A comprehensive 3-dimensional hydrodynamic and eutrophication model, the environmental fluid dynamics code model (EFDC) with three functional phytoplankton groups, was applied to simulate the algal dynamics in a mesotrophic P-limited subtropical plateau lake, Lake Erhai, Southwestern China. Field investigations revealed the seasonal patterns in external total phosphorus (TP) input and TP concentration, as well as the composition of the phytoplankton community. The model was calibrated to reproduce qualitative features and the succession of phytoplankton communities, and the net primary production was calculated. The modeled daily net primary production (NPP) ranged between −16.89 and 15.12 mg C/m²/d and exhibited significant seasonal variation. The competition for phosphorus and temperature was identified as the primary governing factor of NPP by analyzing the parameter sensitivity and limitation factors of the lake. The simulation of four nutrient loading reduction scenarios suggested high phytoplankton biomass and NPP sensitivity to the external TP reduction. A significant positive correlation was found among NPP, total phytoplankton biomass and TP concentration. Overall, this work offers an alternative approach to estimating lake NPP, which has the potential to improve sustainable lake management.

Reservoir water level regulation induces intricate processes of phosphorus (P) migration and release within the water-level fluctuation zone (WLFZ). These dynamic interactions pose significant challenges for effective pollution management strategies. This study focused on two typical tributary WLFZs (narrow gorges type and wide river type) in Three Gorges Reservoir (TGR), aiming to quantify P sources and assess their release potentials across two operational periods. Results showed that the deposition of suspended particulate P (SS-P) transported from the upstream was the dominant P source in the two WLFZs during the low water level period (August 2022). During the drainage period, the main P source of the ‘narrow gorges type’ WLFZ was P loss from slope soil above the WLFZ, but the ‘wide river type’ WLFZ had a mix of P sources, including P loss from slope soil, SS-P depositions from the TGR backwater and the tributary upstream. Among the three P sources, the slope soil source exhibited a relatively higher degree of P saturation (DPS) values (0.5–18.8%), indicating a greater potential for P release. Given that P loss from slope soil is the primary P source in the WLFZs during the drainage period, which coincides with the spring plowing season, it is crucial to implement measures to prevent P loss from slope soils to safeguard water quality in the TGR.

Large-scale hydropower development provides substantial socio-economic and energy benefits but simultaneously introduces complex ecological and environmental challenges that require comprehensive scientific assessment. This study systematically evaluates the effects of the leading reservoir (Longpan hydropower station, referring to the uppermost and principal flow-regulating dam in the cascade) in the middle reaches of the Jinsha River’s operation on the water environment of the mainstream Yangtze River, China, with the aim of clarifying its water quality responses and supporting evidence-based basin management. Based on an analysis of the current water quality conditions of the Yangtze River and a comparative review of the operational experience of the Three Gorges Reservoir, this research explores the mechanisms through which large reservoirs alter hydrological and ecological processes. These mechanisms include reduced flow velocity, prolonged water residence time, weakened pollutant dispersion, and increased risk of algal blooms in tributaries. To quantitatively assess these impacts, an improved river dilution–mixing model was developed and applied to simulate the water quality response during the dry season (February–April) under different discharge scenarios. Key downstream monitoring sections were examined. The modeling results indicate that the operation of the Leading reservoir can moderately reduce dry-season concentrations of key pollutants (e.g., total phosphorus, permanganate index) at downstream sections by approximately 2–5% on average, with spatially heterogeneous effects. Although the overall improvement magnitude remains limited, the combined effects of sediment deposition and in situ degradation may yield more pronounced real-world benefits. The findings underscore the importance of optimizing the regulatory function of the Longpan Reservoir through coordinated operation within the cascade reservoir system. It is recommended to integrate water resource allocation, water quality management, and aquatic ecosystem protection, alongside enhanced pollution control and ecological restoration in key zones. The methodology and findings provide a referenced framework for assessing the water-environmental implications of large-scale reservoir regulation in other major river systems.

Qionghai Lake is an important freshwater source in the Yunnan–Guizhou Plateau. However, cyanobacterial blooms have been observed recently in Qionghai Lake, but their formation mechanism and control management are not well understood. Herein, phytoplankton, zooplankton, eutrophication, nutrients, and biochemical indices were measured in Qionghai Lake from May 2022 to April 2023. The results showed that cyanobacterial blooms in Qionghai Lake predominated in Anabaena sp. with a density of 1.11 × 10⁷–18.87 × 10⁷ cells/L. Anabaena blooms started in the northwestern area of Qionghai Lake in November 2022 and then expanded to the entire lake until it peaked and subsided in February 2023. Protozoa dominated in zooplankton while having no significant relationship with Anabaena blooms in Qionghai Lake. The trophic level index and chlorophyll a showed similar spatiotemporal trends with Anabaena sp. density, and water quality in the northwest of the Qionghai Lake was worse than in other parts. Total nitrogen (TN) and total phosphorus (TP) were 0.41–0.54 and 0.021–0.045 mg/L from November 2022 to February 2023. TN and TP were positively correlated with Anabaena sp. density, but TP was the most significant environmental factor affecting Anabaena bloom in Qionghai Lake. These findings might provide essential information for improving bloom control and water quality remediation in Qionghai Lake.