Nonlinear response of methane release to increased trophic state levels coupled with microbial processes in shallow lakes

nosZ II/nosZ I ratio regulates the N2O reduction rates in the eutrophic lake sediments

Nitrous oxide (N2O) is a more potent greenhouse gas with an atmospheric lifetime of 121 years, contributing significantly to climate change and stratospheric ozone depletion. Lakes are hotspots for N2O release due to the imbalance between N2O sources and sinks. N2O-reducing bacteria are the only biological means to mitigate N2O emission, yet their roles in lakes are not well studied. This study investigated the potential for N2O reduction, keystones of typical and atypical N2O-reducing bacterial communities, and their correlations with environmental factors in the sediments of Lake Taihu through microcosm experiments, high-throughput sequencing of the nosZ gene, and statistical modeling. The results showed that potential N2O reduction rates in sediments ranged from 13.71 to 76.95 μg N2O g-1 d-1, with lower rates in December compared to March and July. Correlation analysis indicated that the nosZ II/nosZ I ratio and the trophic lake index (TLI) were the primary factors influencing N2O reduction rates and N2O-reducing bacterial community structures. The genera Pseudogulbenkiania and Ardenticatena were identified as the most abundant typical and atypical N2O-reducing bacteria, respectively, and were also recognized as the keystone taxa. Quantitative real-time PCR (qPCR) results revealed that nosZ II was more abundant than nosZ I in the sediments. Partial least squares path modeling (PLS-PM) further demonstrated that atypical N2O-reducing bacteria had significant positive effects on N2O reduction process in the sediments (p < 0.05). Overall, this study highlights the crucial ecological roles of atypical N2O-reducing bacteria in the sediments of the eutrophic lake of Taihu, underscoring their potential in mitigating N2O emissions.

Lake ecological restoration of vegetation removal mitigates algal blooms and alters landscape patterns of water and sediment bacteria

Elucidating the influences of ecological restoration measure of lakeshore vegetation removal on water quality and biological community is an important but underestimated subject. We adopted molecular and statistical tools to estimate ecological restoration performance in a plateau lake receiving vegetation removal and simultaneously investigated variabilities of bacterial communities in water and sediment. Significant decreases in lake trophic level and algal bloom degree followed notable decreases in water total nitrogen and total phosphorus after vegetation removal. Non-significant changes in sediment nutrients accompanied remarkable variabilities of abundance and composition of nutrient-cycling functional genes (NCFGs) of sediment bacteria. Taxonomic and phylogenetic α-diversities, functional redundancies, and dispersal potentials of bacteria in water and sediment decreased after vegetation removal, and community successions of water and sediment bacteria were separately significant and non-significant. There were opposite changes in ecological attributes of bacteria in water and sediment in response to vegetation removal, including niche breadth, species replacement, richness difference, community complexity, and community stability. Species replacement rather than richness difference affected more on taxonomic β-diversities of bacteria in water and sediment before and after vegetation removal, and determinism rather than stochasticity dominated bacterial community assemblage. Our results highlighted vegetation removal mitigated algal bloom and affected differently on landscapes of water and sediment bacteria. These findings point to dominant ecological mechanisms underlying landscape shifts in water and sediment bacteria in a disturbed lake receiving vegetation removal and have the potential to guide lake ecological restoration.

Characteristics of nutrients and heavy metals release from sewage sludge biochar produced by industrial-scale pyrolysis in the aquatic environment and its potential as a slow-release fertilizer and adsorbent

