he lakes in the Qinghai-Tibet Plateau are greatly affected by climate change. The lake expansion and salinity decline caused by rapid warming are changing the physical environment inside the lake, which in turn affects its ecological process. However, the differences in the vertical distribution of phytoplankton biomass between the thermal stratification period and the mixing period in plateau lakes are still not fully understood. Based on the comprehensive observation data of 10 large lakes in the plateau during the thermal stratification period and the mixing period from 2024 to 2025, combined with the historical high-resolution and continuous lake water temperature profile, this study analyzed the difference of phytoplankton biomass ( represented by chlorophyll a ) between the thermal stratification period and the mixing period, and discussed the regulation mechanism of thermal stability and density barrier driven by salinity gradient on the temporal and spatial displacement of chlorophyll a. The results showed that the change of lake thermal structure dominated the vertical differentiation of chlorophyll a, and the concentration of chlorophyll a in most lakes during the thermal mixing period was significantly lower than that during the thermal stratification period. During the thermal stratification period, the chlorophyll a in the lakes of Bamu Co, Selin Co and Zhari Namco showed a typical deep chlorophyll maximum ( DCM ) phenomenon in the vertical direction ( the peak concentration of chlorophyll a in Bamu Co was 6.15 μg / L and 4.10 μg / L in 2024 and 2025, respectively ). In 2024, the peak concentration of chlorophyll a in Zhari Namco was 7.10 μg / L ; in 2024, the peak concentration of chlorophyll a in Selin Co was 3.97 μg / L ), that is, the surface concentration was low, and a significant peak was formed in the thermocline with the increase of depth and reached the maximum value, and then gradually decreased in the deep water layer. Not all lakes follow this rule. For example, although there is thermal stratification in Peiku Co and Tangra Yumco, there is no significant difference in the vertical distribution of chlorophyll a concentration. In the thermal mixing period, the vertical distribution of chlorophyll a concentration in the lake was in a uniform distribution state, and the overall chlorophyll a concentration was at a low level ( the maximum chlorophyll a concentration in the mixing period of Zhari Namco in 2025 was 2.61 μg / L, and the maximum chlorophyll a concentration in Selin Co was 2.40 μg / L ). In addition, the high-salinity water in the lower layer ( below 20 m ) of Dagze Co formed a stable salinity thermocline, resulting in the lake maintaining a density stratification state throughout the year. This unique physical structure makes the vertical distribution pattern of chlorophyll a significantly different from other lakes only controlled by thermal stratification. The results of this study show that the change of thermal structure controls the vertical distribution pattern of chlorophyll a in plateau lakes, while the stable salinity stratification of Dagze Co can form a perennial effective physical barrier and dominate its unique vertical distribution pattern. Therefore, the vertical distribution characteristics of phytoplankton biomass in plateau lakes are determined by the thermal stratification and high salinity of lakes.