Selection of water source for water transfer based on algal growth potential to prevent algal blooms

Influences of lower pH on phytoplankton growth in alkaline lakes after water transfer: Insights from a coupled hydrodynamic-algal ecological model and experimental analysis

Alkaline lakes with high pH and unique ecological communities often face water-level drawdown and ecological degradation problems due to climatic and hydrologic factors. Water transfer is becoming a popular method for solving these problems. However, a high pH is often considered the key to maintaining the stability of alkaliphilic algal communities, and a lower pH induced by water transfer from a neutral-pH river may threaten ecosystems in alkaline lakes. To explore the response characteristics of phytoplankton in alkaline lakes to pH changes, we conducted cultivation experiments on one species of dominant Cyanobacteria and one species of dominant Chlorophyta from alkaline lakes under different pH conditions. Subsequently, we constructed a coupled hydrodynamic and algal mathematical model considering the effect of pH to predict the dynamic changes in phytoplankton in a typical alkaline lake under water-transfer conditions. Both species are basophilic, and pH has a "low-inhibition and high-promotion" effect on their growth. A lower pH is detrimental to cyanobacterial growth and competitiveness, which may cause Cyanobacteria to lose their dominance in weakly alkaline environments with a pH < 8.5; additionally, water transfer causes a decrease in the total biomass and proportion of Cyanobacteria in Lake Chenghai, with decreases induced by pH changes accounting for 13.4% and 70.1%, respectively. The decrease in pH is the main reason for the decrease in dominance of Cyanobacteria after water transfer. These results provide a basic summary of the effects of pH changes on the algal growth in alkaline lakes and are a useful for formulating ecological water-transfer strategies for alkaline lakes.

Spatio-temporal variation of toxin-producing gene abundance in Microcystis aeruginosa from Poyang Lake

Microcystis aeruginosa (M. aeruginosa) causes massive blooms in eutrophic freshwater and releases microcystin. Poyang Lake is the largest freshwater lake in China and has kept a mid-nutrient level in recent years. However, there is little research on microcystin production in Poyang Lake. In this study, water and sediment samples from ten sampling sites in Poyang Lake were collected from May to December in 2020, and from January to April in 2021 respectively. Microcystis genes (mcyA, mcyB, 16 s rDNA) were quantified by real-time fluorescence quantitative PCR analysis, and then the spatial and temporal variation of mcy genes, physicochemical factors, and bacterial population structure in the lake was analyzed. The relationship between the abundance of mcy genes and physicochemical factors in water column was also revealed. Results indicated that the microcystin-producing genes mcyA and mcyB showed significant differences in spatial and temporal levels as well, which is closely related to the physicochemical factors especially the water temperature (p < 0.05) and the nitrogen content (p < 0.05). The abundance of mcy genes in the sediment in December affected the abundance of mcy genes in the water column in the next year, while the toxic Microcystis would accumulate in the sediment. In addition to the toxic Microcystis, we also found a large number of non-toxic Microcystis in the water column and sediment, and the ratio of toxic to non-toxic species can also affect the toxicity production of M. aeruginosa. Overall, the results showed that M. aeruginosa toxin-producing genes in Poyang Lake distributed spatially and temporally which related to the physicochemical factors of Poyang Lake.

Assessing risks of algal blooms in water transfer based on algal growth potential

Water transfer is an effective way to solve the unequal distribution of water resources to meet the needs of urban residents and industry. Annual wet weight data indicated that there may be algal blooms during water transfer. We explored the ecological risk of water transfer from Xiashan to the Jihongtan reservoir through algae growth potential (AGP) tests. The results showed that the Jihongtan reservoir had certain self-regulation abilities. When the total dissolved phosphorus (TDP) concentration was not more than 0.04 mg/L, the risk of algal bloom was low. When the N/P ratio (by mass) was less than 40, the ecological imbalance of algal growth may be caused. When the N/P ratio was 20, it was the best environment for algal growth. Under the current nutrient condition of the Jihongtan reservoir, the volume of ecological safety threshold in water transfer was 60% of the Jihongtan reservoir's capacity. If the nutrient level was further increased, the water transfer threshold would rise to 75%. In addition, water transfer may cause the homogenization of water quality and accelerate the eutrophication process of reservoirs. Regarding risk assessment, we believe that controlling nitrogen and phosphorus are more consistent with the natural evolution of reservoirs than controlling phosphorus alone for solving the problem of eutrophication.

Modelling of pH changes in alkaline lakes with water transfer from a neutral river

While water transfer from rivers to alkaline lakes has been proposed to solve lake water level drawdown and ecological degradation problems, its effectiveness for achieving ecological goals is often questionable. A sudden pH decline in alkaline lakes due to water transfer is considered likely to harm the lake ecology. However, it remains unclear to what extent water transfer affects alkaline lake pH. Thus, a three-dimensional numerical model coupling a pH calculation method considering the carbonate balance with the MIKE3 hydrodynamic model was developed to predict pH changes in an alkaline lake. Laboratory and field measurements verified the model reliability. The model accurately simulated the mixed-water pH during water transfer, with a root mean square error of 0.03-0.07 and a coefficient of determination of 0.894-0.998. The model was then applied to predict the pH response to water transfer in Lake Chenghai. The results showed that the pH response to water transfer demonstrated spatial and temporal variability, and a low-pH diffusion zone (pH ≤ 9) formed in the northern parts of the lake during annual water transfer; the effects of water transfer on the pH in the lake were cumulative over time, and the average pH in Lake Chenghai after five years decreased by 0.2 units; strong wind and low inflow could effectively reduce the low-pH diffusion area; and daily thermal stratification of the plateau region threatened the low-pH diffusion area control in Lake Chenghai. Our results provide a new reference for formulating ecological water transfer strategies for alkaline lakes and similar water bodies.

