unknown features of its thermohaline structure

Three field campaigns on Lake Issyk-Kul in 2015, 2016 and 2017 yielded 75 CTD profiles and water samples at 34 locations across the lake, with surface biogeochemical and thermohaline parameters mapped at high resolution along the ship's track and 147-day temperature records from three moored thermistor chains in the eastern littoral. While the active layer's thermal state and some biogeochemical characteristics varied markedly between years under atmospheric forcing, the lake's haline structure was remarkably stable; the data do not confirm reported warming of the deep waters but do show a positive salinity trend in the interior over recent decades. A weak but persistent salinity maximum at 70–120 m is newly described and attributed, with simple model estimates, to littoral water advected into the deep layer along submerged ancient riverbeds during differential cooling.