Deep Subtropical Reservoir Research Articles

Impoundment-induced stoichiometric imbalance exacerbated phosphorus limitation in a deep subtropical reservoir: implications for eutrophication management

Impoundment-induced stoichiometric imbalance exacerbated phosphorus limitation in a deep subtropical reservoir: implications for eutrophication management

Thermocline stratification favors phytoplankton spatial overlap and species diversity in a subtropical deep reservoir

Spatial segregation of species along opposing resource gradients is a major research topic in ecology as it enables the coexistence and maintenance of high diversity. Thermocline stratification causes heterogeneous resource distribution, however, the effect of thermocline stratification on phytoplankton dynamic in-depth profiles is still unclear. To understand the underlying mechanism behind the effects of thermocline stratification on phytoplankton species diversity in stratified lakes, the monthly changes in thermocline parameters, deep chlorophyll maxima (DCM) parameters, spatial overlap (SO) among major phytoplankton taxonomic groups, and phytoplankton species diversity were evaluated in Lake Qiandaohu in the Zhejiang Province of China from April 2017 to December 2018. Thermocline depth (TD) was significantly negatively related to thermocline strength (TS). The monthly air temperature was the main driver behind the thermocline and the seasonal thermal-stratification cycle was divided into two stratification stages. Significant linear relationships were observed between the DCM parameters (depth, concentration, and thickness) and TD during the strong stratification period. TD was significantly positively related to phytoplankton species diversity and the SO between Cryptophyta and Chlorophyta as well as between Cryptophyta and Bacillariophyta during weak stratification periods. Significant positive correlations were observed between SO and Shannon diversity during both periods. Structural equation modeling (SEM) showed that air temperature significantly decreased TD and increased species diversity by increasing SO during the strong stratification period. Strong stratification under warming favored the formation of shallower and thinner DCMs, leading to phytoplankton coexistence and maintenance of high species diversity through a preclusion of dominance. This study characterized the temporal dynamics of phytoplankton dynamic in-depth profiles in response to strong stratification from warming.

Influences of hydrodynamics on dissolved inorganic carbon in deep subtropical reservoir: Insights from hydrodynamic model and carbon isotope analysis

Dam construction significantly impacts river hydrodynamics, subsequently influencing carbon biogeochemical processes. However, the influence of hydrodynamic conditions on the migration and transformation of Dissolved Inorganic Carbon (DIC) remains uncertain. To bridge this knowledge gap, we integrated hydrochemistry, isotopic composition (δ13CDIC), and a hydrodynamic model (CE-QUAL-W2) to examine the distinctions, control mechanisms, and environmental effects of DIC biogeochemical processes in a typical large and deep reservoir (Hongjiadu Reservoir) under different hydrodynamic conditions. We evaluated hydrodynamic alterations through the Schmidt stability index and relative water column stability. The analysis disclosed that during weak hydrodynamics periods, the energy necessary for complete mixing the surface and deep water was 34 times higher (3615.32 J/m2 vs.106.86 J/m2), and stability was 13 times greater (312.96 vs. 24.69) compared to periods of strong hydrodynamics. Additionally, the spatiotemporal heterogeneity of DIC concentrations (1.4 % to -9.1 %) and δ13CDIC (-1.7 % to -19.5 %) from the dry to wet seasons reflected disparities in DIC control mechanisms under varied hydrodynamic conditions. Based on model simulations, our calculations indicate that during weak hydrodynamics periods, the enhancement of the biological carbon pump effect resulted in substantial sequestration of DIC, reaching up to 379.6 t-DIC·d−1 in the water. Conversely, during strong hydrodynamics periods, DIC retention capacity decreased by 69.2 t·d−1, resulting in reservoir CO2 emissions of 22.7 × 104 t, which were more than 7 times higher than during weak hydrodynamics periods (3.2 × 104 t). Our findings emphasize the discernible impact of hydrodynamic conditions on reservoir biogeochemical processes related to DIC. Considering the increasing construction of reservoirs globally, understanding and controlling hydrodynamic conditions are crucial for mitigating CO2 emissions and optimizing reservoir management.

