Changes in Water Chemistry Associated with Rainstorm Events Increase Carbon Emissions from the Inflowing River Mouth of a Major Drinking Water Reservoir

Floating-leaved and submerged macrophytes suppress filamentous cyanobacteria blooms and 2-MIB episodes in eutrophic shallow lakes

Although macrophytes can control filamentous cyanobacteria that produce 2-methylisoborneol (2-MIB) in lakes, there is a lack of evidence regarding the inhibitory effects of different macrophyte growth forms on 2-MIB producers, as well as the underlying mechanisms. To address this knowledge gap, this study compared the impact of floating-leaved and submerged macrophytes on Pseudanabaena growth and 2-MIB release, combing a field investigation and culture experiments. The field survey showed that both the Pseudanabaena cell density and 2-MIB concentrations were significantly lower in areas dominated by floating-leaved or submerged macrophytes than in phytoplankton-dominated areas. In the culture experiments, floating-leaved macrophytes exhibited overall stronger inhibitory effects on Pseudanabaena than submerged macrophytes. Allelopathic effects emerged as a more critical mechanism than nutrient competition and light limitation in controlling Pseudanabaena growth and 2-MIB release by regulating the photosynthetic activity, gene abundance, and cell density. Allelopathic experiments further confirmed that the dissolved organic matter released from Nymphoides peltate, Trapa bispinosa and Myriophyllum spicatum contained higher concentrations of allelochemicals than that released from Vallisneria natans and Ceratophyllum demersum, driving the photosynthetic inhibition pathway. These findings demonstrate that biological control has great promise as an effective method for odorant management in eutrophic shallow lakes.

The role of reservoir size in driving methane emissions in China

Reservoirs play a crucial role as sources of methane (CH₄) emissions, with emission rates and quantities varying widely depending on reservoir size due to factors such as surface area, water depth, usage, operational methods, and spatial distribution. Gaining insights into emission characteristics across different reservoir sizes can aid in designing and managing reservoirs to mitigate CH₄ emissions effectively. In this study, machine learning models were applied to estimate both diffusive and ebullitive CH₄ emissions across 97,435 reservoirs in China, spanning five categories of storage capacity. This comprehensive assessment covers nearly all reservoirs within the country, revealing total CH₄ emissions of approximately 5,414 Gg. Reservoirs > 0.01 km3 are responsible for about 90 % of these emissions, primarily due to high diffusive flux rates and extensive surface areas. Elevated CH₄ diffusion in reservoirs > 0.01 km3 is largely influenced by their thermal stratification and capacity for organic matter accumulation. Furthermore, these reservoirs are particularly vulnerable to climate warming, which could accelerate CH₄ emission rates more rapidly in larger reservoirs than in smaller ones (below 0.01 km³). Consequently, prioritising CH₄ management in reservoirs > 0.01 km3 is imperative. Nevertheless, the high ebullitive flux of CH₄ in reservoirs < 0.01 km3, linked to their shallow depth, highlighting the potential for significant CH₄ ebullition from smaller aquatic systems. Given large and small-ranged reservoirs' distinct spatial distribution patterns, targeted management strategies are recommended: project-level management for large reservoirs and basin-level approaches for smaller reservoirs.

Metabolic processes and their effects on carbon cycling in a karst reservoir, south China

Metabolic processes of aquatic photosynthetic organisms significantly influence the multi-time scale cycle of dissolved inorganic carbon (DIC) in aquatic environments. Running karst water characterized by high DIC content are becoming lentic water body due to damming activities around the world, which could benefit for aquatic metabolic processes and change initial carbon cycle processes. However, despite the availability of high-resolution monitoring data, there is a lack of comprehensive studies examining the characteristics of the metabolic processes of aquatic photosynthetic organisms, controlling factors, and their relationship with changes in reservoir thermal structure and carbon cycling. Based on this, this study selected the Dalongdong (DLD) reservoir, a typical karst reservoir located in Southwest China, as the research area for multi-parameter high-resolution continuous monitoring conducted in 2021. Seasonal sampling of relevant indicators was conducted in June, August, and December. The results indicate that the average net ecosystem production (NEP) of surface-layer during mixing period was 0.40 mg L-1 d-1, significantly lower than the thermal stratification period (4.65 mg L-1 d-1). The reservoir demonstrated a net autotrophic state, with the estimated carbon sink of approximately 81.54 t C km-2 y-1. Seasonal and diurnal variations in metabolic processes were primarily influenced by photosynthetically active radiation, which subsequently affected the hydrochemical characteristics. During the thermal stratification period, high temperatures and significant DIC fertilization effect sustained high production efficiency among aquatic photosynthetic organisms. In contrast, during the mixing period, biological growth was more dependent on upwelling nutrient inputs from the reservoir bottom. Through physical disturbances and the inhibition of biological photosynthesis, CO2 emissions increased during rainfall, while NEP rose following the rainfall. Combining high-resolution continuous data with seasonal sampling data, the multi-scale effects of aquatic photosynthetic metabolism on the carbon cycle in aquatic systems are elucidated, which is essential for accurately calculating the carbon budget in reservoir basins.

