Effects of phytoplankton blooms on fluxes and emissions of greenhouse gases in a eutrophic lake

Ecological restoration reduces greenhouse gas emissions by altering planktonic and sedimentary microbial communities in a shallow eutrophic lake

Ecological restoration is a promising approach to alleviate eutrophication. However, its impacts on greenhouse gas (GHG) emissions and the underlying microbial mechanisms in different habitats of lakes remain unclear. To address this knowledge gap, we measured carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes at both water-air and sediment-water interfaces of eutrophic (Caohai) and restored area (Dapokou) of Dianchi Lake, a typical eutrophic lake in China. Meanwhile, we investigated the responses of planktonic and sedimentary bacterial and fungal communities by high-throughput sequencing. Our results indicated that 6 years of ecological restoration significantly reduced CO2 and N2O fluxes by 1.0-3.6 and 2.2-2.8 folds respectively, with more pronounced variations at the water-air interface than the sediment-water interface. Ecological restoration also shifted the structures of planktonic bacterial and fungal communities remarkably, leading to a significant reduction in the relative abundances of Actinobacteriota (by 70.94%), Bacteroidota (by 61.65%), Planctomycetota (by 74.18%) and Chytridiomycota (by 95.44%). Correlation analyses further suggested that GHG fluxes at the water-air interface were significantly correlated with planktonic microbial community composition (P < 0.05), and the significant reduction of CO2 and N2O fluxes under ecological restoration could be attributed to the decreased abundances of organic matter decomposers (such as hgcI_clade, Sporichthyaceae and Acidibacter) and increased abundances of autotrophs (such as Hydrogenophaga and Cyanobium_PCC-6307) in water. Collectively, our findings verify the importance of ecological restoration in reducing GHG emissions in inland lake ecosystems, providing new insights for addressing global climate change and advancing carbon neutrality.

Optimizing Watershed Land Use to Achieve the Benefits of Lake Carbon Sinks while Maintaining Water Quality

Greenhouse gas emissions and water quality decline are two major issues currently affecting lakes worldwide. Determining how to control both greenhouse gas emissions and water quality decline is a long-term challenge. We compiled data on the annual average carbon dioxide (CO2) flux and water quality parameters for 422 global lakes, revealing that 82.42% of the lakes act as carbon sources and that 66.56% have experienced water quality deterioration. Carbon sources and eutrophication trends were observed for lakes from the 1990s to 2020s, with further deterioration expected over the next 80 years. Unmanaged land use change in lake watersheds could exacerbate the CO2 flux into lakes and water quality degradation. In this study, a watershed land use planning (WLUP) framework was established, and a 24.83% reduction in the CO2 flux into lake water, a 5.07% reduction in chlorophyll a (Chl-a), a 4.70% reduction in total phosphorus, and a 12.92% increase in Secchi depth were achieved. The WLUP framework identifies Asia and Europe as the regions experiencing the greatest demands for land use transformation, where optimization leads to the most significant improvements. Metagenomic analysis revealed that forests enhance carbon fixation and that grasslands reduce carbon degradation and phosphorus metabolism in lake watersheds, explaining and supporting the possibility of WLUP. This work provides a win-win solution for improving CO2 fluxes and water quality in global lakes to mitigate the effects of climate change and promote lake protection.

Invasive primary producers modulate carbon fluxes and associated carbon budgets in temperate shallow lakes

Lowland shallow lakes are the receiving environments of nutrients and organic carbon from the catchment area. In temperate areas, the synergic action of nutrients and mild temperatures induce carbon emissions from these systems. However, this trend might be modulated by the trophic state of lakes and by their productivity. In this study, we consider blooms of invasive submerged aquatic vegetation (SAV) and cyanobacteria as a valuable proxy for eutrophication and explore their role in carbon pools and associated budgets in temperate shallow lakes. We calculated the mass carbon budget of two large shallow lakes, characterized by different trophic states and colonized by varying degrees of invasive SAV and cyanobacteria, basing on annual carbon pools (input, output, gas exchange, burial) and aquatic metabolism. The oligo-mesotrophic lake behaved as an annual CO2 and CH4 source toward the atmosphere (81.2 ± 14.8 g C m-2 yr-1), mainly due to dominant benthic heterotrophic metabolism, whereas the mesotrophic lake behaved as an annual sink (-6.7 ± 9.7 g C m-2 yr-1), mainly because of a much higher net carbon uptake by invasive SAV and cyanobacteria. In the mesotrophic lake, the fast-growing metabolism of the invasive primary producers also resulted in a strong buffer capacity with respect to the carbon export from the lake. Our study highlights the major role played by the littoral lacustrine zones in the control of regional/global carbon cycle, especially in densely vegetated systems. We suggest that the interplay between eutrophication and biological invasions can switch lakes from carbon source to sink.

Potential of submerged macrophytes restoration for reducing CH4 and CO2 emissions in a typical urban lake

As one of the areas most affected by human activities, urban lakes play a crucial role in global carbon cycle. Currently, submerged macrophyte restoration is a common ecological practice in urban lakes, primarily aimed at improving water quality and enhancing the aesthetic value of lake environments. Despite its widespread application, its contribution to carbon emission reduction has not received sufficient attention. In this study, we quantified the fluxes, concentrations and isotope signatures of CH4 and CO2 in the restoration zone (RL), the unrestored zone (UR) and the inflow rivers (IR) of Lake Xuanwu, along with potential environmental and dissolved organic matter (DOM) factors over the course of a year. The results indicated that the restoration of submerged macrophytes significantly diminished the emission of CH4 and CO2 from the lake. Compared to the UR and IR zone, the CH4 flux in the RL zone was reduced by 82.27 % and 92.18 %, while the CO2 flux decreased by 464.95 % and 133.12 %, respectively. Further investigation revealed distinct eutrophication levels between the RL zone with submerged macrophytes compared to the UR zone, and higher eutrophication levels were associated with reduced carbon sequestration stability. Nitrogen and phosphorus played critical roles in the emission of CH4 and CO2, respectively. Submerged macrophytes directly reduce carbon emissions through photosynthesis and significantly influence the long-term carbon sequestration capacity of lakes by secreting oxygen, modifying the ecological characteristics of the aquatic environment, and altering the production and mineralization processes of CH4 and CO2 in sediment porewater. These results underscore the potential of submerged macrophytes restoration as a viable strategy for reducing local emissions of CH4 and CO2 in urban lakes.

