Quantifying Lake Water Quality Evolution: Coupled Geochemistry, Hydrodynamics, and Aquatic Ecology in an Acidic Pit Lake

Hydrological characteristics strongly dominate the spatiotemporal variation of bacterioplankton sub-communities in the Yangtze River Estuary

Seasonal variations in river discharge in estuarine ecosystems drive dynamic interactions between freshwater and seawater, significantly impacting the structure and functionality of bacterioplankton sub-communities. Seasonal saltwater intrusion during the dry season intensifies the dynamic variations in water quality and salinity between the northern and southern branches of the Yangtze River Estuary, further complicating the ecological processes governing bacterioplankton communities. Nevertheless, the mechanisms underlying these processes and their effects on distinct bacterioplankton sub-communities remain insufficiently explored. In this study, the composition, functions, and assembly mechanisms of bacterioplankton communities in the Yangtze River Estuary during different hydrological periods were examined using molecular technique and various statistical analysis methods. The results showed that rare sub-communities exhibited the highest α-diversity, abundant and transient sub-communities primarily contributed to the diversity differences across hydrological periods. Saltwater intrusion into the northern branch altered bacterioplankton community and weakened the distance-decay pattern of the transient and rare sub-communities. Additionally, stochastic processes governed the assembly of rare sub-communities, and saltwater intrusion disrupted their migration and dispersal patterns. In contrast, abundant and transient sub-communities maintain their stability by adjusting their ecological strategies in response to salinity changes. Functional analysis indicated that external nutrient inputs and hydrodynamic changes in the wet season promoted community functional diversity and activity, while abundant sub-communities in the dry season were more effective at occupying resources and performing specific functions related to carbon cycling. These findings highlight the relationships between bacterioplankton sub-communities and environmental changes in estuarine ecosystems, underscoring their key roles in biogeochemical cycling.

Seasonal variation drives species coexistence and community succession in microbial communities of stratified acidic pit lakes

Acidic pit lakes (APLs) are a special type of ecosystem and represent a significant environmental issue worldwide. While previous studies have explored the structure and function of microbial communities in APLs stratification, natural attenuation, and remediation processes, little is known about the succession patterns of microbial association networks and the underlying assembly mechanisms during seasonal succession. In this study, the distribution characteristics and succession patterns of prokaryotic and eukaryotic microorganisms in APLs across different seasons were investigated using 16S rRNA and 18S rRNA high-throughput sequencing technologies, combined with ecological and multivariate statistical methods. The diversity, composition and structure of prokaryotic and eukaryotic microbial communities showed obvious seasonal differences, and the surface waters were more susceptible to seasonal disturbances. Temperature is the most critical factor influencing the seasonal succession of microbial communities. During the year-round succession, variable selection (40.86 %) dominated in the prokaryotic community and homogeneous selection (69.64 %) dominated in the eukaryotic community. Moreover, the proportion of deterministic processes increased with increasing water temperature differences. Co-occurrence networks were more complex and inter-kingdom exchanges were more frequent during the warm seasons (summer and autumn), and microbial communities were more stable during the cool seasons (spring and winter). Meanwhile, the inter-kingdom interactions between eukaryotes and prokaryotes are predominantly positive in all seasons except autumn, which may serve as a strategy to resist environmental stress. These findings indicate that there is a significant seasonal heterogeneity between eukaryotes and prokaryotes in APLs, providing valuable insights into the ecological processes of microbial community succession in extreme environments.

The Hydraulic Evolution of Groundwater-Fed Pit Lakes After Mine Closure

Open pit mining frequently requires regional water tables to be lowered to access ore deposits. When mines close, dewatering ceases allowing the water table to recover. In arid and semi-arid mining regions, the developing pit lakes are predominantly fed by groundwater during this recovery phase and pit lakes develop first into "terminal sinks" for the surrounding groundwater system. With time, the re-establishment of regional hydraulic gradients can cause pit lakes to develop into throughflow systems, in which pit lake water outflows into adjacent aquifers. In this study, we use numerical groundwater modeling to aid process understanding of how regional hydraulic gradients, aquifer properties, net evaporation rates, and pit geometry determine the hydraulic evolution of groundwater-fed pit lakes. We find that before the recovery of the regional water table to its new equilibrium, pit lakes frequently transition to throughflow systems. Throughflow from pit lakes to downstream aquifers can develop within two decades following cessation of dewatering even under low hydraulic gradients (e.g., 5 × 10-4) or high net evaporation rates (e.g., 2.5 m/year). Pit lakes remain terminal sinks only under suitable combinations of high evaporation rates, low hydraulic gradients, and low hydraulic conductivities. In addition, we develop an approximate analytical solution for a rapid assessment of the hydraulic status of pit lakes under steady-state conditions. Understanding whether pit lakes remain terminal sinks or transition into throughflow systems largely determines the long-term water quality of pit lakes and downstream aquifers. This knowledge is fundamental for mine closure and planning post-mining land use.