Persistent response of the bottom free-living bacteria to typhoon events in a subtropical reservoir

Typhoon-induced perturbations can result in long-lasting effects on aquatic communities in subtropical lakes or reservoirs. However, the responses of bacterial communities and their related nutrient cycling to episodic typhoon events throughout the water column in deep waters remain largely unknown. Here, we conducted a four-year field study to reveal the depth-specific responses of both free-living (FL) and particle-attached (PA) bacteria to typhoon events in a subtropical deep reservoir from 2015 to 2018. By comparing the depth-specific responses of FL and PA bacteria, we found that typhoon-induced inputs of organic matter and microorganisms significantly increased FL bacterial diversity and changed FL bacterial community composition in bottom waters perhaps through the density current or undercurrent. Typhoon events had a more persistent effect on FL than PA bacterial communities, especially in bottom waters of the reservoir. Free-living bacteria were more associated with nutrient cycling in bottom waters than particle-attached bacteria. These findings provide deep understanding of how FL and PA bacteria respond to typhoon events at community level in subtropical deep reservoir and thus help us to improve reservoir management in a rapidly changing world.

Diatom evidence of ecological change in a deep subtropical reservoir (southeastern China) over the past 60 years

Diatom evidence of ecological change in a deep subtropical reservoir (southeastern China) over the past 60 years

Core taxa drive microeukaryotic community stability of a deep subtropical reservoir after complete mixing.

Microeukaryotes are key for predicting the change of ecosystem processes in the face of a disturbance. However, their vertical responses to multiple interconnected factors caused by water mixing remain unknown. Here, we conducted a 12-month high-frequency study to compare the impacts of mixing disturbances on microeukaryotic community structure and stability over different depths in a stratified reservoir. We demonstrate that core and satellite microeukaryotic compositions and interactions in surface waters were not resistant to water mixing, but significantly recovered. This was because the water temperature rebounded to the pre-mixing level. Core microeukaryotes maintained community stability in surface waters with high recovery capacity after water mixing. In contrast, the changes in water temperature, chlorophyll-a, and nutrients resulted in steep and prolonged variations in the bottom core and satellite microeukaryotic compositions and interactions. Under low environmental fluctuation, the recovery of microbial communities did not affect nutrient cycling in surface waters. Under high environmental fluctuation, core and satellite microeukaryotic compositions in bottom waters were significantly correlated with the multi-nutrient cycling index. Our findings shed light on different mechanisms of plankton community resilience in reservoir ecosystems to a major disturbance over depths, highlighting the role of bottom microeukaryotes in nutrient cycling.

The influence of the deep subtropical reservoir on the karstic riverine carbon cycle and its regulatory factors: Insights from the seasonal and hydrological changes

Reservoirs are widely established worldwide with considerable environmental impacts, especially on the riverine carbon cycle. However, the influence of reservoirs on the cycling of different forms of carbon and its regulation factors (e.g., seasonal variations versus hydrological management) have not been simultaneously studied. To fill this knowledge gap, seasonal water samples from the deep subtropical reservoir (Longtan reservoir) in the Pearl River were collected, and the concentrations and stable carbon isotopes of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC) were determined. The variations in stable carbon isotopes of DIC (-11.4‰ to -5.2‰), DOC (-32.2‰ to -26.2‰), and POC (-38.9‰ to -25.3‰) in the river-reservoir system indicated active production and degradation processes in different layers. We estimated that up to 23.0% of DIC, 20.5% of DOC, and most POC were intercepted or degraded within the reservoir. Our results further illustrated that hydrological management (water storage regulation) and seasonal variations from different perspectives controlled the cycling of different forms of carbon in the reservoir. In addition, with the gradual increase in the number of reservoirs, hydrological management can be considered as a potentially effective strategy to adjust the carbon biogeochemical cycling of reservoirs in the future.

Microbial one‑carbon and nitrogen metabolisms are beneficial to the reservoir recovery after cyanobacterial bloom

Cyanobacterial blooms have profound effects on the structure and function of plankton communities in inland waters, but few studies have focused on the effects of microbial-based processes in one‑carbon and nitrogen cycling on water quality improvement following the bloom. Here, we compared the structure and function of the bacterial community, focusing on microbial one‑carbon and nitrogen metabolisms during and after a cyanobacterial Microcystis bloom in a deep subtropical reservoir. Our data showed that microbial one‑carbon and nitrogen cycles were closely related to different periods of the bloom, and the changes of functional genes in microbial carbon and nitrogen cycling showed the same consistent trend as that of Methylomonas sp. With the receding of the bloom, the abundance of Methylomonas as well as the functional genes of microbial one‑carbon and nitrogen cycling reached the peak and then recovered. Our results indicate that microbial one‑carbon and nitrogen metabolisms were beneficial to the recovery of water quality from the cyanobacterial bloom. This study lays a foundation for a deep understanding of the cyanobacterial decomposition mediated by microbes in one‑carbon and nitrogen cycles in inland freshwaters.