To assess the application potential of sewage sludge biochar produced by industrial-scale pyrolysis (ISB), the release characteristics of nutrients (NH4+, PO43-, K, Ca, Mg and Fe) and heavy metals (Mn, Cu, Zn, Pb, Ni and Cr) were investigated. Their release amounts increased with decreasing initial pH and increasing solid-liquid ratios (RS-L) and temperature. The release types of NH4+, K, Mg, and Mn were diffusion/dissolution, while those of Cu, Zn, Pb, Ni, and Cr were diffusion/resorption. The release types of PO43- and Ca varied with initial pH and RS-L, respectively. The chemical actions played dominant roles in their release, while particle surface diffusion and liquid film diffusion determined the rates of diffusion and resorption phases, respectively. The release of NH4+, PO43-, K, Ca, Mg, Mn and Zn was a non-interfering, spontaneous (except PO43-), endothermic, and elevated randomness process. The release efficiency of NH4+, PO43- and K met the Chinese standard for slow-release fertilizers, while the total risk of ISB was low. The eutrophication and potential ecological risks of ISB were acceptable when the dose was less than 3 g L-1 and the initial pH was no lower than 3. In conclusion, ISB had potential as a slow-release fertilizer and adsorbent.

Insights into priming effects of dissolved organic matter degradation in urban lakes with different trophic states

Priming effect (PE) is recognized as an important potential mechanism for dissolved organic matter (DOM) degradation in aquatic ecosystems. However, the priming effects (PEs) of various priming substances on the degradation of DOM pools in urban lakes along diverse trophic states remain unknown. To address this knowledge gap, the PEs and drivers of glucose and plant leachate of lake water with three trophic states were investigated. We reveal differences in the bioavailability of DOM in lake water, glucose, and plant leachate. The PE of the same priming substance was significantly higher in highly-eutrophic lake water than in mesotrophic lake. The priming intensity induced by glucose was significantly higher when compared to plant leachate. Regarding the addition of glucose, humic-like components (C1 and C3) showed slight PE, while the tyrosine-like component C2 showed negative PE. However, the positive PEs were observed on three components after adding plant leachate. The driver of PE by glucose shifted from nutrients to DOM components with increasing trophic levels. The PEs induced by plant leachate were affected by nutrients, chlorophyll-a (Chl-a), water chemistry, and DOM components in lightly/moderately-eutrophic lake water. This study revealed the intensities, directions, and drivers of PEs, providing essential insights into uncovering the DOM biogeochemical process in urban lakes.

Changes in CO2 concentration and degassing of eutrophic urban lakes associated with algal growth and decline

Urban lakes are numerous in the world, but their role in carbon storage and emission is not well understood. This study aimed to answer the critical questions: How does algal growing season influence carbon dioxide concentration (cCO2) and exchange flux (FCO2) in eutrophic urban lakes? We investigated trophic state, seasonality of algal productivity, and their association with CO2 dynamics in four urban lakes in Central China. We found that these lightly-to moderately-eutrophic urban lakes showed a shifting pattern of CO2 source-sink dynamics. In the non-algal bloom phase, the moderately-eutrophic lakes outgassed on average of 12.18 ± 24.37 mmol m-2 d-1 CO2; but, during the algal bloom phase, the lakes sequestered an average 1.07 ± 6.22 mmol m-2 d-1 CO2. The lightly-eutrophic lakes exhibited lower CO2 emission in the algal bloom (0.60 ± 10.24 mmol m-2 d-1) compared to the non-algal bloom (3.84 ± 12.38 mmol m-2 d-1). Biological factors such as Chl-a (chlorophyll a) and AOU (apparent oxygen utilization), were found to be important factors to potentially affect the shifting pattern of lake CO2 source-sink dynamics in moderately-eutrophic lakes, explaining 48% and 34% of the CO2 variation in the non-algal and algal bloom phases, respectively. Moreover, CO2 showed positive correlations with AOU, and negative correlations with Chl-a in both phases. In the lightly-eutrophic lakes, biological factors explained a higher proportion of CO2 variations (29%) in the non-algal bloom phase, with AOU accounting for 19%. Our results indicate that algal growth and decline phases largely affect dissolved CO2 level and exchange flux by regulating in-lake respiration and photosynthesis. Based on the findings, we conclude that shallow urban lakes can act as both sources and sinks of CO2, with algal growth seasonality and trophic state playing pivotal roles in controlling their carbon dynamics.