Multi-omics insights into biogeochemical responses to organic matter addition in an acidic pit lake: Implications for bioremediation

Acidic pit lakes (APLs) emerge as reservoirs of acid mine drainage in flooded open-pit mines, representing extreme ecosystems and environmental challenges worldwide. The bioremediation of these oligotrophic waters necessitates the addition of organic matter, but the biogeochemical response of APLs to exogenous organic matter remains inadequately comprehended. This study delves into the biogeochemical impacts and remediation effects of digestate-derived organic matter within an APL, employing a multi-omics approach encompassing geochemical analyses, amplicon and metagenome sequencing, and ultra-high resolution mass spectrometry. The results indicated that digestate addition first stimulated fungal proliferation, particularly Ascomycetes and Basidiomycetes, which generated organic acids through lignocellulosic hydrolysis and fermentation. These simple compounds further supported heterotrophic growth, including Acidiphilium, Acidithrix, and Clostridium, thereby facilitating nitrate, iron, and sulfate reduction linked with acidity consumption. Nutrients derived from digestate also promoted the macroscopic development of acidophilic algae. Notably, the increased sulfate reduction-related genes primarily originated from assimilatory metabolism, thus connecting sulfate decrease to organosulfur increase. Assimilatory and dissimilatory sulfate reduction collectively contributed to sulfate removal and metal fixation. These findings yield multi-omics insights into APL biogeochemical responses to organic matter addition, enhancing the understanding of carbon-centered biogeochemical cycling in extreme ecosystems and guiding organic amendment-based bioremediation in oligotrophic polluted environments.

Decadal evolution of an acidic pit lake: Insights into the biogeochemical impacts of microbial community succession

Acidic pit lakes represent hydrological features resulting from the accumulation of acid mine drainage in mining operations. Long-term monitoring is essential for these extreme and contaminated environments, yet tracking investigations integrating microbial geochemical dynamics in acidic pit lakes have been lacking thus far. This study integrated historical data with field sampling to track decadal biogeochemical changes in an acidic pit lake. With limited artificial disturbance, significant and sustained biogeochemical changes were observed over the past decade. Surface water pH slowly increased from 2.8 to a maximum of 3.6, with a corresponding increase in bottom water pH to around 3.9, despite the accumulation of externally imported sulfate and metals. Elevated nutrient levels stimulated the macroscopic growth of Chlorophyta, resulting in a shift from reddish-brown to green water with floating algal bodies. Furthermore, microalgae-fixed organic carbon promoted the transition from the initial chemolithotrophy-based population dominated by Acidiphilium and Ferrovum to a heterotrophic community. The increase in heterotrophic iron- and sulfate-reducers may cause an elevation in ferrous levels and a decline in copper concentrations. However, most metals were not removed from the water column, potentially due to insufficient biosulfidogenesis or sulfide reoxidation. These findings offer novel insights into microbial succession in extreme ecosystem evolution and contribute to the management and remediation of acidic pit lakes.

Assessing the validity and sensitivity of microbial processes within a hydrodynamic model

Eutrophication due to excess anthropogenic nutrients in waterways is a significant issue worldwide. The pressure-stressor-response of a waterway to excessive nutrient loading is reliant on numerous physical and biological factors, including hydrodynamics and microbial processing. While substantial progress has been made towards simulating these mechanisms there are limited multi-disciplinary studies that relate the physical hydrodynamics of a site with the ecological response from linked laboratory and field studies. This paper presents the development of a coupled hydrodynamic and aquatic ecosystem response model, expanded to include an integrated microbial loop, that allows the explicit representation of heterotrophic bacteria growth and dissolved organic nutrient mineralisation. A unique long-term water quality dataset at an estuary in south-eastern Australia was used to validate and assess the model's sensitivity to complex biophysical processes driving the observed water quality variability. Results indicate that explicit time-varying bacterial mineralisation rates provide a substantially improved understanding of the broader aquatic ecosystem response than assigned fixed bulk rate parameter values, which are typically derived from non-local literature. Implementation of a microbial loop at the study site indicated that the model is sensitive to the boundary conditions, in particular catchment loads, with both net transport rates and the net growth rates of heterotrophic bacteria demonstrating different responses. Under average flow conditions, a smaller net transport and reduced nutrient availability has a pronounced effect of lowering net growth rates through the applied limitation factors. During high flow conditions, freshwater inflows increased net transport and nutrient loads, which resulted in higher net growth rates. Further, temporal variability in water temperature had a compounding effect on the model's response sensitivity. This approach has broader application in other riverine systems subject to eutrophication, and in interrogating linkages in hydrodynamic and microbial mediated processes (e.g., productivity). Future studies are recommended to better understand the sensitivity of aquatic ecosystem response models to microbial net growth rate kinetics at different temperatures and from top-down predation (e.g., zooplankton grazing).