Dynamics of nitrogen and phosphorus profile and its driving forces in a subtropical deep reservoir.

Nitrogen and phosphorus stratification is crucial for ecosystem dynamics in deep lakes and reservoirs. It is critical for water quality management to understand the characteristics of nitrogen and phosphorus stratification and its driving forces. In this study, high-frequency total nitrogen (TN) and total phosphorus (TP) from January 2017 to October 2019 were estimated using the datasets of high-frequency buoy parameters, including water temperature, pH, chlorophyll-a, oxidation-reduction potential, dissolved oxygen, and fluorescent dissolved organic matter. The results revealed that both nitrogen and phosphorus in water column were periodically stratified. Specifically, the stratification of nitrogen and phosphorus occurred from April to December or January of the following year. Moreover, indices of TN stratification (IC-TN) and TP stratification (IC-TP) were - 0.29 ~ 0.05 and - 0.78 ~ 0.28, respectively. Significant (P < 0.01) positive correlations were observed between RWCS (an index of thermal stability) and IC-TN (or IC-TP), indicating thermal stratification may be the main driving force of nutrient stratification at inter-month scales. Further analysis indicated that the thermal stratification may affect nitrogen and phosphorus stratification though (1) influencing algal growth and (2) affecting the release of internal sources and the material exchange between water columns. Furthermore, precipitation is also suggested as an important factor affecting the stability of nitrogen and phosphorus vertical profile in the flood season. These findings may provide important information for optimizing water quality management efforts in Qiandaohu and other subtropical deep reservoirs. In addition, the knowledge of the effect of temperature and precipitation on nutrient stratification are essential to understand future ecosystem dynamics of deep reservoirs.

Extreme Weather Events Enhance DOC Consumption in a Subtropical Freshwater Ecosystem: A Multiple-Typhoon Analysis.

Empirical evidence suggests that the frequency/intensity of extreme weather events might increase in a warming climate. It remains unclear how these events quantitatively impact dissolved organic carbon (DOC), a pool approximately equal to CO2 in the atmosphere. This study conducted a weekly-to-biweekly sampling in a deep subtropical reservoir in the typhoon-prevailing season (June to September) from 2004 to 2009, at which 33 typhoons with distinctive precipitation (<1~362 mm d−1) had passed the study site. Our analyses indicated that the phosphate (i.e., DIP; <10~181 nMP) varied positively with the intensity of the accumulated rainfall 2-weeks prior; bacteria growth rate (0.05~3.68 d−1) behaved as a positive function of DIP, and DOC concentrations (54~119 µMC) changed negatively with bacterial production (1.2~26.1 mgC m−3 d−1). These implied that the elevated DIP-loading in the hyperpycnal flow induced by typhoons could fuel bacteria growth and cause a significant decline of DOC concentrations. As the typhoon’s intensity increases, many mineral-limited lentic freshwater ecosystems might become more like a CO2 source injecting more CO2 back to the atmosphere, creating a positive feedback loop that might generate severer extreme weather events.

Particulate organic matter as causative factor to eutrophication of subtropical deep freshwater: Role of typhoon (tropical cyclone) in the nutrient cycling

Intense storms pose a serious threat to ecosystem functioning and services. However, the effects of typhoons (tropical cyclones) on the biogeochemical processes mediating risk of eutrophication in deep freshwater ecosystems remain unclear. Here, we conducted a three-year study to elucidate linkages between environmental change, stable isotopes and the stoichiometry of particulate organic matter (POM), and nutrient cycling (i.e., carbon, nitrogen and phosphorus) in a subtropical deep reservoir subjected to typhoon events. The typhoons significantly changed the nutrient levels in the deep waters as well as the thermocline position. Increased typhoon-driven organic matter input, algae sinking and heterotrophic decomposition interacted with each other to cause steep and prolonged increases of total nitrogen, ammonium nitrogen and total phosphorus in the bottom waters of the reservoir. Small-sized or pico-sized POM (i.e., 0.2–3 μm) showed a substantial increase in bottom waters, and it exhibited stronger response than large-sized POM (i.e., 3–20, 20–64, 64–200 μm) to the typhoons. Our results also indicated that typhoons boost the nutrient cycling in deep waters mainly through pico-sized POM.

