<p>Understanding hydrophysical mechanisms and biogeochemical processes in lakes during the spring–summer transition is essential to reduce&#xa0;the risks of eutrophication and harmful algal blooms, which are serious environmental problems in recent decades. While there are extensive data on the effect of the thermal bar phenomenon in large lakes (Baikal, Ladoga, Michigan, and others), this phenomenon remains poorly understood in medium-sized lakes (with depths of up to 100 m) due to its short lifespan. In this article, the effect of water mineralization on the temperature and plankton distributions was studied during the spring thermal bar in a medium-sized lake—Lake Dolgoe, a natural landmark of the Ice Age and the deepest lake in Belarus—using a coupled physical–biological model. The model takes into account the intraday variability of atmospheric condition at the water–air interface. The simulations performed showed that the variation in plankton concentrations for different mineralizations was maximal near the lake surface. Higher mineralization promoted more intensive warming of the near-surface waters and a stronger vertical temperature gradient above a depth of 10 m. In addition, the influence of intraday variations in solar radiation on plankton dynamics became significant after the establishment of direct thermal stratification, the timing of which depended on the content of mineralization.</p>