Effects of wind-driven current and thermal dynamics in a temperate monomictic reservoir: Implications for manganese transport and treatment in water supply systems.

Increasing manganese (Mn) concentrations in source water contribute to aesthetic and health-related concerns in drinking water. The challenges with Mn in drinking water primarily arise from elevated Mn concentrations in the water supply reservoir, with the inefficacy of Mn treatment largely attributed to fluctuating Mn levels in the water source. A three-dimensional Mn cycle model in a temperate monomictic reservoir, Tarago Reservoir, and a decision support system reflecting Mn variations in the local water treatment plant have been established in previous research. This study aimed to examine Mn variations from the reservoir to raw water and treated water under the influence of wind conditions during different stages of thermal structure, and discover valuable recommendations for Mn treatment in the local water supply system. We crafted 12 scenarios to scrutinize the impact of varying intensities of offshore and onshore winds on hydrodynamic processes and Mn transport during strong thermal stratification, weak thermal stratification, and turnover. The scenario analysis revealed that, during the gradual weakening of thermal stratification, offshore wind induced a substantial amount of Mn to the upper layers near the water intake point. Conversely, onshore wind hindered the upward transport of Mn. The simulated Mn in the raw water under the 12 scenarios indicated that the timing of turnover in the Tarago Reservoir is the primary concern for Mn treatment in the water treatment plant. Additionally, close attention should be given to the frequency and intensity of offshore winds during the weakening of thermal stratification.

Dynamic evolution of antibiotic resistance genes in plastisphere in the vertical profile of urban rivers.

Microplastics (MPs) can vertically transport in the aquatic environment due to their aging and biofouling, forming distinct plastisphere in different water layers. However, even though MPs have been regarded as hotspots for antibiotic resistance genes (ARGs), little is known about the propagation and transfer of ARGs in plastisphere in waters, especially in the vertical profile. Therefore, this study investigated the dynamic responses and evolution of ARGs in different plastisphere distributed vertically in an urbanized river. The biofilm biomass in the polylactic acid (PLA) plastisphere was relatively higher than that in the polyethylene terephthalate (PET), showing depth-decay variations. The ARGs abundance in plastisphere were much higher than that in the surrounding waters, especially for the PLA. In the vertical profiles, the ARGs abundance in the PET plastisphere increased with water depths, while the highest abundance of ARGs in the PLA mostly appeared at intermediate waters. In the temporal dynamic, the ARGs abundance in plastisphere increased and then decreased, which may be dominated by the MP types at the initial periods. After long-term exposure, the influences of water depths seemed to be strengthened, especially in the PET plastisphere. Compared with surface waters, the microbiota attached in plastisphere in deep waters showed high species richness, strong diversity, and complex interactions, which was basically consistent with the changes of nutrient contents in different water layers. These vertical variations in microbiota and nutrients (e.g., nitrogen) may be responsible for the propagation of ARGs in plastisphere in deep waters. The host bacteria for ARGs in plastisphere was also developed as water depth increased, leading to an enrichment of ARGs in deep waters. In addition, the abundance of ARGs in plastisphere in bottom waters was positively correlated with the mobile genetic elements (MGEs) of intI1 and tnpA05, indicative of a frequent horizontal gene transfer of ARGs. Overall, water depth played a critical role in the propagation of ARGs in plastisphere, which should not be ignored in a long time series. This study provides new insights into the dynamic evolution of ARGs propagation in plastisphere under increasing global MPs pollution, especially in the vertical profile.

River connectivity determines microbial assembly processes and leads to alternative stable states in river networks.

River network is a common form of lotic ecosystems. Variances in river connection modes would form networks with significantly different structures, and further affect aquatic organisms. Microbial communities are vital organisms of river networks, they participate in numerous biogeochemical processes. Identifying associations between microbial community and structural features of river networks are essential for maintaining environmental quality. Thus, dendritic (DRN) and trellised river networks (TRN) were studied by combining molecular biological tools, ecological theory and hydrodynamic calculation. Results illustrated that river connectivity, a vital structural feature exhibiting mass transport ability of river network, increased relative importance of homogeneous selection processes in microbial assembly, which would further shape community with alternative stable states. Between the two researched river networks, DRN possessed higher connectivity, which made homogeneous selection as the driving force in community assembly. The microbial communities in DRN were consisted of species occupying similar ecological niche, and exhibited two alternative stable states, which can decrease influences of environmental disturbance on community composition. On the contrary, lower connectivity of TRN decreased proportions of homogeneous selection in community assembly, which further led to species occupying varied ecological niche. The microbial community exhibited only one stable state, and environmental disturbance would cause loss of ecological niche and significantly alter community composition. This study could provide useful information for the optimization of river connection engineering.

Hydraulic characteristics in channel confluence affect the nitrogen dynamics through altering interactions among multi-trophic microbiota.

Identifying the distribution of multi-trophic microbiota under the complicated hydrodynamic characteristics of channel confluences and evaluating the microbial contributions to biogeochemical processes are vital for river regulation and ecological function protection. However, relevant studies mainly focus on bacterial community distribution in confluence, neglecting the essential role of multi-trophic microbiota in the aquatic ecosystems and biogeochemical processes. To address this knowledge gap, this study investigated the distribution of multi-trophic microbiota and the underlying assembly process under the hydraulic characteristics in the confluence and described the direct and indirect effects of multi-trophic microbiota on the nitrogen dynamics. Results revealed that, in a river confluence, eukaryotic communities were governed by deterministic processes (52.4%) and bacterial communities were determined by stochastic processes (74.3%). The response of higher trophic levels to environmental factors was intensively higher than that of lower trophic microbiota, resulting in higher trophic microbiota were significantly different between regions with varied environmental conditions (P < 0.05). Flow velocity was the driving force controlling the assembly and composition of multi-trophic microbiota and interactions among multi-trophic levels, and further made a significant difference to nitrogen dynamics. In regions with lower flow velocity, interactions among multi-trophic levels were more complex. There were intense nitrate and nitrite reduction and anammox reactions via direct impacts of protozoan and metazoan and the top-down control (protozoan and metazoan prey on heterotrophic bacteria) among multi-trophic microbiota. Results and findings reveal the ecological effect on river nitrogen removal in a river confluence under complex hydraulic conditions and provide useful information for river management.