Effects of rainfall on thermal stratification and dissolved oxygen in a deep drinking water reservoir

AbstractThermal stratification is crucial for water quality and ecological processes in deep lakes and reservoirs and can be substantially affected by meteorological and hydrological processes in the catchment. However, how thermal stratification responds to rainfalls of different intensities and changing hydrological processes has not been documented very well. Here, high frequency water column profiles at three stations in a large subtropical deep reservoir (Lake Qiandaohu, China) in 2017 were used to elucidate the impacts of rainfall on lake physical process and chemical environment. The impact of rainfalls on the thermal stratification and dissolved oxygen in riverine zone was more impressive than that in transitional and lacustrine zones. The effect on thermal stratification by rainfall was largely affected by the magnitude of rainfall. Moderate and heavy rainfall events could reduce the thermal stability of water column, deepen the mixing layer depth, and shape the thermocline, resulting from decrease of surface water temperature and increased inflows. While rainstorms could totally break up thermoclines in the riverine zone by high volume inflow flushing. In addition, we found that the hypoxia and anoxia initial depths increased during rainfall events in this reservoir, which were well related to the changes of mixing layer depths. This research highlights that quantifying the effects of rainfalls on thermal stratification and dissolved oxygen will be beneficial for optimizing reservoir management.

Spatial Variations of Subsurface Chlorophyll Maxima During Thermal Stratification in a Large, Deep Subtropical Reservoir

AbstractSubsurface chlorophyll maxima (SCM) are found in stratified lakes, reservoirs, and oceans. Spatial variations of the SCM magnitude, depth, and thickness during stratification and related factors were examined in a large, deep subtropical reservoir, Lake Qiandaohu. Significant spatial differences in the SCM parameters were found throughout the lake as thermal stratification developed in the late spring of 2014. SCM depth and thickness were positively correlated with euphotic depth and mixed layer depth but negatively correlated with epilimnetic nutrient concentrations. SCM magnitude was negatively correlated with euphotic depth and mixed layer depth and positively correlated with epilimnetic nutrient concentrations. Seasonal variations in SCM differed among subregions of the lake, which were related to seasonal differences in environmental parameters. SCM (averaging 66.2 mg/m2 in spring and 96.4 mg/m2 in summer) accounted for about 82% of the total areal chlorophyll of the water column (Tchl), and the SCM magnitude was better correlated with Tchl than the surface chlorophyll concentration. This study contributes to understanding of factors causing spatial variations in of SCM in the deep subtropical lakes.

Isotope Constraints on the Sources of Particulate Organic Carbon in a Subtropical Deep Reservoir

AbstractReservoirs are active sites in the terrestrial environment in terms of carbon transformation and storage. Continuous reservoir construction reinforces the importance of such artificial storage; however, knowledge pertaining to the sources, cycling, and preservation of autochthonous/allochthonous particulate organic carbon (POC) in reservoirs remains incomplete. We monitored the flux and carbon isotope composition of sinking particles in a subtropical deep reservoir, and the results were compared with the isotopic signatures of potential POC sources (soil) and the sedimentary sink. We found that the typhoon‐induced POC flux was transported laterally at intermediate water depths within the reservoir and that it contributed more than 70% of the annual deposition. An isotope ternary mixing model showed that over 87% of the POC preserved in sediments was allochthonous. Although the autochthonous POC from primary production was preferentially degraded, the preserved autochthonous POC (42 ± 14 gC m−2 yr−1) was comparable to the observed carbon dioxide uptake (35 gC m−2 yr−1) by the reservoir, suggesting a high efficiency of carbon preservation in subtropical deep reservoirs.

Spatial distribution and filtering efficiency of Daphnia in a deep subtropical reservoir

Studies of Daphnia distribution and function could help people manage and protect water quality. We investigated how spatial distribution and filtering efficiency of Daphnia in the transition and lacustrine zones of the Nanwan Reservoir (China). Samplings were conducted seasonally for 2 years from six sites in the reservoir. Daphnia abundance and biomass were significantly higher in the lacustrine zone than in the transition zone. Similar composition and biomass of edible phytoplankton were found in the two zones, suggesting that food quantity could not explain high Daphnia distribution in the lacustrine zone. The variations of water velocity and food quality could help explaining Daphnia patchy distribution in the reservoir. On the one hand, rapid water velocity can cause the Daphnia decrement in the transition zone. On the other hand, the ratio of particulate organic carbon (POC) to chlorophyll-a (chl-a ) concentration was significantly higher in the transition zone, indicating more allochthonous material constituted the food source for Daphnia. The lower quality food likely suppressed Daphnia development in the transition zone. A linear regression between Daphnia abundance and Secchi depth (SD) may suggest a cause-effect relationship where increased filtering efficiency was responsible for increased water clarity to some extent.

