Background <p>Dianshan Lake is the largest freshwater lake in Shanghai and is the major water source for the Upper Huangpu River, which supplies raw water to the Jinze Reservoir. In this study involving continuously monitoring, we tested the water quality of Dianshan Lake to understand the main water pollution indicators and the status of algal toxins and to evaluate the quality of water in Dianshan Lake.</p> Objective <p>The objectives of the study were to understand the changes in lake eutrophication status and algal toxin distribution patterns, the metabolism and accumulation of algal toxins in aquatic organisms in the lake area, and the changes in liver and kidney functions in healthy people after water treatment in Dianshan Lake in Qingpu District, Shanghai.</p> Methods <p>We collected water samples from 5 monitoring points in Dianshan Lake every month in 2021. The test methods were based on the national standards of China: MC-RR and MC-CR were analyzed by high-performance liquid chromatography combined with mass spectrometry. The Carlson index was calculated to assess eutrophication. Additionally, we collected 10 types of aquatic organisms and examined them for algal toxins once a month. We selected healthy human volunteers for experiments in which we collected fasting venous blood from each individual in both observation and control groups to measure renal function impairment.</p> Results <p>All seasonal conditions, including high water, stagnant water, and dry periods, were represented in the 600 water samples taken from Dianshan Lake in 2021. The permanganate index (COD_Mn) was within the values for Chinese national surface water quality standard III, while the total nitrogen and total phosphorus levels did not meet the surface water quality standards. According to the Carlson index, Dianshan Lake exhibited moderate eutrophication. MC-LR was the dominant toxin in the high-temperature season, when the amount of algal toxins increased. From June through August, extremely high levels of MC-RR were found, whereas MC-LR dominated from July through October. In aquatic organisms, both MC-LR and MC-RR were detected in the monthly samples. The levels of toxins in local healthy humans differed significantly between the observation group and the control group. Individuals in the observation group exhibited renal function impairment.</p> Conclusion <p>The water in Dianshan Lake is eutrophic. We detected both MC-LR and MC-RR in aquatic organisms. Our results indicate that long-term exposure to the polluted water of the lake may lead to health risks.</p>