Identification and relative contributions of environmental driving factors for abundant and rare bacterial taxa to thermal stratification evolution

The thermal stratification of reservoir affects water quality, and water quality evolution is largely driven by microorganisms. However, few studies have been conducted on the response of abundant taxa (AT) and rare taxa (RT) to thermal stratification evolution in reservoirs. Here, using high-throughput absolute quantitative techniques, we examined the classification, phylogenetic diversity patterns, and assembly mechanisms of different subcommunities during different periods and investigated the key environmental factors driving community construction and composition. The results showed that community and phylogenic distances of RT were higher than AT (P < 0.001), and community and phylogenic distances of the different subcommunities were significantly positively correlated with the dissimilarity of environmental factors (P < 0.001). Nitrate (NO₃^--N) was the main driving factor of AT and RT in the water stratification period, and Mn was the main driving factor in the water mixing period (MP) based on redundancy analysis (RDA) and random forest analysis (RF). The interpretation rate of key environmental factors based on the selected indicator species in RT by RF was higher than that of AT, and Xylophilus (10.5%) and Prosthecobacter (0.1%) had the highest average absolute abundance in AT and RT during the water stable stratification period (SSP), whereas Unassigned had the highest abundance during the MP and weak stratification period (WSP). The network of RT and environmental factors was more stable than that of AT, and stratification made the network more complex. NO₃^--N was the main node of the network during the SSP, and manganese (Mn) was the main node during the MP. Dispersal limitation dominated community aggregation, the proportion of AT was higher than that of RT. Structural Equation Model (SEM) showed that NO₃^--N and temperature (T) had the highest direct and total effects on β-diversity of AT and RT for the SP and MP, respectively.

Controlling spring Dinoflagellate blooms in a stratified drinking water reservoir via artificial mixing: Effects, mechanisms, and operational thresholds

Water-lifting aerators (WLAs) are often applied in stratified reservoirs to activate artificial mixing to inhibit harmful algal blooms (HABs). Here, the effects, mechanisms, and operational thresholds of spring Dinoflagellate control via artificial mixing were studied using a combination of taxonomic and functional groups and boundary line models. Algal cell density at two sampling sites (i.e., S1 and S2) decreased to below 1.0 × 10⁶ cells/L (corresponding chlorophyll-a content under 10 μg/L) during artificial mixing, with a Dinoflagellate removal efficiency of 97.1 % at S1 and 95.5 % at S2, respectively. The succession patterns of main phytoplankton taxonomic and functional groups changed greatly during artificial mixing at the sites: from Dinoflagellate and motile Chlorophyta to Bacillariophyta from groups A/L_O/P to A, respectively. Water temperature (WT), light availability (Z_eu/Z_mix), and mixing depth (Z_mix) were more effective factors influencing phytoplankton dynamics at a short-term scale, followed by total phosphorus (TP). A decrease in surface WT and Z_eu/Z_mix, and increase in Z_mix alongside the improvement of TP levels, which were induced by WLAs, drove the Dinoflagellate bloom control by a shift of phytoplankton structure from large, motile, and low surface to volume ratio (S/V) to small, immobile, and high S/V algae. The operational threshold values of WT, Z_eu/Z_mix, Z_mix and TP concentration for the suppression of Dinoflagellate growth using mixing systems are recommended as 9.6 °C, 0.17, 11.5 m, and 0.020 mg/L, respectively, based on a boundary line analysis. This work can help improve the cognition of mechanisms controlling HABs using mixing and aeration techniques in reservoirs.

Hypolimnetic deoxygenation enhanced production and export of recalcitrant dissolved organic matter in a large stratified reservoir

Global impoundment of river systems represents a major anthropogenic forcing to carbon cycling in reservoirs with seasonal thermal stratification. Currently, a quantitative and mechanistic understanding of how hypolimnetic deoxygenation in stratified reservoirs alters dissolved organic matter (DOM) cycling and lateral transport along the river continuum remains unresolved. Herein, we used optical and high-resolution mass spectrometric analyses to track seasonal and spatial compositional changes of DOM from a large, subtropical impounded river in southeast China. Aliphatic compounds were contributed by algal blooms to epilimnetic DOM during the spring/summer and by baseflow to the overall DOM pool during low-discharge periods. Deoxygenation-driven hypolimnetic mineralization enhanced in situ production of bio-refractory molecules and humic-like fluorescent DOM (FDOM_H) by utilizing bio-labile DOM and settling biogenic particles during periods of stratification. Production efficiency of hypolimnetic FDOM_H was 159-444% higher than that of the global dark ocean, and was strongly regulated by temperature and possibly substrate supply. The in situ production rate of hypolimnetic FDOM_H was four to five orders-of-magnitude higher than the dark ocean, with much faster turnover rates in dark inland waters versus the dark ocean. Collectively, these findings indicate that the hypolimnion is a hotspot for microbial carbon transformations, and hence an important source and pool of refractory DOM in aquatic systems. The lateral FDOM_H flux increased 10.8-32.1% due to hypolimnetic reservoir release during periods of stratification, highlighting the importance of incorporating hypolimnetic carbon transformations into models for carbon cycling of inland waters and the land-sea interface.

Laboratory investigation on the influence of factors on the outflow temperature from stratified reservoir regulated by temperature control curtain

A temperature control curtain can effectively mitigate the negative effect of outflow temperature on the river eco-environment downstream. To investigate the response of outflow temperature to influence factors (i.e., installation position of temperature control curtain, submerged depth, temperature distribution, and outflow discharge), experiments were conducted in a nonlinearly stratified fluid. The important degree of influence factors was determined by entropy weight method. The results indicated that the effect extent of influence factors on the outflow temperature was temperature distribution, submerged depth, outflow discharge, and installation position in turn. The installation position had little effect on the outflow temperature. Increasing the outflow discharge could withdraw more warm water near the surface and increase the outflow temperature. The outflow temperature also rose with decreasing submerged depth, and more warm water above the temperature control curtain level tended to be extracted when the submerged depth was enough. Although the outflow temperature increased, its variation amplitude depended on the temperature gradient of temperature distribution and was not affected by the structural form of selective withdrawal. From the point of operation management, the minimum submerged depth was determined using sensitivity analysis to obtain maximum improvement of outflow temperature.