Thermal stratification dynamics in a large and deep subtropical reservoir revealed by high-frequency buoy data

We measure the thermal stratification dynamics in Lake Qiandaohu, China, a deep subtropical reservoir, to better understand the mixing mechanism and its response to lake warming. A high-frequency monitoring buoy dataset from February 2016 to October 2017 is used to evaluate variations in the water temperature profile, Schmidt stability (SS) and thermocline parameters, such as the thermocline depth (TD), bottom depth (TB), thickness (TT), and strength (TS), and elucidate the potential effects of thermal stratification on the lake's ecosystem. High-frequency observation data demonstrate that the lake's thermal-stratification cycle can be divided into three stages: formation, stationary and weakening periods. Consequently, a significant positive correlation between the TB and TT during the formation period and a significant negative correlation between the TD and TT are found during the stationary and weakening periods. Additionally, strong positive correlations exist among the TS, TT and SS for all the data. Our data indicated that an increase in the air temperature caused the surface water temperature, TT, TS and SS to increase. Furthermore, thermal stratification affected the vertical distribution of dissolved oxygen and expanded the area of the hypoxic-anoxic zone. The incomplete mixing of the water from December 2016 to February 2017 because of the high air temperature, which was 2.49 °C higher than the mean air temperature of 1966–2015 (6.44 °C), created the hypoxia hypolimnion from March to May 2017. Under the background of global warming, the thermal stratification of Lake Qiandaohu will likely intensify and further significantly affect the lake's ecosystem.

Temporal and spatial distribution of phytoplankton functional groups and role of environment factors in a deep subtropical reservoir

Phytoplankton and environment factors were investigated in 2015 and phytoplankton functional groups were used to understand their temporal and spatial distribution and their driving factors in Wanfeng Reservoir. Seventeen functional groups (B, D, E, F, G, J, Lo, MP, P, S1, T, W1, W2, X1, X2, Xph, Y) were identified based on 34 species. The dominant groups were: J/B/P/D in dry season, X1/J/Xph/G/T in normal season and J in flood season. Phytoplankton abundance ranged from 5.33×104 cells/L to 3.65×107 cells/L, with the highest value occurring in flood season and lowest in dry season. The vertical profile of dominant groups showed little differentiation except for P, which dominated surface layers over 20 m as a result of mixing water masses and higher transparency during dry season. However, the surface waters presented higher values of phytoplankton abundance than other layers, possibly because of greater irradiance. The significant explaining variables and their ability to describe the spatial distribution of the phytoplankton community in RDA differed seasonally as follows: dry season, NH4-N, NO3-N, NO2-N, TN:TP ratio and transparency (SD); normal season, temperature (WT), water depth, TN, NH4-N and NO3-N; flood season, WT, water depth, NO3-N and NO2-N. Furthermore, nitrogen, water temperature, SD and water depth were significant variables explaining the variance of phytoplankton communities when datasets included all samples. The results indicated that water physical conditions and hydrology were important in phytoplankton community dynamics, and nitrogen was more important than phosphorus in modifying phytoplankton communities. Seasonal differences in the relationship between the environment and phytoplankton community should be considered in water quality management.

Summer profundal hypoxia determines the coupling of methanotrophic production and the pelagic food web in a subtropical reservoir

Summary We investigated how profundal redox conditions determine the inter‐annual variation in methane oxidising bacteria (MOB) contribution to zooplankton production in a deep subtropical reservoir. Two hydrological regimes which affect MOB activity are considered: (i) reduced MOB activity resulting from high profundal oxygen saturation promoted by water column disturbance during the period of summer stratification; and (ii) increased MOB activity due to high profundal oxygen saturation during winter mixing. Four years of field stable‐isotope analyses revealed that oxygen saturation of profundal waters during summer stratification was negatively correlated with winter MOB contribution. This relationship is consistent with our theoretical ecosystem modelling. Although the ecosystem model simulation predicted positive effects of profundal oxygen supply during winter mixing on MOB contribution, the stable isotope mixing model indicated that the effects were secondary. Winter oxygen supply did not enhance MOB activity when methane accumulation during the preceding summer was low. Our findings suggest that summer profundal hypoxia plays the primary role in determining methanotrophic food‐web activity and methane‐derived carbon cycling in deep lakes.