Utilization patterns strongly dominated the dynamics of CO2 and CH4 emissions from small artificial lakes

Small lakes are significant sources of CO2 and CH4 emissions to atmosphere. The dynamics and controls of CO2 and CH4 emissions from human-dominated small lakes with diverse functions remain poorly understood. We investigated the spatiotemporal dynamics of CO2 and CH4 concentrations and fluxes in 33 small lakes around the urban area with different landscape properties and utilization patterns, to clarify the impact of human-dominated functional shift on their greenhouse gas emissions. Meanwhile, we used microcosm cultivation methods to assess the CO2 and CH4 production rates of sediments in these lakes. The results indicated that the utilization ways significantly influence the CO2 and CH4 emissions in these lakes, with urban landscape lakes and aquaculture lakes showing significantly higher emissions compared to irrigation water-supplying lakes and drinking-water lakes. Extensive urbanization and aquaculture practices could increase the risk of that small lakes turn into hotspots of CO2 and CH4 emissions, and further complicate their spatial heterogeneity. Meanwhile, the production potential of CO2 and CH4 in sediments, as well as gas fluxes in small lakes, exhibited consistent functional differentiation across different utilization patterns. They were mainly driven by changes in sediment organic carbon and microbial carbon. Additionally, the difference of organic carbon and nitrogen loads were another drives for the variability in CO2 and CH4 emissions. We highlighted that the continuous accumulation of nutrient loads in water and sediments in human-dominated small lakes has greatly enhanced the potential for carbon gas emissions. We also found that utilization ways can significantly affect the key controls of CO2 and CH4 emission from small lakes, and also influence the reliability of carbon emission prediction models based on water chemistry parameters. To accurately estimate the contribution of small lakes to the global greenhouse gas inventory, it is essential to establish adaptive predictive models that consider the uncertainties in lake carbon emissions resulting from human utilization patterns.

A plateau freshwater shallow lake as a significant CO2 sink during the ice-covered period: A case study of Lake Wuliangsuhai, China

Lakes are essential for estimating the global CO2 budget. However, approximately 50 % of lakes undergo periodic freezing, and there is limited research on the factors influencing the CO2 cycle and ice formation in freshwater lakes located in middle- and high-latitude plateaus during ice-covered periods. Using high-frequency meteorological-flux data collected over six consecutive months during the 2018-2019 freezing period of Lake Wuliangsuhai, this study explored the diurnal variation, daily accumulation, and monthly accumulation of the CO2 cycle and its influencing mechanisms at a half-hour scale. The key findings are as follows. Lakes are CO2 sinks during the ice-covered period, with the fluxes being -1.28 ± 4.79 gCm-2d-1. Net CO2 exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (RECO) during the monitoring period were -116.93 gCm-2,190.36 gCm-2, and 86.09 gCm-2, respectively. The lake ice-air CO2 cycle exhibited significant (p < 0.05) diurnal variation, with daytime contributing 92.89 %, 78.31 %, and 56.91 % to NEE, GPP, and RECO, respectively. From December 2018 to March 2019, the monthly total GPP in the whole lake exceeded 10,000 tons, and high autotrophy was observed in February 2019. During the freezing period, 45.22 % of plant assimilation was consumed by autotrophic and heterotrophic respiration. The capacity of the lake CO2 sink was primarily driven by evaporation, latent heat, radiative sensible heat, and air pressure inhibition, whereas snow cover reduced the CO2 sink capacity by 73.01 %. CO2 capture by surface ice was mainly affected by external factors such as snow cover and solar radiation, whereas bottom ice capture was influenced by internal factors such as acid-base balance, the carbonate pump, and biological primary production. Bubble storage and ice crevice transport significantly affected CO2 migration. In summary, further research should focus on elucidating the CO2 capture mechanisms in seasonally frozen lakes located at middle and high latitudes, within subsequent lake‑carbon sink studies.

Lacustrine groundwater discharge-derived carbon and nitrogen to regulate biogeochemical processes as revealed by stable isotope signals in a large shallow eutrophic lake

Eutrophic shallow lakes are hotspots of carbon (C) and nitrogen (N) accumulation and transformation, and are increasingly recognized as important sources of greenhouse gases (GHGs: CO2, CH4 and N2O). Lacustrine groundwater discharge (LGD) is a crucial component of the water budget and terrestrial material delivery for lakes, but its interplays with intrinsic CN biogeochemical processes remain less tackled. In this study, C and N ingredients and multiple stable isotopes (δ2H, δ18O, δ13C, and δ15N) were measured seasonally in groundwater, river water and lake water of a large eutrophic shallow lake in eastern China. The results revealed that groundwater is enriched with various forms of C and N that have similar sources and pathways as surface water in the lake and rivers. The isotope balance model also indicated that LGD derived C and N contribute significantly to lake inventories in addition to river runoff. These allochthonous C and N provide extra substrates for related biogeochemical processes, such as algae proliferation, organic matter degradation, methanogenesis and denitrification. Simultaneously, the excess oxygen consumption leads to depletion and hypoxia in the lake, further facilitating the processes of methanogenesis and denitrification. LGD functions not only as an external source of C and N that directly increases GHG saturations, but also as a mediator of internal CN pathways, which significantly affect hypoxia formation, GHG productions and emissions in the eutrophic lake. This study highlights the unrevealed potential regulation of LGD on biogeochemical processes in the eutrophic lake, and underscores the need for its consideration in environmental and ecological studies of lakes both regionally and globally.

Cyanobacteria decay alters CH4 and CO2 produced hotspots along vertical sediment profiles in eutrophic lakes

Cyanobacteria-derived organic carbon has been reported to intensify greenhouse gas emissions from lacustrine sediments. However, the specific processes of CH4 and CO2 production and release from sediments into the atmosphere remain unclear, especially in eutrophic lakes. To investigate the influence of severe cyanobacteria accumulation on the production and migration of sedimentary CH4 and CO2, this study examined the different trophic level lakes along the middle and lower reaches of the Yangtze River. The results demonstrated that eutrophication amplified CH4 and CO2 emissions, notably in Lake Taihu, where fluxes peaked at 929.9 and 7222.5 μmol/m2·h, mirroring dissolved gas levels in overlying waters. Increased sedimentary organic carbon raised dissolved CH4 and CO2 concentrations in pore-water, with isotopic tracking showing cyanobacteria-derived carbon specifically elevated CH4 and CO2 in surface sediment pore-water more than in deeper layers. Cyanobacteria-derived carbon deposition on surface sediment boosted organic carbon and moisture levels, fostering an anaerobic microenvironment conducive to enhanced biogenic CH4 and CO2 production in surface sediments. In the microcosm systems with the most severe cyanobacteria accumulation, average CH4 and CO2 concentrations in surface sediments reached 6.9 and 2.3 mol/L, respectively, surpassing the 4.7 and 1.4 mol/L observed in bottom sediments, indicating upward migration of CH4 and CO2 hotspots from deeper to surface layers. These findings enhance our understanding of the mechanisms underlying lake sediment carbon emissions induced by eutrophication and provide a more accurate assessment of lake carbon emissions.