Carbon emissions affected by real-time reservoir operation: a hydrodynamic modeling approach coupled with air-water mass transfer

Air-water diffusive carbon fluxes (e.g., CO₂ and CH₄) in reservoirs, particularly those dammed in river valleys, are the major pathway of reservoir carbon emissions. Hydrodynamic conditions caused by real-time reservoir operation could potentially affect air-water transfer of these greenhouse gases (GHGs), yet still under explored. Here, we proposed an estimation method of gross carbon emissions based on a computational fluid dynamic (CFD) modelling approach. The model assumed that air-water mass transfer was primarily regulated by surface turbulence, and disregarded contributions from biogeochemical processes as well as seasonality of meteorological parameters (i.e., wind speed and direction; air temperature). Through the hydrodynamic modeling, reservoir water level, flow velocity, surface turbulence, and air-water transfer velocity of carbon fluxes were elaborated. Gross carbon emissions were integrated by the carbon fluxes in each discrete cell and time under real-time reservoir operation. The Xiangjiaba Reservoir (XJB), located in the upper Yangtze Basin, was selected as the case of the study. Based on daily hydrological data in 2018, such as reservoir inflow, outflow and water level, the gross CO₂ and CH₄ emissions in the reservoir were approximately 6.7 Gg and 5.6 Mg. Variations of daily water level and discharge induced by reservoir operation could evidently affect carbon emissions. In particular, when reservoir initiated its impoundment, the discharge could be the probably critical factor that affected mass transfer velocity and carbon emissions in the reservoir. Our model could provide a new vision for evaluating the effect of real-time reservoir operation on carbon emissions.

Eichhornia crassipes-rhizospheric biofilms contribute to nutrients removal and methane oxidization in wastewater stabilization ponds receiving simulative sewage treatment plants effluents

Wastewater stabilization ponds (WSPs) have been used in treating sewage treatment plants (STPs) effluents. However, little is known about the role of rhizospheric biofilms on methane release in WSPs with floating plants. In the present study, the nutrient removal, CH₄ fluxes, CH₄ oxidization potential and rhizospheric bacterial community were investigated in WSPs with Eichhornia crassipes under simulate STPs effluents for 31 days. At the end of the experiment, E. crassipes biomass was 5.60-8.81 times of initial weight and increased with increasing nutrients concentration. E. crassipes effectively reduced methane release and nutrients. Compared to control, E. crassipes reduced 52.30%-83.21% of CH₄ fluxes at water-atmosphere interface and had better inhibition effect on CH₄ fluxes in treatments with high nutrients. However, methane oxidization rates of E. crassipes roots were higher in low nutrients (0.83 ± 0.046 mg CH₄ (kg fresh plant)^-1 day^-1) than high nutrients (0.12 ± 0.04 mg CH₄ (kg fresh plant)^-1 day^-1). Structural equation modeling revealed that biomass of E. crassipes has negative effect on CH₄ fluxes (-0.453, p = 0.000). Proteobacteria, Bacteroidetes, Planctomycetes, Chloroflexi and Actinobacteria were the predominant phyla in the rhizospheric biofilm of E. crassipes and contributed to nutrients removal. Aerobic methanotrophs and pomA abundances were higher in rhizospheric biofilm exposed to high nutrients than low nutrients and aerobic methanotrophs had close interactions with other microorganisms and participated in the carbon and nitrogen cycle, demonstrating that many bacteria harboring pmoA gene did not fully involve in methane oxidization. These data highlight plants E. crassipes have an important role in both reducing methane release and nutrients removal.