Diazotrophic bacterial community variability in a subtropical deep reservoir is correlated with seasonal changes in nitrogen.

Nitrogen-fixing microorganisms (diazotrophs) play important roles in aquatic biogeochemistry and ecosystem functioning. However, little is known about the spatiotemporal variation of diazotrophic microbial communities in deep subtropical reservoirs. In this study, denaturing gradient gel electrophoresis (DGGE), clone libraries, quantitative PCR, and quantitative reverse transcription (RT)-PCR were used together to examine the vertical and seasonal patterns of diazotrophic microbial communities based on nitrogenase (nifH) gene sequences in the Dongzhen Reservoir, China, across time (every 3 months for 1 year) and space (five different water depths). In general, the numbers of DGGE bands increased with water depth during the stratification seasons (spring, summer, and autumn), with the clone-library-based operational taxonomic unit (OTU) number and nifH gene diversity being highest in autumn (6 OTUs at depth 0 m; 15 OTUs at 33 m) and winter (12 OTUs at 0 m, 13 OTUs at 33 m) but decreasing drastically in spring (2 OTUs at 0 m, 3 OTUs at 33 m) and summer (3 OTUs at 0 m, 2 OTUs at 33 m). The nifH gene abundance was lowest in the water mixing season (winter average, 5.17 × 10(7) copies/L) but increased in the three other seasons (9.03 × 10(9) copies/L). Cyanobacteria (dominated by filamentous thermophilic cyanobacteria and Cylindrospermopsis raciborskii) were the most dominant diazotrophic group at all depths and seasons, while both alphaproteobacteria and gammaproteobacteria were co-dominant in the bottom waters in autumn and winter. The distinct seasonal and spatial patterns in diazotrophic communities were significantly related to total nitrogen (TN) and ammonium nitrogen (NH4-N) in the reservoir (P < 0.01). Further, TN showed a significant positive correlation with nifH RNA copy number (P < 0.05) and DGGE band number (P < 0.01), whereas the NH4-N was negatively correlated with nifH DNA copy number (P < 0.01) and positively with both RNA/DNA ratio (P < 0.01) and DGGE band number (P < 0.01). Our data indicated that water stratification, mixing, and nitrogen might drive the diazotrophic community structure and activity in complex ways, thereby influencing the aquatic nitrogen cycle. Therefore, adaptive reservoir management strategies should carefully consider the effects of water stratification for protecting drinking water quality and for controlling the potential for diazotrophic cyanobacteria blooms.

Dissolved oxygen stratification and response to thermal structure and long-term climate change in a large and deep subtropical reservoir (Lake Qiandaohu, China)

From January 2010 to March 2014, detailed depth profiles of water temperature, dissolved oxygen (DO), and chromophoric dissolved organic matter (CDOM) were collected at three sites in Lake Qiandaohu, a large, deep subtropical reservoir in China. Additionally, we assessed the changes in DO stratification over the past 61 years (1953–2013) based on our empirical models and long-term air temperature and transparency data. The DO concentration never fell below 2 mg/L, the critical value for anoxia, and the DO depth profiles were closely linked to the water temperature depth profiles. In the stable stratification period in summer and autumn, the significant increase in CDOM in the metalimnion explained the decrease in DO due to the oxygen consumed by CDOM. Well-developed oxygen stratification was detected at the three sites in spring, summer and autumn and was associated with thermal stratification. Oxycline depth was significantly negatively correlated with daily air temperature and thermocline thickness but significantly positively correlated with thermocline depth during the stratification weakness period (July–February). However, there were no significant correlations among these parameters during the stratification formation period (March–June). The increase of 1.67 °C in yearly average daily air temperature between 1980 and 2013 and the decrease of 0.78 m in Secchi disk depth caused a decrease of 1.65 m and 2.78 m in oxycline depth, respectively, facilitating oxygen stratification and decreasing water quality. Therefore, climate warming has had a substantial effect on water quality through changing the DO regime in Lake Qiandaohu.