The shifting pattern of CO2 source sink in a subtropical urbanizing lightly eutrophic lake

Globally, numerous freshwater lakes exist, and rapid urbanization has impacted carbon biogeochemical cycling at the interface where water meets air in these bodies. However, there is still a limited understanding of CO2 absorption/emission in eutrophic urbanizing lakes. This study therefore involved biweekly in-situ monitoring to evaluate fluctuations in the partial pressure (pCO2) and flux (fCO2) of CO2 and associated parameters from January to September 2020 (7:00-17:00 CST) in an urbanizing lake in southwestern China. Our study revealed that during the daylight hours of the 11 sampling days, both pCO2 and fCO2 consistently demonstrated decreasing trends from the early morning period to the late afternoon period, with notable increases on May 7th and August 15th, respectively. Interestingly, unlike our previous findings, an nonsignificant difference (p > 0.05) in mean pCO2 and fCO2 was observed between the morning period and the afternoon period (n = 22). Furthermore, the mean pCO2 in January (~105 μatm; n = 4) and April (133-212 μatm; n = 8) was below the typical atmospheric CO2 level (C-sink), while that in the other months surpassed 410 μatm (C-source), although the average values (n = 44) of pCO2 and fCO2 were 960 ± 841 μatm and 57 ± 85 mmol m-2 h-1, respectively. Moreover, the pCO2 concentration was significantly greater in summer (May to August, locally reaching 1087 μatm) than in spring (January to April at 112 μatm), indicating a seasonal shift between the C-sink (spring) and the C-source (summer). In addition, a significant positive correlation in pCO2/fCO2 with chlorophyll-a/nitrate but a negative correlation in dissolved oxygen and total phosphorus were recorded, suggesting that photosynthesis and respiration were identified as the main drivers of CO2 absorption/emissions, while changes in nitrate and phosphorus may be attributed to urbanization. Overall, our investigations indicated that this lightly eutrophic lake demonstrated a distinct shifting pattern of CO2 source-sink variability at daily and seasonal scales.

Epilimnetic oligotrophication increases contribution of oxic methane production to atmospheric methane flux from stratified lakes

Although considerable attention has been paid to the effects of eutrophication on aquatic methane (CH4) emissions to the atmosphere, the ecosystem-level effects of oligotrophication/re-oligotrophication on aquatic CH4 production and subsequent ecological responses remain to be elucidated. It has been hypothesized that dissolved inorganic phosphorus (DIP)-deficient conditions drive the ecosystem to utilize poorly bioavailable organic phosphorus for biomass formation, thereby generating CH4 as a by-product. To test this hypothesis, a mass balance approach was used to estimate in situ oxic methane production (OMP) in an oligotrophic, deep Lake Fuxian. The isotopic signature of dissolved 13C-CH4, the potential substrates for OMP, and the phnJ/phnD genes associated with microbial demethylation of organic phosphorus compounds were analyzed. Our results indicate that CH4 accumulation was maximal in the surface mixed layer (SML, i.e., Epilimnion) during lake stratification, and ∼ 86 % of the total CH4 flux to the atmosphere was due to OMP. Decomposition of methylphosphonate (MPn) by Alphaproteobacteria (genera Sphingomonas and Mesorhizobium) contributed significantly to OMP. Furthermore, water temperature (Temp), chlorophyll a (Chla), and DIP were the most critical predictors of water OMP potential. Meta-analysis of currently available global data showed that OMP had a negative exponential distribution with DIP (OMP = 2.0 e-0.71DIP, R2 = 0.57, p < 0.05). DIP concentrations below a threshold of 3.40 ∼ 9.35 μg P L-1 triggered OMP processes and increased the atmospheric CH4 emissions. Under future warming scenarios, stratification and catchment management induced oligotrophication or re-oligotrophication may systematically affect the biogeochemical cycling of phosphorus and the OMP contribution to CH4 emission in stratified lakes.

Unexpected increase of sulfate concentrations and potential impact on CH4 budgets in freshwater lakes

The continuous increase in sulfate (SO42-) concentrations discharged by anthropogenic activities lacks insights into their dynamics and potential impact on CH4 budgets in freshwater lakes. Here we conducted a field investigation in the lakes along the highly developed Yangtze River basin, China, additionally, we analyzed long-term data (1950-2020) from Lake Taihu, a typical eutrophic lake worldwide. We observed a gradual increase in SO42- concentrations up to 100 mg/L, which showed a positive correlation with the trophic state of the lakes. The annual variations indicated that eutrophication intensified the fluctuation of SO42- concentrations. A random forest model was applied to assess the impact of SO42- concentrations on CH4 emissions, revealing a significant negative effect. Synchronously, a series of microcosms with added SO42- were established to simulate cyanobacteria decomposition processes and explore the coupling mechanism between sulfate reduction and CH4 production. The results showed a strong negative correlation between CH4 concentrations and initial SO42- levels (R2 = 0.83), indicating that higher initial SO42- concentrations led to lower final CH4 concentrations. This was attributed to the competition for cyanobacteria-supplied substrates between sulfate reduction bacteria (SRB) and methane production archaea (MPA). Our study highlights the importance of considering the unexpectedly increasing SO42- concentrations in eutrophic lakes when estimating global CH4 emission budgets.