Climate Warming Does Not Override Eutrophication, but Facilitates Nutrient Release from Sediment and Motivates Eutrophic Process

The climate is changing. The average temperature in Wuhan, China, is forecast to increase by at least 4.5 °C over the next century. Shallow lakes are important components of the biosphere, but they are sensitive to climate change and nutrient pollution. We hypothesized that nutrient concentration is the key determinant of nutrient fluxes at the water-sediment interface, and that increased temperature increases nutrient movement to the water column because warming stimulates shifts in microbial composition and function. Here, twenty-four mesocosms, mimicking shallow lake ecosystems, were used to study the effects of warming by 4.5 °C above ambient temperature at two levels of nutrients relevant to current degrees of lake eutrophication levels. This study lasted for 7 months (April–October) under conditions of near-natural light. Intact sediments from two different trophic lakes (hypertrophic and mesotrophic) were used, separately. Environmental factors and bacterial community compositions of overlying water and sediment were measured at monthly intervals (including nutrient fluxes, chlorophyll a [chl a], water conductivity, pH, sediment characteristics, and sediment-water et al.). In low nutrient treatment, warming significantly increased chl a in the overlying waters and bottom water conductivity, it also drives a shift in microbial functional composition towards more conducive sediment carbon and nitrogen emissions. In addition, summer warming significantly accelerates the release of inorganic nutrients from the sediment, to which microorganisms make an important contribution. In high nutrient treatment, by contrast, the chl a was significantly decreased by warming, and the nutrient fluxes of sediment were significantly enhanced, warming had considerably smaller effects on benthic nutrient fluxes. Our results suggest that the process of eutrophication could be significantly accelerated in current projections of global warming, especially in shallow unstratified clear-water lakes dominated by macrophytes.

Sources, Migration, Transformation, and Environmental Effects of Organic Carbon in Eutrophic Lakes: A Critical Review

Organic carbon (OC) plays a leading role in the carbon cycle of lakes and is crucial to carbon balances at regional and even global scales. In eutrophic lakes, in addition to external river inputs, the decomposition of endogenous grass and algae is a major source of organic carbon. Outbreaks of algal blooms (algal eutrophication) and the rapid growth of aquatic grasses (grass eutrophication) can lead to the accumulation and decay of large amounts of algae and aquatic grass debris, which increases the intensity of the carbon cycle of lakes and greatly impacts aquatic environments and ecosystems. The structures, decomposition processes, and distribution characteristics of algae and higher aquatic plant debris in eutrophic lakes are different from mesotrophic and oligotrophic lakes. Studying their accumulation dynamics and driving mechanisms is key to further understanding lake carbon cycles and their many interdependent pathways. This paper focuses on the carbon sources, tracing technologies, migration and transformation processes, and environmental effects of OC in eutrophic lakes. Based on the existing knowledge, we further combed the literature to identify the most important knowledge gaps preventing an in-depth understanding of the processes and driving mechanisms of the organic carbon cycle in eutrophic lakes.

Frequent algal blooms dramatically increase methane while decrease carbon dioxide in a shallow lake bay

Freshwater ecosystems play a key role in global greenhouse gas estimations and carbon budgets, and algal blooms are widespread owing to intensified anthropological activities. However, little is known about greenhouse gas dynamics in freshwater experiencing frequent algal blooms. Therefore, to explore the spatial and temporal variations in methane (CH₄) and carbon dioxide (CO₂), seasonal field investigations were performed in the Northwest Bay of Lake Chaohu (China), where there are frequent algal blooms. From the highest site in the nearshore to the pelagic zones, the CH₄ concentration in water decreased by at least 80%, and this dynamic was most obvious in warm seasons when algal blooms occurred. CH₄ was 2-3 orders of magnitude higher than the saturated concentration, with the highest in spring, which makes this bay a constant source of CH₄. However, unlike CH₄, CO₂ did not change substantially, and river mouths acted as hotspots for CO₂ in most situations. The highest CO₂ concentration appeared in winter and was saturated, whereas at other times, CO₂ was unsaturated and acted as a sink. The intensive photosynthesis of rich algae decreased the CO₂ in the water and increased dissolved oxygen and pH. The increase in CH₄ in the bay was attributed to the mineralization of autochthonous organic carbon. These findings suggest that frequent algal blooms will greatly absorb more CO₂ from atmosphere and increasingly release CH₄, therefore, the contribution of the bay to the lake's CH₄ emissions and carbon budget will be major even though it is small. The results of this study will be the same to other shallow lakes with frequent algal bloom, making lakes a more important part of the carbon budget and greenhouse gases emission.

