This study investigates the role of water level fluctuations (WLF) in shaping phytoplankton community structure and gross primary productivity (GPP) in Hongfeng Reservoir, a karst reservoir in Southwest China, through long-term monitoring (2016–2023) across seven sampling sites. Results revealed substantial seasonal and interannual variations in water levels (maximum fluctuation range: 9.78 m) and nutrient concentrations, with total nitrogen (TN) and phosphorus (TP) peaking during wet seasons. Chlorophyta dominated species diversity (42–48%), while Cyanophyta exhibited the highest relative abundance. GPP fluctuated markedly between the dry (586.09 mg C/m²/d) and wet seasons (3018.66 mg C/m²/d), driven by phytoplankton abundance and community stability. The Bray-Curtis (BC) dissimilarity index declined over time, showing a negative correlation with WLF (r = -0.03) and a significant negative impact on GPP. WLF indirectly enhanced GPP by altering community dynamics. Findings highlight WLF as a critical regulator of phytoplankton structure and productivity, emphasizing the importance of hydrological management in mitigating algal blooms and balancing ecological stability. This study provides actionable insights for optimizing water level regulation to sustain reservoir ecosystem health in karst regions.Supplementary InformationThe online version contains supplementary material available at 10.1038/s41598-025-25966-6.

In situ high-resolution insights into the dynamics of arsenic (As) species and heavy metals across the sediment-water interface in a deep karst reservoir.

Effectively mitigated eutrophication risk by strong biological carbon pump (BCP) effect in karst reservoirs.

Quantitatively tracing the decomposition of endogenous particulate organic carbon during sinking in (sub-)deep reservoirs: Using radiocarbon isotopes Δ14C.

Freshwater ecosystem is a significant natural source of CH4 emission in the atmosphere. To fully understand the dynamics of methane emissions in reservoirs, it is essential to grasp the temporal and vertical distribution patterns, as well as the factors that influence the methanogenic bacterial communities within the sediments. This study investigates the methane dynamics, carbon isotope fractionation (δ13CH4), and abundance of functional microorganisms along the geochemical gradient in the in situ sedimentary column of Hongfeng Reservoir (China). Notably, the methane concentration in sediment in summer ranged in 15.39-127.22µmol/L, which is twice as high as wintertime concentrations in the surface layer near the sediment-water interface (0-10cm depth). Illumina sequencing of the sediments identified 11 genera affiliated with methanogenic archaea, with dominant genus Methanosaeta reaching a relative abundance of 34.95% in summer. The total carbon (TOC) content in sedimentary columns in different seasons is positively correlated with Methanosarcina (P < 0.05). In addition, seasonal discrepancies are observed in the sediment profiles for total nitrogen (TN), sulfate (SO42-), and ferrous iron (Fe2+) concentrations. The concentration of total nitrogen (TN) is higher in summer than in winter. In summer, sulfate accumulates in the middle layer of the sedimentary column, while in winter, the maximum concentration of sulfate in the surface layer reaches 0.65mmol/L. These geochemical gradients drive the biological transformation of nitrogen, sulfur, and iron, may also be linked to the consumption of methane. Thus, it is established that the temporal and spatial dynamics of methanogenic communities in sediments significantly influence the fluctuations in methane release fluxes within reservoirs, highlighting the necessity to account for seasonal biological variations when assessing greenhouse gas emissions from reservoirs.