Strengthen the purification of eutrophic water and improve the characteristics of sediment by functional ecological floating bed suspended calcium peroxide and sponge iron jointly

To overcome the shortcomings of conventional ecological floating bed (CEFB) in purifying landscape water, this study constructed a functional ecological floating bed (FEFB) through the suspension of calcium peroxide (CP) and sponge iron (SI) jointly below the CEFB. The purification effect of water quality and influence of sediment were compared in control check, CEFB, and FEFB systems, which were loaded the same sediment and reclaimed water in a field experiment. Results showed that the FEFB suspended with CP and SI had evident purification effect on the quality of landscape water supplied with reclaimed water and can maintain stably the nutrient status of the water body at mesotrophic levels and low turbidity. The FEFB promoted the degradation of humus, thus eliminating the chroma risk in water body caused by the decay of plants from the CEFB. Moreover, the FEFB can control the sediment mass produced, reduce the total nitrogen (TN) mass of sediment, and decrease the transformable TN (TTN) content in the sediment. The FEFB enhanced the stability of phosphorus (P) in the sediment, where the relative content of Ca-P and stable P reached 42.18% and 64.27%, respectively. To sum up, the FEFB suspended with SI and CP can not only effectively control the eutrophication and sensory index of landscape water but also change the TTN content and P forms in sediment, making the sediment more stable. Thus, the FEFB provides an innovative approach to reduce endogenous nutrient release for landscape water along with recharging with reclaimed water.

Seasonal Variation and Spatial Heterogeneity of Water Quality Parameters in Lake Chenghai in Southwestern China

Seasonal dynamics and the vertical stratification of multiple parameters, including water temperature (WT), dissolved oxygen (DO), pH, and chlorophyll-a (Chl-a), were analyzed in Lake Chenghai, Northern Yunnan, based on monitoring data collected in 2015 (October), 2016 (March, May, July), 2017 (March, June, October), 2018 (August), and 2020 (June, November). The results indicate that the lake water was well mixed in winter and spring when the water quality was stable. However, when WT becomes stratified in summer and autumn, the Chl-a content and pH value changed substantially, along with the vertical movement of the thermocline. With rising temperature, the position of the stratified DO layer became higher than the thermocline, leading to a thickening of the water body with a low DO content. This process induced the release of nutrients from lake sediments and promoted eutrophication and cyanobacteria bloom. The thermal stratification structure had some influence on changes in DO, pH, and Chl-a, resulting in the obvious stratification of DO and pH. In summer, with an increase in temperature, thermal stratification was significant. DO and pH achieved peak values in the thermocline, and exhibited a decreasing trend from this peak, both upward and downward. The thermocline was anoxic and the pH value was low. Although Chl-a maintained a low level below the thermocline and was not high, there was a sudden increase in the surface layer, which should be urgently monitored to prevent large-scale algae reproduction and even local outbreaks in Lake Chenghai. Moreover, Lake Chenghai is deeper in the north and shallower in the south: this fact, together with the stronger wind–wave disturbance in the south, results in surface WT in the south being lower than that in the north year-round. This situation results in a gradual diminution of aquatic plants from north to south. Water quality in the lake’s southern extent is better than that in the north, exhibiting obvious spatial heterogeneity. It is recommended that lake water quality monitoring should be strengthened to more fully understand lake water quality and take steps to prevent further deterioration.

Severe cyanobacteria accumulation potentially induces methylotrophic methane producing pathway in eutrophic lakes

Although cyanobacteria blooms lead to an increase in methane (CH₄) emissions in eutrophic lakes have been intensively studied, the methane production pathways and driving mechanisms of the associated CH₄ emissions are still unclear. In this study, the hypereutrophic Lake Taihu, which has extreme cyanobacteria accumulation, was selected to test hypothesis of a potential methylotrophic CH₄ production pathway. Field observation displayed that the CH₄ emission flux from the area with cyanobacteria accumulation was 867.01 μg m^-2·min^-1, much higher than the flux of 3.44 μg m^-2·min^-1 in the non-cyanobacteria accumulation area. The corresponding abundance of methane-producing archaea (MPA) in the cyanobacteria-concentrated area was 77.33% higher than that in the non-concentrated area via RT-qPCR technologies. Synchronously, sediments from these areas were incubated in anaerobic bottles, and results exhibited the high CH₄ emission potential of the cyanobacteria concentrated area versus the non-concentrated area (1199.26 vs. 205.76 μmol/L) and more active biological processes (CO₂ emission, 2072.8 vs. -714.62 μmol/L). We also found evidence for the methylotrophic methane producing pathway, which contributed to the high CH₄ emission flux from the cyanobacteria accumulation area. Firstly, cyanobacteria decomposition provided the prerequisite of abundant methyl thioether substances, including DMS, DMDS, and DMTS. Results showed that the content of methyl thioethers increased with the biomass of cyanobacteria, and the released DMS, DMDS, and DMTS was up to 96.35, 3.22 and 13.61 μg/L, respectively, in the highly concentrated 25000 g/cm³ cyanobacteria treatment. Then, cyanobacteria decomposition created anaerobic microenvironments (DO 0.06 mg/L and Eh -304.8Mv) for methylotrophic methane production. Lastly, the relative abundance of Methanosarcinales was increased from 7.67% at the initial stage to 36.02% at the final stage within a sediment treatment with 10 mmol/L N(CH₃)₃. Quantitatively, the proportion of the methylotrophic methane production pathway was as high as 32.58%. This finding is crucial for accurately evaluating the methane emission flux, and evaluating future management strategies of eutrophic lakes.