Indices and models of surface water quality assessment: Review and perspectives

Many technologies have been designed to monitor, evaluate, and improve surface water quality, as high-quality water is essential for human activities including agriculture, livestock, and industry. As such, in this study, we investigated water quality indices (WQIs), trophic status indices (TSIs), and heavy metal indices (HMIs) for assessing surface water quality. Based on these indices, we summarised and compared water assessment models using expert system (ES) and machine learning (ML) methods. We also discussed the current status and future perspectives of water quality management. The results of our analyses showed that assessment indices can be used in three aspects of surface water quality assessment: WQIs are aggregated from multiple parameters and commonly used in surface water quality classification; TSIs are calculated from the concentrations of different nutrients required for algae and bacteria, and employed to evaluate the eutrophication levels of lakes and reservoirs; HMIs are mainly applied for human health risk assessment and the analysis of correlation of heavy metal sources. ES- and ML-based assessment models have been developed to efficiently generate assessment indices and predict water quality status based on big data obtained from new techniques. By implementing dynamic monitoring and analysis of water quality, we designed a next-generation water quality management system based on the above indices and assessment models, which shows promise for improving the accuracy of water quality assessment.

Eutrophic levels and algae growth increase emissions of methane and volatile sulfur compounds from lakes

Eutrophic lakes are hot spots of CH₄ and volatile sulfur compound (VSC) emissions, especially during algal blooms and decay. However, the response of CH₄ and VSC emissions to lake eutrophication and algae growth as well as the underlying mechanisms remain unclear. In this study, the emissions of CH₄ and VSCs from four regions of Lake Taihu with different eutrophic levels were investigated in four months (i.e., March, May, August and December). The CH₄ emissions ranged from 20.4 to 126.9 mg m^-2 d^-1 in the investigated sites and increased with eutrophic levels and temperature. H₂S and CS₂ were the dominant volatile sulfur compounds (VSCs) emitted from the lake. The CH₄ oxidation potential of water ranged from 2.1 to 14.9 μg h^-1 L^-1, which had positive correlations with trophic level index and the environmental variables except for the NH₄⁺-N concentration. Eutrophic levels could increase the abundances of bacteria and methanotrophs in lake water. α-Proteobacteria methanotroph Methylocystis was more abundant than γ-Proteobacteria methanotrophs in March and May, while the latter was more abundant in August and November. The relative abundance of Cyanobacteria, including Microcystis, A. granulata var. angustissima and Cyanobium had significantly positive correlations with temperature, turbidity, SO₄^2--S, and total sulfur. Partial least squares path modelling revealed that the algal growth could promote VSC emissions, which had a positive correlation with CH₄ oxidation potential, likely due to the positive correlation between the CH₄ and VSC emissions from lakes. These findings indicate that water eutrophication and algae growth could increase the emissions of CH₄ and VSCs from lakes. Controlling algae growth might be an effective way to mitigate the emissions of CH₄ and VSCs from freshwater lakes.

Production and transformation of organic matter driven by algal blooms in a shallow lake: Role of sediments