Re-estimating China's lake CO2 flux considering spatiotemporal variability

The spatiotemporal variability of lake partial carbon dioxide pressure (pCO2) introduces uncertainty into CO2 flux estimates at the lake water-air interface. Knowing the variation pattern of pCO2 is important for obtaining accurate global estimation. Here we examine seasonal and trophic variations in lake pCO2 based on 13 field campaigns conducted in Chinese lakes from 2017 to 2021. We found significant seasonal fluctuations in pCO2, with decreasing values as trophic states intensify within the same region. Saline lakes exhibit lower pCO2 levels than freshwater lakes. These pCO2 dynamics result in variable areal CO2 emissions, with lakes exhibiting different trophic states (oligotrophication > mesotrophication > eutrophication) and saline lakes differing from freshwater lakes (-23.1 ± 17.4 vs. 19.3 ± 18.3 mmol m-2 d-1). These spatiotemporal pCO2 variations complicate total CO2 emission estimations. Using area proportions of lakes with varying trophic states and salinity in China, we estimate China's lake CO2 flux at 8.07 Tg C yr-1. In future studies, the importance of accounting for lake salinity, seasonal dynamics, and trophic states must be noticed to enhance the accuracy of large-scale carbon emission estimates from lake ecosystems in the context of climate change.

Experimental Ecosystem Eutrophication Causes Offsetting Effects on Emissions of CO2, CH4, and N2O from Agricultural Reservoirs

Despite decades of research and management efforts, eutrophication remains a persistent threat to inland waters. As nutrient pollution intensifies in the coming decades, the implications for aquatic greenhouse gas (GHG) emissions are poorly defined, particularly the responses of individual GHGs: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The biogeochemical controls of each gas can differ, making it difficult to predict the overall effect of nutrient pollution on the net radiative forcing of aquatic ecosystems. Here, we induced eutrophication of small nitrogen (N)-limited agricultural reservoirs and measured changes in diffusive GHG emissions within a before-after-control-impact (BACI) study design during June to September 2021. Each gas exhibited a unique response to 300% increases in primary production, with a shift from an overall CO2 source to a sink, a modest increase in N2O flux, and, unexpectedly, no significant change in CH4 emissions. The lack of net directional change in CO2-equivalent GHG emissions in fertilized reservoirs during the summer contrasts findings from empirical studies of eutrophic lakes. Our findings illustrate the difficulty in extrapolating among different sized ecosystems and suggest that forecast 2-fold increases in agricultural N fertilization by 2050 may not result in consistently elevated GHG emissions during summer, at least from small reservoirs in continental grassland regions.

Seasonal and spatial variations of greenhouse gas (CO2, CH4 and N2O) emissions from urban ponds in Brussels

Freshwaters have been recognized as important sources of greenhouse gases (GHG) to the atmosphere. However, urban ponds have received little attention even though their number is increasing due to expanding urbanisation globally. Ponds are frequently associated to urban green spaces that provide several ecosystemic services such as cooling local climate, regulating the water cycle, and acting as small carbon sinks This study aims to identify and understand the processes producing GHGs (CO2, CH4, and N2O) in the urban ponds of the temperate European city of Brussels in Belgium. 22 relatively small ponds (0.1-4.6 ha) surrounded by contrasted landscape (strictly urban, bordered by cropland or by forest), were sampled during four seasons in 2021-2022. The mean ± standard deviation was 3,667 ± 2,904 ppm for the partial pressure of CO2 (pCO2), 2,833 ± 4,178 nmol L-1 for CH4, and 273 ± 662% for N2O saturation level (%N2O). Relationships of GHGs with oxygen and water temperature suggest that biological processes controlled pCO2, CH4 concentration and%N2O. However, pCO2 was also controlled by external inputs as indicated by the higher values of pCO2 in the smaller ponds, more subject to external inputs than larger ones. The opposite was observed for CH4 concentration that was higher in larger ponds, closer to the forest in the city periphery, and with higher macrophyte cover. N2O concentrations, as well as dissolved inorganic nitrogen, were higher closer to the city center, where atmospheric nitrogen deposition was potentially higher. The total GHG emissions from the Brussels ponds were estimated to 1kT CO2-eq per year and were equivalent to the carbon sink of urban green spaces.

Ecological water diversion activity changes the fate of carbon in a eutrophic lake

Lake eutrophication mitigation measures have been implemented by ecological water diversion, however, the responses of carbon cycle to the human-derived hydrologic process still remains unclear. With a famous river-to-lake water diversion activity at eutrophic Lake Taihu, we attempted to fill the knowledge gap with integrative field measurements (2011-2017) of gas carbon (CO2 and CH4) flux, including CO2-equivalent, and dissolved carbon (DOC and DIC) at water-receiving zone and reference zone. Overall, results showed the artificial water diversion activity increased gas carbon emissions. At water-receiving zone, total gas carbon (expressed as CO2-equivalent) emissions increased significantly due to the occurring of water diversion, with CO2 flux increasing from 9.31 ± 16.28 to 18.16 ± 12.96 mmol C m-2 d-1. Meanwhile, CH4 emissions at water-receiving zone (0.06 ± 0.05 mmol C m-2 d-1) was double of that at reference zone. Water diversion decreased DOC but increased DIC especially at inflowing river mouth. Temporal variability of carbon emissions and dissolved carbon were linked to water temperature, chlorophyll a, and nutrient, but less or negligible dependency on these environment variables were found with diversion occurring. Water diversion may increase gas carbon production via stimulating DOC mineralization with nutrient enrichment, which potentially contribute to increasing carbon emissions and decreasing DOC at the same time and the significant correlation between CO2 flux and CH4 flux. Our study provided new insights into carbon biogeochemical processes, which may help to predict carbon fate under hydrologic changes of lakes.

Influence of hydrological features on CO2 and CH4 concentrations in the surface water of lakes, Southwest China: A seasonal and mixing regime analysis