Implications of bacterial mineralisation in aquatic ecosystem response models

Widespread wastewater pollution is a major barrier to the sustainable management of freshwater and coastal marine ecosystems worldwide. Integrated multi-disciplinary studies are necessary to improve waterway management and protect ecosystem integrity. This study used the Generalised Likelihood Uncertainty Estimation (GLUE) methodology to link microbial community ecotoxicology laboratory data to a mechanistic aquatic ecosystem response model. The generic model provided good predictive skill for major water quality constituents, including heterotrophic bacteria dynamics (r2 = 0.91). The model was validated against observed data across a gradient of effluent concentrations from community whole effluent toxicity (WET) laboratory tests. GLUE analysis revealed that a combined likelihood measure increased confidence in the predictive capability of the model. This study highlights the importance of calibrating aquatic ecosystem response models with net growth rates (i.e., sum of the growth minus loss rate parameter terms) of biological functional groups. The final calibrated net growth rate value of heterotrophic bacteria determined using the GLUE analysis was selected to be 0.58, which was significantly greater than the average literature value of -0.15. This finding demonstrated that use of literature parameter values without a good understanding of the represented processes could create misleading outputs and result in unsatisfactory conclusions. Further, fixed bulk mineralisation rate literature values are typically higher than realistically required in aquatic ecosystem response models. This indicates that explicitly including bacterial mineralisation is crucial to represent microbial ecosystem functioning more accurately. Our study suggests that improved data collection and modelling efforts in real-world management applications are needed to better address nutrients released into the natural environment. Future studies should aim to better understand the sensitivity of aquatic ecosystem response models to bacterial mineralisation rates.

Hydrochemical evolution of the Reocín mine filling water (Spain)

The Reocín mine is located in Cantabrian region, in northern Spain. Its exploitation ended in 2003 due to the exhaustion of its reserves. In November 2004, the controlled flooding of the openpit began. During this process, both the qualities of stored water and piezometric levels have been monitored to control the possible water detraction from the Saja-Besaya hydrographic system. This paper deals with the water chemistry analysis of the pit lake surface, as well as the different conditions of the area. Geological and hydrogeological contexts play an important role in the lake water chemistry. The lake water quality continues improving. The sulphate content and zinc concentrations are already below the permitted pouring limits. Three factors are distinguished: the washing of the mine shafts is completed; the water supply from the aquifer contributes to the dissolution of the salt content and the bedrock, and dolomite, which neutralizes acid waters and improves the water quality during the flooding process with a pH value of 8. Owing to these conditions, the stored water meets the necessary conditions for discharge and provides the opportunities to use it.

Vertical environmental gradient drives prokaryotic microbial community assembly and species coexistence in a stratified acid mine drainage lake

Acid mine drainage (AMD) lakes are typical hydrologic features caused by open pit mining and represent extreme ecosystems and environmental challenges. Little is known about microbial distribution and community assembly in AMD lakes, especially in deep layers. Here, we investigated prokaryotic microbial diversity and community assembly along a depth profile in a stratified AMD lake using 16S rRNA gene sequencing combined with multivariate ecological and statistical methods. The water column in the AMD lake exhibited tight geochemical gradients, with more acidic surface water. Coupled with vertical hydrochemical variations, prokaryotic microbial community structure changed significantly, and was accompanied by increased diversity with depth. In the surface water, heterogeneous selection was the most important assembly process, whereas stochastic processes gained importance with depth. Meanwhile, microbial co-occurrences, especially positive interactions, were more frequent in the stressful surface water with reduced network modularity and keystone taxa. The pH was identified as the key driver of microbial diversity and community assembly along the vertical profile based on random forest analysis. Taken together, environmental effects dominated by acid stress drove the community assembly and species coexistence that underpinned the spatial scaling patterns of AMD microbiota in the lake. These findings demonstrate the distinct heterogeneity of local prokaryotic microbial community in AMD lake, and provide new insights into the mechanism to maintain microbial diversity in extreme acidic environments.

Engineered river flow-through to improve mine pit lake and river values

Mine pit lakes may develop at mine closure when mining voids extend below groundwater levels and fill with water. Acid and metalliferous drainage (AMD) and salinity are common problems for pit lake water quality. Contaminated pit lake waters can directly present significant risk to both surrounding and regional communities and natural environmental values and limit beneficial end use opportunities. Pit lake waters can also discharge into surface and groundwater; or directly present risks to wildlife, stock and human end users. Riverine flow-through is increasingly proposed to mitigate or remediate pit lake water contamination using catchment scale processes. This paper presents the motivation and key processes and considerations for a flow-through pit lake closure strategy. International case studies as precedent and lessons for future application are described from pit lakes that use or propose flow-through as a key component of their mine closure design. Chemical and biological processes including dilution, absorption and flocculation and sedimentation can sustainably reduce pit lake contaminant concentrations to acceptable levels for risk and enable end use opportunities to be realised. Flow-through may be a valid mine closure strategy for pit lakes with poor water quality. However, maintenance of existing riverine system values must be foremost. We further suggest that decant river water quality may, in some circumstances, be improved; notably in examples of meso-eutrophic river waters flowing through slightly acidic pit lakes. Flow-through closure strategies must be scientifically justifiable and risk-based for both lake and receptors potentially affected by surface and groundwater transport. Due to the high-uncertainty associated with this complex strategy, biotic and physico-chemical attributes of both inflow and decant river reaches as well as lake should be well monitored. Monitoring should directly feed into an adaptive management framework discussed with key stakeholders with validation of flow-through as a sustainable strategy prior to mine relinquishment.