The generation of organic matter (OM) occurs synchronously with phytoplankton growth. Characterization of the generated particulate and dissolved OM during algal blooms in eutrophic lakes is crucial for better understanding the carbon cycle but remains limited. We speculate that sediments play a critical role in the biogeochemical transformation of OM derived from algal blooms in shallow lakes. In this study, changes in OM quantity and quality and the concentrations of biogenic elements (nutrients and metals) during algal blooms, were studied in situ in a shallow eutrophic lake (Lake Chaohu, China). Two enclosure treatments in the presence and absence of sediments were compared, and the cause-effect relationships among sediment, nutrients, metals, phytoplankton, particulate OM (POM), and dissolved OM (DOM) were revealed by a partial least square-path model (PLS-PM). The results showed that the changes in nutrients and metals concentrations over time were consistent with that of chlorophyll a (Chl a), and at the end of the treatment, the concentrations of Chl a, nutrients, and metals in Treatment S (with sediments) were approximately 3-5 times of those in Treatment N (without sediments). The high concentration of Chl a in Treatment S resulted in a high quantity of POM, which showed low molecular weight, low humification, and was enriched in protein-like components (∼ 70%). For DOM, the quantity increased after the decrease in POM, and DOM quality showed a significantly higher abundance of humic-like components and a higher molecular weight than POM did. The PLS-PM results showed that the significant positive effects of sediment on nutrients, metals, phytoplankton, POM, and DOM were 0.28, 0.37, 0.28, 0.25, and 0.25, respectively, suggesting that sediment had an important role in the biogeochemical cycles of these substances. The significant negative relationship between POM and DOM (-0.62) and the distinct difference in POM and DOM quality implied the efficient transformation of the freshly generated OM to those with a higher molecular weight, higher humification, and potentially refractory. Our results depicted the quick biogeochemical transformation of nutrients, metals, and the potential formation of refractory organic carbon in water column, as driven by the couple of the algae pump with the microbial carbon pump.

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.

Pollution alters methanogenic and methanotrophic communities and increases dissolved methane in small ponds

Small ponds have become a hotspot of greenhouse gas emissions, but our understanding of methane (CH₄) cycling and its biological regulation in small polluted ponds remains limited. To assess how pollution affects CH₄ content, we investigated dissolved CH₄ concentrations, water and sediments properties, methanogenic and methanotrophic communities in two types of small polluted ponds. Compared with low pollution (LP) ponds, high pollution (HP) ponds showed significantly (P < 0.05) higher dissolved CH₄ in water. Sequencing of methyl coenzyme M reductase (mcrA) and particulate methane monooxygenase (pmoA) genes showed that HP led to significant (P < 0.05) shifts of CH₄-cycling microbial communities, with increased Shannon index of sediment methanogenic communities and water methanotrophic communities. There were also strong negative associations (P < 0.05) between dissolved CH₄ concentrations and interdomain methanogen-methanotroph network connectivity in water and sediments, respectively. The partial least squares path modeling indicated that dissolved oxygen, total organic carbon, ammonium nitrogen and nitrate nitrogen of water, and total nitrogen and total carbon of sediment, and CH₄-cycling microbes could regulate the CH₄ content. This study clarified the effects of environmental deterioration on CH₄ cycling in small ponds, highlighting the use of methanogen-methanotroph network connectivity to assess the CH₄ production.

Estimation of total flux of polycyclic aromatic hydrocarbons facilitated by methane ebullition into water column from global lake sediments

Methane ebullition and contamination are two typical characteristics from lakes, however, these two are generally studied independently. In fact, the exchange of matter and energy between methane bubbles and their surrounding environment can be very active to enhance the contaminant transport. There is limited research on understanding the characteristics and trends of gas ebullition facilitated contaminant emissions in large areas considering water and air as receptors. We herein estimate the transport capacity of methane ebullition for polycyclic aromatic hydrocarbons (PAHs) out of the sediment from global lakes, which may reach an average of 71 (up to 159) t yr^-1. Methane bubbles could transfer one third of the total PAH flux from sediments, or equivalent of 1.3-3.0 ng L^-1 of additional PAHs, into the water column with the rest going into air, offsetting from 52 to 118% of dry PAH deposition flux into global lakes sediment per year. Given the PAH concentration in lake water is often in the range of 0.1-100 ng L^-1, ebullition facilitated PAH flux may increase PAH concentration by a factor of 1.4 to 2.4 until 2,100, being a significant contributor for the PAH increment in lake waters.