Due to the large spatiotemporal variability in the processes controlling carbon emissions from lakes, estimates of global lake carbon emission remain uncertain. Identifying the most reliable predictors of CO2 and CH4 concentrations across different hydrological features can enhance the accuracy of carbon emission estimates locally and globally. Here, we used data from 71 lakes in Southwest China varying in surface area (0.01‒702.4 km2), mean depth (< 1‒89.6 m), and climate to analyze differences in CO2 and CH4 concentrations and their driving mechanisms between the dry and rainy seasons and between different mixing regimes. The results showed that the average concentrations of CO2 and CH4 in the rainy season were 23.9 ± 18.8 μmol L-1 and 2.5 ± 4.9 μmol L-1, respectively, which were significantly higher than in the dry season (10.5 ± 10.3 μmol L-1 and 1.8 ± 4.2 μmol L-1, respectively). The average concentrations of CO2 and CH4 at the vertically mixed sites were 24.1 ± 21.8 μmol L-1 and 2.6 ± 5.4 μmol L-1, being higher than those at the stratified sites (14.8 ± 13.4 μmol L-1 and 1.7 ± 3.5 μmol L-1, respectively). Moreover, the environmental factors were divided into four categories, i.e., system productivity (represented by the contents of total nitrogen, total phosphorus, chlorophyll a and dissolved organic matter), physicochemical factors (water temperature, Secchi disk depth, dissolved oxygen and pH value), lake morphology (lake area, water depth and drainage ratio), and geoclimatic factors (altitude, wind speed, precipitation and land-use intensity). In addition to the regression and variance partitioning analyses between the concentrations of CO2 and CH4 and environmental factors, the cascading effects of environmental factors on CO2 and CH4 concentrations were further elucidated under four distinct hydrological scenarios, indicating the different driving mechanisms between the scenarios. Lake morphology and geoclimatic factors were the main direct drivers of the carbon concentrations during the rainy season, while they indirectly affected the carbon concentrations via influencing physicochemical factors and further system productivity during the dry season; although lake morphology and geoclimatic factors directly contributed to the carbon concentrations at the vertically mixed and stratified sites, the direct effect of system productivity was only observed at the stratified sites. Our results emphasize that, when estimating carbon emissions from lakes at broad spatial scales, it is essential to consider the influence of precipitation-related seasons and lake mixing regimes.

Methane ebullition fluxes and temperature sensitivity in a shallow lake

Inland waters are important sources of atmospheric methane (CH4), with a major contribution from the CH4 ebullition pathway. However, there is still a lack of CH4 ebullition flux (eFCH4) and their temperature sensitivity (Q10) in shallow lakes, which might lead to large uncertainties in CH4 emission response from aquatic to climate and environmental change. Herein, the magnitude and regulatory of two CH4 pathways (ebullition and diffusion) were studied in subtropical Lake Chaohu, China, using the real-time portable greenhouse gas (GHG) analyzer-floating chamber method at 18 sites over four seasons. eFCH4 (12.06 ± 4.10 nmol m-2 s-1) was the dominant contributing pathway (73.0 %) to the two CH4 emission pathways in Lake Chaohu. The whole-lake mass balance calculation demonstrated that 56.6 % of the CH4 emitted from the sediment escaped through the ebullition pathway. eFCH4 was significantly higher in the western (WL: 16.54 ± 22.22 nmol m-2 s-1) and eastern lake zones (EL: 11.89 ± 15.43 nmol m-2 s-1) than in the middle lake zone (ML: 8.86 ± 13.78 nmol m-2 s-1; p < 0.05) and were significantly higher in the nearshore lake zone (NL: 15.94 ± 19.58 nmol m-2 s-1) than in the pelagic lake zone (PL: 6.64 ± 12.37 nmol m-2 s-1; p < 0.05). eFCH4 was significantly higher in summer (32.12 ± 13.82 nmol m-2 s-1) than in other seasons (p < 0.05). eFCH4 had a strong temperature dependence. Sediment total organic carbon (STOC) is an important ecosystem level Q10 driver of eFCH4. The meta-analysis also verified that across ecosystems the ecosystem-level Q10 of eFCH4 was significantly positively correlated with STOC and latitude (p < 0.05). This study suggests that eFCH4 will become increasingly crucial in shallow lake ecosystems as climate change and human activities increase. The potential increase in ebullition fluxes in high-latitude lakes is of great importance.

Urbanization influences dissolved organic matter characteristics but microbes affect greenhouse gas concentrations in lakes

Recognition and prediction of dissolved organic matter (DOM) properties and greenhouse gas (GHG) emissions is critical to understanding climate change and the fate of carbon in aquatic ecosystems, but related data is challenging to interpret due to covariance in multiple natural and anthropogenic variables with high spatial and temporal heterogeneity. Here, machine learning modeling combined with environmental analysis reveals that urbanization (e.g., population density and artificial surfaces) rather than geography determines DOM composition and properties in lakes. The structure of the bacterial community is the dominant factor determining GHG emissions from lakes. Urbanization increases DOM bioavailability and decreases the DOM degradation index (Ideg), increasing the potential for DOM conversion into inorganic carbon in lakes. The traditional fossil fuel-based path (SSP5) scenario increases carbon emission potential. Land conversion from water bodies into artificial surfaces causes organic carbon burial. It is predicted that increased urbanization will accelerate the carbon cycle in lake ecosystems in the future, which deserves attention in climate models and in the management of global warming.

Lake browning counteracts cyanobacteria responses to nutrients: Evidence from phytoplankton dynamics in large enclosure experiments and comprehensive observational data

Lakes worldwide are affected by multiple stressors, including climate change. This includes massive loading of both nutrients and humic substances to lakes during extreme weather events, which also may disrupt thermal stratification. Since multi-stressor effects vary widely in space and time, their combined ecological impacts remain difficult to predict. Therefore, we combined two consecutive large enclosure experiments with a comprehensive time-series and a broad-scale field survey to unravel the combined effects of storm-induced lake browning, nutrient enrichment and deep mixing on phytoplankton communities, focusing particularly on potentially toxic cyanobacterial blooms. The experimental results revealed that browning counteracted the stimulating effect of nutrients on phytoplankton and caused a shift from phototrophic cyanobacteria and chlorophytes to mixotrophic cryptophytes. Light limitation by browning was identified as the likely mechanism underlying this response. Deep-mixing increased microcystin concentrations in clear nutrient-enriched enclosures, caused by upwelling of a metalimnetic Planktothrix rubescens population. Monitoring data from a 25-year time-series of a eutrophic lake and from 588 northern European lakes corroborate the experimental results: Browning suppresses cyanobacteria in terms of both biovolume and proportion of the total phytoplankton biovolume. Both the experimental and observational results indicated a lower total phosphorus threshold for cyanobacterial bloom development in clearwater lakes (10-20 μg P L-1 ) than in humic lakes (20-30 μg P L-1 ). This finding provides management guidance for lakes receiving more nutrients and humic substances due to more frequent extreme weather events.

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.

