Abstract: Yuehai Lake is a nationally significant wetland, where water pH influences the stability of the lake ecosystem. To investigate factors influencing Yuehai Lake's elevated summer pH, we analyzed monthly water quality data from provincial monitoring sections and daily data from the automatic monitoring stations for the lake in 2020-2024, and detailed surveys of the water quality of Yellow River replenishment area and Yuehai Lake during the summer of 2025. We then examined the spatiotemporal distribution of Yuehai Lake's pH as well as the effects of Yellow River replenishment, evaporative concentration, and biological activities on pH levels. The results showed: 1) During 2020-2024, summer pH levels in Yuehai Lake exceeded winter values, with daily pH readings occasionally exceeding the 9.0 limit stipulated by the Environmental Quality Standards for Surface Water (GB 3838-2002). During the summer of 2025, pH ranged from 8.23 to 9.24, spatially distributed with lower pH near the southeastern inflow point and higher values towards the lake center. 2) Correlation analysis revealed significant positive correlations between pH and dissolved oxygen (DO), and significant negative correlations with Ca²⁺ and H\mathrmCO_3^- . Principal component analysis (PCA) showed that pH had positive loadings on components representing aquatic photosynthetic activity intensity and calcium carbonate (CaCO₃) precipitation-dissolution equilibrium. Yuehai Lake's pH was influenced by Yellow River inflow, evaporative concentration, and biological activity. 3) Based on hydrochemical analysis, Yuehai Lake exhibited a Cl⁻-\mathrmSO_4^2- -Na⁺-Mg²⁺ ion composition, whereas Yellow River inflow water displayed an H\mathrmCO_3^- -\mathrmSO_4^2- -Ca²⁺-Mg²⁺ composition, indicating ionic concentration disparities. During summer, the pH of Yellow River inflow water was 8.02, but the Ca²⁺+Mg²⁺/\mathrmCO_3^2- +H\mathrmCO_3^- +H₂\mathrmCO_3^* ratio of the Yellow River inflow exceeded 1. Upon entering the lake, this inflow consumed dissolved inorganic carbon, causing the pH of the lake to rise. Gibbs diagrams indicated evaporative concentration in Yuehai Lake, with saturation indices for calcite and dolomite exceeding zero in lake water samples. This demonstrated that evaporative concentration promoted carbonate precipitation, consuming \mathrmCO_3^2- and H\mathrmCO_3^- to elevate lake pH. Furthermore, the partial pressure of carbon dioxide index remained below zero during summer, while dissolved oxygen saturation exceeded 100%, indicating that biological photosynthesis was the dominant process, leading to CO₂ consumption and a consequent rise in pH. This study suggests that the replenishment of water in the Yellow River, evaporation concentration, and biological activities contribute to the high pH values in Yuehai Lake during summer. To alleviate the increase in pH, we recommend adjusting the water replenishment scheme and closely managing the nutrient status of the lake.