Nonlinear pattern and algal dual-impact in N2O emission with increasing trophic levels in shallow lakes

Shallow lakes are considered important contributors to emissions of nitrous oxide (N₂O), a powerful greenhouse gas, in aquatic ecosystems. There is a large degree of uncertainty regarding the relationship between N₂O emissions and the progress of lake eutrophication, and the mechanisms underlying N₂O emissions are poorly understood. Here, N₂O emission fluxes and environmental variables in different lakes along a trophic state gradient in the Yangtze River basin were studied. N₂O emission fluxes were -1.0-53.0 μg m^-2 h^-1 and 0.4-102.9 μg m^-2 h^-1 in summer and winter, respectively, indicating that there was marked variation in N₂O emissions among lakes of different trophic state. The non-linear exponential model explained differences in N₂O emission fluxes by the degree of eutrophication (p < 0.01). TN and chl-a both predicted 86% of the N₂O emission fluxes in shallow lakes. The predicted N₂O emission fluxes based on the IPCC EF_5r overestimated the observed fluxes, particularly those in hyper-eutrophic lakes. These findings demonstrated that nutrient-rich conditions and algal accumulation are key factors determining N₂O emission fluxes in shallow lakes. Furthermore, this study also revealed that temperature and algae accumulation-decomposition determine an N₂O emission flux in an intricate manner. A low temperature, i.e., winter, limits algae growth and low oxygen consumption for algae decomposition. The environment leaves a high dissolved oxygen concentration, slowing down N₂O consumption as the final step of denitrification. In summer, with the oxygen consumed by excess algal decomposition, the N₂O production is limited by the complete denitrification as well as the limited substrate supply of nitrate by nitrification in hypoxic or anoxic conditions. Such cascading events explained the higher N₂O emission fluxes from shallow lakes in winter compared with summer. This trend was amplified in hyper-eutrophic shallow lakes after algal disappearance. Collectively, algal accumulation played a dual role in stimulating and impeding N₂O emissions, especially in hyper-eutrophic lakes. This study expands our knowledge of N₂O emissions from shallow lakes in which eutrophication is underway.

Candidatus Methylumidiphilus Drives Peaks in Methanotrophic Relative Abundance in Stratified Lakes and Ponds Across Northern Landscapes

Boreal lakes and ponds produce two-thirds of the total natural methane emissions above the latitude of 50° North. These lake emissions are regulated by methanotrophs which can oxidize up to 99% of the methane produced in the sediments and the water column. Despite their importance, the diversity and distribution of the methanotrophs in lakes are still poorly understood. Here, we used shotgun metagenomic data to explore the diversity and distribution of methanotrophs in 40 oxygen-stratified water bodies in boreal and subarctic areas in Europe and North America. In our data, gammaproteobacterial methanotrophs (order Methylococcales) generally dominated the methanotrophic communities throughout the water columns. A recently discovered lineage of Methylococcales, Candidatus Methylumidiphilus, was present in all the studied water bodies and dominated the methanotrophic community in lakes with a high relative abundance of methanotrophs. Alphaproteobacterial methanotrophs were the second most abundant group of methanotrophs. In the top layer of the lakes, characterized by low CH4 concentration, their abundance could surpass that of the gammaproteobacterial methanotrophs. These results support the theory that the alphaproteobacterial methanotrophs have a high affinity for CH4 and can be considered stress-tolerant strategists. In contrast, the gammaproteobacterial methanotrophs are competitive strategists. In addition, relative abundances of anaerobic methanotrophs, Candidatus Methanoperedenaceae and Candidatus Methylomirabilis, were strongly correlated, suggesting possible co-metabolism. Our data also suggest that these anaerobic methanotrophs could be active even in the oxic layers. In non-metric multidimensional scaling, alpha- and gammaproteobacterial methanotrophs formed separate clusters based on their abundances in the samples, except for the gammaproteobacterial Candidatus Methylumidiphilus, which was separated from these two clusters. This may reflect similarities in the niche and environmental requirements of the different genera within alpha- and gammaproteobacterial methanotrophs. Our study confirms the importance of O2 and CH4 in shaping the methanotrophic communities and suggests that one variable cannot explain the diversity and distribution of the methanotrophs across lakes. Instead, we suggest that the diversity and distribution of freshwater methanotrophs are regulated by lake-specific factors.