Quantification of Diffusive Methane Emissions from a Large Eutrophic Lake with Satellite Imagery

Lakes are major emitters of methane (CH4); however, a longstanding challenge with quantifying the magnitude of emissions remains as a result of large spatial and temporal variability. This study was designed to address the issue using satellite remote sensing with the advantages of spatial coverage and temporal resolution. Using Aqua/MODIS imagery (2003-2020) and in situ measured data (2011-2017) in eutrophic Lake Taihu, we compared the performance of eight machine learning models to predict diffusive CH4 emissions and found that the random forest (RF) model achieved the best fitting accuracy (R2 = 0.65 and mean relative error = 21%). On the basis of input satellite variables (chlorophyll a, water surface temperature, diffuse attenuation coefficient, and photosynthetically active radiation), we assessed how and why they help predict the CH4 emissions with the RF model. Overall, these variables mechanistically controlled the emissions, leading to the model capturing well the variability of diffusive CH4 emissions from the lake. Additionally, we found climate warming and associated algal blooms boosted the long-term increase in the emissions via reconstructing historical (2003-2020) daily time series of CH4 emissions. This study demonstrates the great potential of satellites to map lake CH4 emissions by providing spatiotemporal continuous data, with new and timely insights into accurately understanding the magnitude of aquatic greenhouse gas emissions.

Uncertainty in projections of future lake thermal dynamics is differentially driven by lake and global climate models

Freshwater ecosystems provide vital services, yet are facing increasing risks from global change. In particular, lake thermal dynamics have been altered around the world as a result of climate change, necessitating a predictive understanding of how climate will continue to alter lakes in the future as well as the associated uncertainty in these predictions. Numerous sources of uncertainty affect projections of future lake conditions but few are quantified, limiting the use of lake modeling projections as management tools. To quantify and evaluate the effects of two potentially important sources of uncertainty, lake model selection uncertainty and climate model selection uncertainty, we developed ensemble projections of lake thermal dynamics for a dimictic lake in New Hampshire, USA (Lake Sunapee). Our ensemble projections used four different climate models as inputs to five vertical one-dimensional (1-D) hydrodynamic lake models under three different climate change scenarios to simulate thermal metrics from 2006 to 2099. We found that almost all the lake thermal metrics modeled (surface water temperature, bottom water temperature, Schmidt stability, stratification duration, and ice cover, but not thermocline depth) are projected to change over the next century. Importantly, we found that the dominant source of uncertainty varied among the thermal metrics, as thermal metrics associated with the surface waters (surface water temperature, total ice duration) were driven primarily by climate model selection uncertainty, while metrics associated with deeper depths (bottom water temperature, stratification duration) were dominated by lake model selection uncertainty. Consequently, our results indicate that researchers generating projections of lake bottom water metrics should prioritize including multiple lake models for best capturing projection uncertainty, while those focusing on lake surface metrics should prioritize including multiple climate models. Overall, our ensemble modeling study reveals important information on how climate change will affect lake thermal properties, and also provides some of the first analyses on how climate model selection uncertainty and lake model selection uncertainty interact to affect projections of future lake dynamics.

Combined effects of microplastics and warming enhance algal carbon and nitrogen storage

Algae dominate primary production in groundwater and oceans and play a critical role in global carbon dioxide fixation and climate change but are threatened by ongoing global warming events (such as heatwaves) and increasing microplastic (MP) pollution. However, whether and how ecologically important phytoplankton respond to the combined effects of warming and MPs remain poorly understood. We thus investigated the combined effects of these factors on carbon and nitrogen storage and the mechanisms underlying the alterations in the physiological performance of a model diatom, Phaeodactylum tricornutum, exposed to a warming stressor (25 °C compared with 21 °C) and polystyrene MP acclimation. Although warmer conditions decreased the cell viability, the diatoms subjected to the synergistic effects of MPs and warming showed significant increases in the growth rate (1.10-fold) and nitrogen uptake rate (1.26-fold). Metabolomics and transcriptomic analyses revealed that MPs and warming mainly promoted fatty acid metabolism, the urea cycle, glutamine and glutamate production, and the tricarboxylic acid (TCA) cycle due to an increased level of 2-oxoglutarate, which is the hub of carbon and nitrogen metabolism and accounts for the acquisition and utilization of carbon and nitrogen. Our findings emphasize the nonnegligible effects of MPs and HWs on the algal carbon and nitrogen cycles in waters.

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.

The long overlooked microalgal nitrous oxide emission: Characteristics, mechanisms, and influencing factors in microalgae-based wastewater treatment scenarios

Microalgae-based wastewater treatment is particularly advantageous in simultaneous CO₂ sequestration and nutrients recovery, and has received increasing recognition and attention in the global context of synergistic pollutants and carbon reduction. However, the fact that microalgae themselves can generate the potent greenhouse gas nitrous oxide (N₂O) has been long overlooked, most previous research mainly regarded microalgae as labile organic carbon source or oxygenic approach that interfere bacterial nitrification-denitrification and the concomitant N₂O production. This study, therefore, summarized the amount and rate of N₂O emission in microalgae-based systems, interpreted in-depth the multiple pathways that lead to NO formation as the key precursor of N₂O, and the pathways that transform NO into N₂O. Reduction of nitrite could take place in either the cytoplasm or the mitochondria to form NO by a series of enzymes, while the NO could be enzymatically reduced to N₂O at the chloroplasts or the mitochondria respectively under light and dark conditions. The influences of abiotic factors on microalgal N₂O emission were analyzed, including nitrogen types and concentrations that directly affect the nitrogen transformation routes, illumination and oxygen conditions that regulate the enzymatic activities related to N₂O generation, and other factors that indirectly interfere N₂O emission via NO regulation. The uncertainty of microalgae-based N₂O emission in wastewater treatment scenarios were emphasized, which would be particularly impacted by the complex competition between microalgae and ammonia oxidizing bacteria or nitrite oxidizing bacteria over ammonium or inorganic carbon source. Future studies should put more efforts in improving the compatibility of N₂O emission results expressions, and adopting consistent NO detection methods for N₂O emission prediction. This review will provide much valuable information on the characteristics and mechanisms of microalgal N₂O emission, and arouse more attention to the non-negligible N₂O emission that may impair overall greenhouse gas reduction efficiency in microalgae-based wastewater treatment systems.

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.