Chemodiversity and molecular traits of dissolved organic matter driven by cascade reservoirs regulations in the upper Yangtze River

Dissolved organic matter (DOM) plays a fundamental role in biogeochemical cycles within riverine ecosystems. However, the construction and impoundment of dams disrupt the natural biophysical gradients in rivers, potentially leading to alterations and turnover of DOM compositions. This research investigated the composition and chemodiversity of DOM in the cascade reservoirs along the upper Yangtze River. Our findings reveal that DOM in the cascade reservoirs are predominantly composed of lignin-like, lipid-like, and protein-like compounds. The DOM chemodiversity exhibited a significant decrease during the flood season, but recovered during the dry season. Additionally, a general decline in DOM chemodiversity was observed along the longitudinal gradient in the cascade reservoirs. The molecular traits of DOM results indicated high bioavailability and lability of DOM molecules during the flood season. A random forest analysis identified physicochemical indicators as the most important factor influencing DOM chemodiversity. During the flood season, the selected variables were mostly strongly correlated with DOM molecular traits and categories. Specifically, Qin and Qout showed significant negative correlations with easily degradable compounds such as carbohydrates and lipids, as well as with molecular traits. Conversely, during the dry season, few physicochemical indicators and hydrological parameters show significant correlations with proteins and aromatic compounds. Structural equation model further demonstrated that hydrological parameters exert a strong positive influence on DOM molecular categories. This study provides a foundational framework for further evaluation cascade damming effects on DOM biogeochemical processes.

Terrestrial Organic Matter Inputs Modulate Methane Emissions from a Mega-Reservoir

Reservoirs are hotspots for methane (CH4) emissions. However, to date, the effects of terrestrial organic matter (OM) input and degradation on CH4 emissions from large reservoirs remain largely unknown. From May 2020 to April 2021, we conducted monthly sampling campaigns at 100 sites in Lake Qiandao (580 km2), a mega-reservoir in China, and made monthly vertical profile observations from March to September 2023. We estimated an annual mean FCH4 flux of 0.26 g C m-2 yr-1 (1.51 × 108 g C yr-1). Elevated FCH4 and enriched δ13C-CH4 coincided with low dissolved oxygen (DO) concentrations, high levels of organic suspended solids, terrestrial organic matter, nutrients, depleted δ18O-H2O, and low carbon isotope fractionation (αC) in the inflowing lake regions. Dissolved CH4 (cCH4) correlated positively to the relative abundance of aliphatic compounds. Anoxic bioincubation experiments revealed rapid degradation of riverine organic matter, accompanied by a 56-fold increase in cCH4, δ13C-CH4 enrichment (to -32.25‰), and a significant decrease in αC to 1.02. These findings indicate that acetoclastic CH4 production makes a substantial contribution to cCH4 and thus FCH4. Based on multiple lines of evidence, we conclude that input of terrestrial organic matter and its subsequent degradation lead to DO depletion, and their OM degradation byproducts serve as carbon substrates that promote CH4 emissions.

Inflow-modulated inputs of dissolved organic matter fuel carbon dioxide emissions from a large hyper-eutrophic lake

Terrestrial dissolved organic matter (DOM) is potentially reactive and, upon entering lake ecosystems, can be readily degraded to low-molecular-weight organic matter and dissolved CO2. However, to date, there has been limited research on the links between long-term variation in the composition of DOM and CO2 emissions from lakes. Lake Taihu is a large, shallow, and hyper-eutrophic lake where DOM composition is strongly influenced by inputs from the rivers draining cultivated and urbanized landscapes. This study aims to investigate the impact of water chemistry and DOM characteristics, modulated by inflow-discharge dynamics, on the variability of pCO2 in Lake Taihu. Based on long-term seasonal observations from 2000 to 2022, we estimated the annual areal average CO2 flux (FCO2) to be 470 ± 107 gCO2 m-2 yr-1 corresponding to a summed flux of 1.1 ± 0.3 TgCO2 yr-1 from the lake. Elevated levels of FCO2 were found in 2004-2007, and across the dataset, the highest FCO2 was often observed in February and the lowest in August, corresponding with higher algal uptake in summer. Northwestern inflowing riverine DOM, consisting of both soil leachate and rapidly degradable domestic wastewater along with the input of riverine CO2 itself, explained the elevated FCO2 in the northwestern lake regions. Ultra-high resolution mass spectrometry (FT-ICR MS) can resolve the composition of DOM at the molecular level, revealing that CO2 concentrations vary with the relative abundance of terrestrially derived condensed aromatics. Incubation experiments further confirmed the link between the degradation of biologically labile DOM and aerobic CO2 production. We conclude that riverine organic matter inputs and subsequent degradation, modulated by inflow discharge, play a key role in shaping CO2 emissions from Lake Taihu over the past two decades.