Overall spatiotemporal dynamics of greenhouse gasses and oxygen in two subtropical reservoirs with contrasting trophic states

The impact of cultural eutrophication on carbon cycling in subtropical reservoirs was assessed using high-resolution measurements of dissolved gas concentration, atmospheric exchange, and uptake/production rates of methane, carbon dioxide, and oxygen. Seasonal measurements were performed in two reservoirs that pertain to the same hydrological basin but are drastically different in terms of allochthonous carbon input. These results were used to feed a mass balance model, from which a large number of overall parameters were determined to explicitly describe the dynamics and spatial attributes of the carbon cycle in the reservoirs. A single graphical representation of each reservoir was created to facilitate an overall appraisal of the carbon cycle. The impact of cultural eutrophication was profound and resulted in a complete redistribution of how the various bioprocesses participated in the methane, carbon dioxide, and oxygen cycles. Among several identified impacts of eutrophication, it was observed that while eutrophication triggered increased methane production, this effect was followed by a similar increase in methane emissions and methanotrophic rates, while gross primary production was depleted.

The role of freshwater eutrophication in greenhouse gas emissions: A review

Greenhouse gases (GHGs) have long received public attention because they affect the Earth's climate by producing the greenhouse effect. Freshwaters are an important source of GHGs, and the enhancement in their eutrophic status affects GHG emissions. Along with the increasing eutrophication of water bodies, the relevant quantitative and qualitative studies of the effects of freshwater eutrophication on GHG emissions have made substantial progress, particularly in the past 5 years. However, to our knowledge, this is the first critical review to focus on the role of freshwater eutrophication in GHG emissions. In this review, the emissions of common GHGs from freshwater are quantitatively described. Importantly, direct (i.e., dissolved oxygen, organic carbon, and nutrients) and indirect factors (i.e., dominant primary producer and algal blooms) affecting GHG emissions from eutrophic freshwater are systematically analyzed. In particular, the existence and significance of feedback loops between freshwater eutrophication and GHG emissions are emphasized considering the difficulties managing freshwater ecosystems and the Earth's climate. Finally, several future research directions as well as mitigation measures are described to provide useful insight into the dynamics and control of GHG emissions.

Stochastic trophic level index model: A new method for evaluating eutrophication state

The trophic state index (TSI) and trophic level index (TLI) are commonly used methods for evaluating the eutrophication state of lakes and reservoirs. However, they are unable to overcome uncertainties such as calculation errors and spatial heterogeneity of evaluation indicators. To comprehensively evaluate the eutrophication state of a region, we introduce a probability density function and propose the stochastic trophic level index model (STLI). The probability density function of each trophic level is derived through the principle of maximum entropy, and membership vector F (F1, F2, F3, F4, F5) for each trophic level is established to quantify the risk of regional eutrophication. We utilized STLI to evaluate the eutrophication status of Songhua Lake, China, and determined that the method can be used for uncertainty and risk assessment. Our results show that the Jiaohe River backwater area has the highest eutrophication level (light eutropher), with a 0.12 probability of further deterioration to middle eutropher. The eutrophication status of the Main Scenic Area of the Songhua Lake Scenic Resort was shown to be mesotropher, with 0.26 and 0.08 probabilities of further deterioration to light eutropher and middle eutropher, respectively. Finally, the eutrophication status of the Songhua River Three Lakes Reserve Experimental Area was shown to be mesotropher, with a 0.24 probability of further deterioration to light eutropher. Overall, the Songhua River Three Lakes Reserve Experimental Area is the most promising for the lowest level of eutrophication. We recommend that the management department take effective targeted measures against the Jiaohe River backwater area first. The probability density and membership vector of STLI can effectively solve the uncertainties presented by traditional methods for evaluating regional eutrophication status.