Concentrations of dissolved organic matter and methane in lakes in Southwest China: Different roles of external factors and in-lake biota

Many factors have been reported to affect material cycling in lakes, but the combined and cascading impacts of external environmental factors and in-lake biota on lake carbon cycling are poorly understood. We elucidated the influencing pathways of geoclimatic factors, lake morphometry, land-use type, chemical and physical factors, and biological taxa (phytoplankton and macroinvertebrates) on the concentrations of two important components of carbon cycling, i.e., dissolved organic matter (DOM) and methane (CH₄) based on datasets from 64 plateau lakes in Southwest China. Partial least squares path modelling (PLS-PM) indicated that (1) geoclimatic factors influenced DOM and CH₄ by affecting land use and lake physical factors (e.g., water temperature), (2) lake morphometry (water depth and lake area) had a direct and great negative effect on the CH₄ concentration related to the production and oxidation of CH₄ and affected phytoplankton and macroinvertebrates by influencing chemical and physical factors, (3) land-use type affected DOM and CH₄ concentrations in both direct and indirect ways, (4) terrestrial humic-like DOM was mainly discharged from forestland and also affected by macroinvertebrates, while the impacts of agricultural and construction land on autochthonous DOM and CH₄ concentrations mainly occurred by changing nutrients and then the aquatic biota. Moreover, changes in aquatic biota, primarily affected by water quality, influenced DOM spectral properties, and the two biotas affected DOM and CH₄ concentrations differently. Phytoplankton, especially cyanobacteria contributed to (protein-like and humic-like) DOM in both direct and indirect ways related to eutrophication, whereas macroinvertebrates influenced DOM possibly by utilization, bioturbation, and microbial decomposition of feces according to their different relationships with DOM spectral indices. Additionally, CH₄ production can be enhanced by DOM accumulation, and the significant positive correlations of CH₄ concentrations with protein-like DOM and biological index indicate that autochthonous DOM may play an important role for the CH₄ production. Our findings contribute to the understanding of lake carbon cycling under natural conditions and anthropogenic disturbances.

Patterns and drivers of CH4 concentration and diffusive flux from a temperate river-reservoir system in North China

Freshwater reservoirs are regarded as an important anthropogenic source of methane (CH₄) emissions. The temporal and spatial variability of CH₄ emissions from different reservoirs results in uncertainty in the estimation of the global CH₄ budget. In this study, surface water CH₄ concentrations were measured and diffusive CH₄ fluxes were estimated via a thin boundary layer model in a temperate river-reservoir system in North China, using spatial (33 sites) and temporal (four seasons) monitoring; the system has experienced intensive aquaculture disturbance. Our results indicated that the dissolved CH₄ concentration in the reservoir ranged from 0.07 to 0.58 µmol/L, with an annual average of 0.13 ± 0.10 µmol/L, and the diffusive CH₄ flux across the water-air interface ranged from 0.66 to 3.61 μmol/(m²•hr), with an annual average of 1.67 ± 0.75 μmol/(m²•hr). During the study period, the dissolved CH₄ concentration was supersaturated and was a net source of atmospheric CH₄. Notably, CH₄ concentration and diffusive flux portrayed large temporal and spatial heterogeneity. The river inflow zone was determined to be a hotspot for CH₄ emissions, and its flux was significantly higher than that of the tributary and main basin; the CH₄ flux in autumn was greater than that in other seasons. We also deduced that the CH₄ concentration/diffusive flux was co-regulated mainly by water temperature, water depth, and water productivity (Chla, trophic status). Our results highlight the importance of considering the spatiotemporal variability of diffusive CH₄ flux from temperate reservoirs to estimate the CH₄ budget at regional and global scales.

CS2 increasing CH4-derived carbon emissions and active microbial diversity in lake sediments

Lakes are important methane (CH₄) sources to the atmosphere, especially eutrophic lakes with cyanobacterial blooms accompanied by volatile sulfur compound (VSC) emissions. CH₄ oxidation is a key strategy to mitigate CH₄ emission from lakes. In this study, we characterized the fate of CH₄-derived carbon and active microbial communities in lake sediments with CS₂ used as a typical VSC, based on the investigation of CH₄ and VSC fluxes from Meiliang Bay in Lake Taihu. Stable isotope probing microcosm incubation showed that the efficiency of CH₄-derived carbon incorporated into organic matter was 21.1% in the sediment with CS₂ existence, which was lower than that without CS₂ (27.3%). SO₄^2--S was the main product of CS₂ oxidation under aerobic condition, accounting for 59.3-62.7% of the input CS₂-S. CS₂ and CH₄ coexistence led to a decrease of methanotroph and methylotroph abundances and stimulated the production of extracellular polymeric substances. CS₂ and its metabolites including total sulfur, SO₄^2- and acid volatile sulfur acted as the main drivers influencing the active microbial community structure in the sediments. Compared with α-proteobacteria methanotrophs, γ-proteobacteria methanotrophs Methylomicrobium, Methylomonas, Crenothrix and Methylosarcina were more dominant in the sediments. CH₄-derived carbon mainly flowed into methylotrophs in the first stage. With CH₄ consumption, more CH₄-derived carbon flowed into non-methylotrophs. CS₂ could prompt more CH₄-derived carbon flowing into non-methanotrophs and non-methylotrophs, such as sulfur-metabolizing bacteria. These findings can help elucidate the influence of VSCs on microorganisms and provide insights to carbon fluxes from eutrophic lake systems.

Methane-Derived Carbon as a Driver for Cyanobacterial Growth

Methane, a potent greenhouse gas produced in freshwater ecosystems, can be used by methane-oxidizing bacteria (MOB) and can therefore subsidize the pelagic food web with energy and carbon. Consortia of MOB and photoautotrophs have been described in aquatic ecosystems and MOB can benefit from photoautotrophs which produce oxygen, thereby enhancing CH₄ oxidation. Methane oxidation can account for accumulation of inorganic carbon (i.e., CO₂) and the release of exometabolites that may both be important factors influencing the structure of phytoplankton communities. The consortium of MOB and phototroph has been mainly studied for methane-removing biotechnologies, but there is still little information on the role of these interactions in freshwater ecosystems especially in the context of cyanobacterial growth and bloom development. We hypothesized that MOB could be an alternative C source to support cyanobacterial growth in freshwater systems. We detected low δ¹³C values in cyanobacterial blooms (the lowest detected value −59.97‰ for Planktothrix rubescens) what could be the result of the use of methane-derived carbon by cyanobacteria and/or MOB attached to their cells. We further proved the presence of metabolically active MOB on cyanobacterial filaments using the fluorescein isothiocyanate (FITC) based activity assay. The PCR results also proved the presence of the pmoA gene in several non-axenic cultures of cyanobacteria. Finally, experiments comprising the co-culture of the cyanobacterium Aphanizomenon gracile with the methanotroph Methylosinus sporium proved that cyanobacterial growth was significantly improved in the presence of MOB, presumably through utilizing CO₂ released by MOB. On the other hand, ¹³C-CH₄ labeled incubations showed the uptake and assimilation of MOB-derived metabolites by the cyanobacterium. We also observed a higher growth of MOB in the presence of cyanobacteria under a higher irradiance regime, then when grown alone, underpinning the bidirectional influence with as of yet unknown environmental consequences.