The key environmental factors driving the succession of phytoplankton functional groups in Hongfeng Reservoir, southwest China

Organic phosphorus regeneration enhanced since eutrophication occurred in the sub-deep reservoir

Using a structural equation model to assess the spatiotemporal dynamics and driving factors of phytoplankton in the plateau Hongfeng Reservoir in southwest China

(1) The results of fuzzy comprehensive evaluation based on fuzzy mathematics showed that S4 and S6 were unpolluted (I), S3, S5, and S7 were unpolluted to moderately polluted (II), S2, S9, and S10 were moderately polluted (III), and S1 and S8 were moderately to strongly polluted (IV). (2) S1, S8 and S9 were in considerable toxic risk and the value of toxicity risk index was in the range of 8.33-16.11 based on the PEL and TEL effect. (3) Industrial and agricultural wastewater and urban domestic sewage were the main sources of principal

Niche and interspecific associations of Pseudoanabaena limnetica – Exploring the influencing factors of its succession stage

Composition and indication of plankton fatty acids under the influence of environmental factors in the Hongfeng Reservoir, Southwest China

Internal phosphorus (P) input has been proven to be an important cause of eutrophication. The purpose of this study is to explore the process and mechanism of P release from sediments in seasonal hypoxic reservoirs. Six sediment cores were collected from Hongfeng Reservoir, one of the largest reservoirs in southwestern China. Incubation experiments were conducted using the sediment cores under aerobic and anaerobic conditions. The diffusion gradients in thin films (DGT) technique was employed to determine the concentration profiles and release characteristics of labile-P and labile-Fe at the sediment–water interface. The microbial community structure in surface sediments was determined by 16S rRNA sequencing. Compared with the aerobic condition, the P release flux was ~3.75 times under anaerobic condition, which mainly came from BD-P and NaOH-P. In addition, DGT-P and DGT-Fe were significantly positively correlated (R2 > 0.66, p < 0.001). From 16S rRNA sequencing, SRB and PSB were shown to promote P release through sulfate reduction and P dissolution in sediments. Moreover, the control measures of internal P release are discussed due to the potential risk of it in deep-water reservoirs. Dissolved oxygen is the key control factor of P release; thus, anaerobic conditions promoted the release of P from sediments. Fe-P reduction and dissolution are the main processes. SRB and PSB played an important role in the P cycle of sediments. It is necessary to increase oxygen in seasonal hypoxic reservoirs to reduce the risk of internal P release.

Varying water column stability controls the denitrification process in a subtropical reservoir, Southwest China

Freshwater sediments harbor taxonomically and metabolically diverse microbial communities, which play critical roles in aquatic ecosystems. However, little is known about the biogeographical patterns of assembly of such sedimentary communities, especially in cascade reservoirs of various age and hydrological characteristics. Using Illumina sequencing and quantitative real-time polymerase chain reaction targeting the 16S rRNA gene, we investigated spatial patterns of both bacteria and archaeal communities across eight reservoirs (including Hongfeng Reservoir (HFy), Hongjiadu Reservoir (HJDy), Wujiangdu Reservoir (WJDs), Dongfeng Reservoir (DFs), Silin Reservoir (SLd), Pengshui Reservoir (PSd), Suofengying Reservoir (SFYd), and Yinpan Reservoir (YPd)), forming the largest cascade reservoir system located on the Wujiang River, the tributary of the upper Yangtze River. Longer hydrological residual times promoted the proliferation of bacteria while reducing their diversity. In structures of different age, distinct archaeal communities were observed, with more abundant and diverse archaea in the earlier constructed reservoirs (HFy, WJDs). Both environmental factors (redundancy analysis) and spatial scale (non-metric multidimensional scaling analysis) determined community structures. The abundances of bacteria and archaea were both positively correlated with the presence of nutrients. In particular, a shift in the composition of the sedimentary archaeal community was observed along with increasing trophic complexity of these reservoirs, from Thaumarchaeota- towards Euryarchaeota-dominated communities. Also, the preference of several phyla of bacteria, including Latescibacteria and Bacteroidetes, for anaerobic and eutrophic habitats was significantly positively correlated with the presence of nutrients in these reservoirs. We demonstrated the existence of different distribution patterns of bacterial and archaeal communities in reservoir sediments, reflecting both deterministic and stochastic processes distinctly regulating the abundance of bacteria and archaea, in response to hydrological and biogeochemical changes after water impoundment in cascade reservoirs along a large river.

