Dynamics of cyanobacterial bloom and water quality in Lake Taihu under National Class Ⅲ water quality achieved in 2024

根据生态环境部设在太湖的17个国家地表水考核断面周年监测，2024年太湖水体总磷浓度年均值低于0.05 mg/L，是自2007年太湖大规模治理以来首次达到地表水Ⅲ类标准。为揭示太湖总磷浓度下降的生态环境背景与机制，基于太湖湖泊生态系统研究站2005—2024年的长期监测数据，分析了太湖水体总磷浓度与藻情、气象水文条件及其他营养盐之间的协同变化特征。分析发现，2024年太湖总磷浓度低值的出现与2022年以来蓝藻水华强度下降、外源氮磷入湖负荷降低密切关联，特别是2023年蓝藻水华强度明显偏低，惯性延续导致2024年上半年藻情极轻，对全年水体磷浓度低值的出现具有重要贡献；2024年下半年太湖藻情反弹，8月水体蓝藻生物量明显增高，与2024年度降雨量大、外源氮磷负荷增高有关。此外，2024年10—12月蓝藻水华面积也大幅增加，但同期水体藻类生物量、叶绿素a浓度的反弹幅度却较小，与秋冬季气温偏暖、入湖水量大引起的微囊藻团颗粒表层聚积条件较好等蓝藻水华表观强度放大作用有关。研究表明，外源氮磷负荷下降对太湖蓝藻水华强度已经产生了明显遏制作用，但气温、降雨、风浪等气候因子仍对蓝藻水华表观强度具有强烈影响。在蓝藻水华控制策略方面，应进一步加强外源氮、磷入湖负荷削减，持续推进局部湖区草型生态系统恢复工程，调控湖体食物链，以促进太湖水环境的进一步改善。;According to the year-round total phosphorus (TP) monitoring data from 17 national water quality monitoring sites in Lake Taihu, the Class Ⅲ level (China's surface water quality standards) of TP, defined as a TP concentration lower than 0.05 mg/L, has been achieved since 2007. In order to elucidate the concurrent environmental and ecological variation with the decrease of TP, the long-term variation of the monitoring data was analyzed using the ecological monitoring data of the Taihu Laboratory for Lake Ecosystem Research (TLLER) from 2005 to 2024. The findings of this analysis indicated that the low external loads of TP and total nitrogen (TN), coupled with the low intensity of the Microcystis bloom from 2022 to 2023, established a significant foundation for the low TP observed in 2024. The inertia of the low bloom intensity in 2023 resulted in an extremely low bloom intensity in the first half of 2024, which further caused a relatively low annual average of TP in 2024. Conversely, the increase in bloom intensity in the second half of 2024 was due to the increase in external loading of TP and TN, as well as the extremely high autumn precipitation and air temperature. Furthermore, the abnormally large bloom area during November and December in 2024 was inconsistent with the indicators of chlorophyll-a concentration and cyanobacterial biomass. In contrast, the escalation in bloom intensity during the latter half of 2024 can be attributed to the augmentation of external loading of TP and TN, compounded by the occurrence of extreme rainfall and elevated air temperature during the autumn months. Furthermore, the substantial expansion of the bloom area during November and December of 2024 exhibited an inconsistency with chlorophyll-a concentration and cyanobacterial biomass indicators, thereby indicating that the heightened rainfall and augmented air temperature served to amplify the apparent bloom intensity. The findings of this study indicated that a reduction in external TP and TN loading had significantly suppress the bloom intensity in Lake Taihu. However, it is important to note that climatic conditions, such as temperature, rainfall, and wind wave, continue to exert a substantial influence on bloom intensity. To ensure the maintenance of optimal water quality, it is imperative to consider further reductions in external loading of TN and TP. Furthermore, ecological restoration of macrophytes in specific lake zones and optical food chain are necessary to promote the further improvement of the water environment of Lake Taihu.