Increasing sulfate concentration and sedimentary decaying cyanobacteria co-affect organic carbon mineralization in eutrophic lake sediments

Sulfate (SO₄^2-) concentrations in eutrophic lakes are continuously increasing; however, the effect of increasing SO₄^2- concentrations on organic carbon mineralization, especially the greenhouse gas emissions of sediments, remains unclear. Here, we constructed a series of microcosms with initial SO₄^2- concentrations of 0, 30, 60, 90, 120, 150, and 180 mg/L to study the effects of increased SO₄^2- concentrations, coupled with cyanobacterial blooms, on organic carbon mineralization in Lake Taihu. Cyanobacterial blooms promoted sulfate reduction and released a large amount of inorganic carbon. The SO₄^2- concentrations in cyanobacteria treatments significantly decreased and eventually reached close to 0. As the initial SO₄^2- concentration increased, the sulfate reduction rates significantly increased, with maximum values of 9.39, 9.44, 28.02, 30.89, 39.68, and 54.28 mg/L∙d for 30, 60, 90, 120, 150, and 180 mg/L SO₄^2-, respectively. The total organic carbon content in sediments (51.16-52.70 g/kg) decreased with the initial SO₄^2- concentration (R² = 0.97), and the total inorganic carbon content in overlying water (159.97-182.73 mg/L) showed the opposite pattern (R² = 0.91). The initial SO₄^2- concentration was positively correlated with carbon dioxide (CO₂) emissions (R² = 0.68) and negatively correlated with methane (CH₄) emissions (R² = 0.96). The highest CO₂ concentration and lowest CH₄ concentration in the 180 mg/L SO₄^2- treatment were 1688.78 and 1903 μmol/L, respectively. These biogeochemical processes were related to competition for organic carbon sources between sulfate reduction bacteria (SRB) and methane production archaea (MPA) in sediments. The abundance of SRB was positively correlated with the initial SO₄^2- concentration and ranged from 6.65 × 10⁷ to 2.98 × 10⁸ copies/g; the abundance of MPA showed the opposite pattern and ranged from 1.99 × 10⁸ to 3.35 × 10⁸copies/g. These findings enhance our understanding of the effect of increasing SO₄^2- concentrations on organic carbon mineralization and could enhance the accuracy of assessments of greenhouse gas emissions in eutrophic lakes.

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.

Temporal trends in methane emissions from a small eutrophic reservoir: the key role of a spring burst

Waters impounded behind dams (i.e., reservoirs) are important sources of greenhouses gases (GHGs), especially methane (CH₄), but emission estimates are not well constrained due to high spatial and temporal variability, limitations in monitoring methods to characterize hot spot and hot moment emissions, and the limited number of studies that investigate diurnal, seasonal, and interannual patterns in emissions. In this study, we investigate the temporal patterns and biophysical drivers of CH₄ emissions from Acton Lake, a small eutrophic reservoir, using a combination of methods: eddy covariance monitoring, continuous warm-season ebullition measurements, spatial emission surveys, and measurements of key drivers of CH₄ production and emission. We used an artificial neural network to gap fill the eddy covariance time series and to explore the relative importance of biophysical drivers on the interannual timescale. We combined spatial and temporal monitoring information to estimate annual whole-reservoir emissions. Acton Lake had cumulative areal emission rates of 45.6 ± 8.3 and 51.4 ± 4.3 g CH₄ m^-2 in 2017 and 2018, respectively, or 109 ± 14 and 123 ± 10 Mg CH₄ in 2017 and 2018 across the whole 2.4 km² area of the lake. The main difference between years was a period of elevated emissions lasting less than 2 weeks in the spring of 2018, which contributed 17 % of the annual emissions in the shallow region of the reservoir. The spring burst coincided with a phytoplankton bloom, which was likely driven by favorable precipitation and temperature conditions in 2018 compared to 2017. Combining spatially extensive measurements with temporally continuous monitoring enabled us to quantify aspects of the spatial and temporal variability in CH₄ emission. We found that the relationships between CH₄ emissions and sediment temperature depended on location within the reservoir, and we observed a clear spatiotemporal offset in maximum CH₄ emissions as a function of reservoir depth. These findings suggest a strong spatial pattern in CH₄ biogeochemistry within this relatively small (2.4 km²) reservoir. In addressing the need for a better understanding of GHG emissions from reservoirs, there is a trade-off in intensive measurements of one water body vs. short-term and/or spatially limited measurements in many water bodies. The insights from multi-year, continuous, spatially extensive studies like this one can be used to inform both the study design and emission upscaling from spatially or temporally limited results, specifically the importance of trophic status and intra-reservoir variability in assumptions about upscaling CH₄ emissions.

Integrated approach towards quantifying carbon dioxide and methane release from waste stabilization ponds

Accurate estimations of gaseous emissions and carbon sequestration in wastewater processing are essential for the design, operation and planning of treatment infrastructure, particularly considering greenhouse gas reduction targets. In this study, we look at the interplay between biological productivity, hydrodynamics and evasion of carbon-based greenhouse gases (GHG) through diffusion and ebullition in order to provide direction for more accurate assessments of their emissions from waste stabilization ponds (WSPs). The ponds stratified in the day and mixed at night. Buoyancy flux contributed between 40 and 75% to turbulence in the water column during nocturnal cooling events, and the associated mixing lead to increasing carbon dioxide (CO₂) and methane (CH₄) concentrations by up to an order of magnitude in the surface. The onset of stratification and phytoplankton surface blooms, associated with high pH as well as low and variable CO₂ partial pressure resulted in an overall reduction of CO₂ efflux. Ebullition represented between 40 and 99% of the total CH₄ efflux, and up to 95% of the integrated GHG release during wastewater treatment (in CO₂ equivalents). Hydrodynamic conditions, diurnal variability and ebullition need to be accounted for reliable assessments of GHG emissions from WSPs. Our study is an important step towards gaining a deeper understanding in the functioning of these hot spots of carbon processing. The contribution of WSPs to atmospheric GHG budget is likely to increase with population growth unless their performance is improved in this regard.