Dark carbon fixation and chemolithotrophic microbial community in surface sediments of the cascade reservoirs, Southwest China

Flux of organic carbon burial and carbon emission from a large reservoir: implications for the cleanliness assessment of hydropower

In situ, high-resolution evidence of phosphorus release from sediments controlled by the reductive dissolution of iron-bound phosphorus in a deep reservoir, southwestern China

The Guizhou Plateau in SW China is dominated by carbonate rocks and karst landforms. Erosion rates are low, but soils are thin and soil erosion remains a serious problem. Hongfeng Lake is the largest reservoir on the Guizhou Plateau. A 35-cm-long sediment core was retrieved from the reservoir and six soil profiles were sampled in the catchment. 137Cs activity was measured in the core to establish a chronological framework. Sediments and soils were analyzed for particle size, TOC, TN and mineral magnetism. Soil erosion that occurred between 1960 and 2003 was inferred from stratigraphic variations in particle size, TOC, C/N and magnetism of the sediments, aided by similar analyses on soils. Erosion was generally low from 1960 to 1987, but intensified thereafter, until about 1996. Between ca. 1996 and 2003, erosion declined. Land use data were derived from remote sensing images for 1973, 1990, 1993, 1996, 2000, 2003, 2006, 2010 and 2013, and running the Conversion of Land Use and its Effects at Small Region Extent (CLUE-S) model for 1976, 1979, 1982, 1985 and 1988. Precipitation, Digital Elevation Model (DEM) and soil-property data were also collected. Using these data, we determined input variables [rainfall erosivity (R), soil erodibility (K), slope length and gradient (LS), vegetation cover and management (C) and erosion control practices (P) for the Revised Universal Soil Loss Equation (RUSLE) Model]. Annual soil loss (A) was estimated for the Hongfeng Reservoir catchment for each year using the RUSLE model. Annual soil loss for 1973, 1976, 1979, 1982 and 1985 was generally low, with an average of 38.5 t ha−1 a−1. For 1988, 1990, 1993 and 1996, average annual soil loss increased to 57.7 t ha−1 a−1. The average for 2000 and 2003 declined to 46.6 t ha−1 a−1. Variations in the modeled annual soil loss are consistent with what was measured in the sediment record, implying that erosion events were recorded in the sediments despite temporary storage of sediments in the large, relatively flat catchment. Erosion was low and may have declined even more from 2003 to 2013. Despite the temporal variability, erosion was generally more intense in the west and east than in the central part of the catchment. Topography, rainfall and vegetation are the main factors that influenced soil erosion in the catchment.

Lake water level fluctuations (WLF) are an important factor driving the selection and seasonal dynamics of phytoplankton and potentially toxigenic cyanobacteria. Nevertheless, the relative importance of environmental drivers connected to WLF may be completely different, depending on the typology and use of waterbodies, latitude and climatic regimes. In this study, we investigated the impact of WLF in a large subtropical reservoir in south-eastern China (Hongfeng Reservoir, Guizhou Province). The study was based on monthly samplings carried out in 2014 in six stations. The strong increase in the water level observed in early summer caused a radical shift in the phytoplankton community. While in the pre-flooding period phytoplankton was composed of large diatoms, chrysophytes and Oscillatoriales (mostly Limnothrix sp.), the post-flooding period showed an increase in smaller and more competitive chlorophytes, smaller diatoms and cryptophytes better adapted to a fast colonisation of new and nutrient-rich environments. The environmental drivers that drove the change were dilution, flushing and interference with the seasonal water stratification processes. We concluded that, because WLF represents a complex variable integrating different physical effects in one explanatory descriptor, its value as a predictor of phytoplankton and cyanobacteria dynamics in lake ecosystems is difficult to generalise and needs to be investigated on a case-by-case basis. For this reason, considering the year-to-year hydrological variability that potentially characterise reservoirs, definite indications for management should be outlined considering more than 1-year study.

Integrated hydrologic and hydrodynamic modeling to assess water exchange in a data-scarce reservoir