Rainstorm-induced organic matter pulses: A key driver of carbon emissions from inland waters

Numerous rivers and lakes in the monsoon climate zone are heavily influenced by frequent rainstorms that mobilize dissolved organic matter (DOM) from pristine or urbanized environments into downstream lakes. Of particular concern is the mobilization of DOM from anthropogenic effluents, which are commonly enriched in aliphatic compounds that can be easily degraded by microorganisms. Rapid degradation of highly biodegradable DOM, in turn, may cause significant depletion of dissolved oxygen in the water, which, by creating anoxic conditions at the bottom water-sediment interface, promotes microbial production of CO2 and CH4. Further investigations based on high-frequency monitoring and novel techniques such as ultra-high-resolution mass spectrometry and isotopic measurements, are needed to elucidate the processes and mechanisms by which pulsed aliphatic inputs impact lake carbon emissions.

Human Activities Reshape Greenhouse Gas Emissions From Inland Waters

Inland waters are significant sources of greenhouse gases (GHGs) in an increasingly human-dominated world, yet the mechanisms by which human activities reshape GHG emissions from these systems remain poorly understood. Here, we synthesized research from three human-dominated landscapes-agricultural, urban, and impounded river systems-to demonstrate that inland waters within these systems exhibit significantly higher GHG emissions compared to their natural or seminatural counterparts. This is particularly evident for CH4 and N2O emissions, which show median enhancement ratios of 2.0-10 and 2.4-13 across the systems, respectively. In contrast, CO2 emissions exhibit overall lower enhancement (median enhancement ratios of < 2.0-3.1), largely due to simultaneously increased photosynthetic uptake from aquatic eutrophication. These observations underscore a clear human footprint on aquatic GHG emissions and the underlying biogeochemical processes. The observed changes in GHG emissions are driven by increased inputs of sediments, carbon, and nutrients from human-disturbed landscapes, coupled with the expansion of aquatic anoxia resulting from increased aquatic metabolism, fine sediment deposition, and eutrophication. Beyond altering emission rates, human activities also modify the abundance and distribution of inland waters, potentially exerting substantial, yet unquantified, effects on landscape-scale GHG emissions. We highlight the importance of understanding these processes for accurately quantifying and mitigating the human footprint on aquatic GHG emissions. Future research and mitigation efforts should account for the variability and mechanisms discussed in this review to effectively address human-induced GHG emissions from inland waters.

Algal organic matter triggers re-assembly of bacterial community in plumbing system

Algal bloom outbreaks in upstream drinking water reservoirs inevitably lead to algal organic matter (AOM) pollution in downstream drinking water plants and distribution systems. However, the responses of indoor piped drinking water quality and microbial community to AOM remain to be well studied. In this study, we investigated the effects of low and high concentrations of Chlorella organic matters on pipe-based drinking water. We found that AOM introduced nitrogen and phosphorus contamination into drinking water and promoted massive regeneration of bacteria during stagnation, along with increased bacterial metabolic activity. Compared to the Control group, the utilization capacity of alcohols, acids, esters, and amino acids increased under the influence of AOM. In addition, AOM intrusion reduced the bacterial community diversity in drinking water. The bacterial communities became more saturated, interspecific relationships became more complex, and interspecific competition increased. Bacteria with the ability to denitrification, such as Pseudomonas putida, Sphingobium amiense, Delftia tsuruhatensis, and Acidovorax temperans, were the most abundant. Residual chlorine, ammonium, nitrite, and iron had notable effects on the bacterial community under the influence of AOM. The results help elucidate the response mechanism of microbial community to AOM contamination in indoor drinking water pipes and provide a scientific basis for drinking water safety risk management.

Rainstorms drive the carbon dioxide emissions during the algae-growing season in a large eutrophic lake

Lakes are sources of atmospheric carbon dioxide (CO2), contributing to global climate change. Temporal variations in lake CO2 emissions are pronounced, with algal growth and precipitation identified as important drivers. Eutrophic lakes often act as atmospheric CO2 sinks during the growing season. However, these lakes can also emit CO2 during the same period, a paradox that we hypothesize is driven by precipitation. This study tests this hypothesis and examines how rainstorms influence lake CO2 emissions. To investigate, seven buoys were deployed in eutrophic Lake Taihu and a major inflow river to monitor water quality at 4-h intervals throughout 2021. CO2 flux (FCO2) was calculated using integrated methods, including gas diffusion models, the CO2calc program, and machine learning algorithms, based on water quality and meteorological data. Results revealed that only rainstorms (daily rainfall >50 mm) significantly increased FCO2. Although only three rainstorm events occurred, they accounted for 12.15% of the annual CO2 emissions. During the growing season, the lake was a net CO2 source, but without rainstorm-induced emissions, it would have functioned as a CO2 sink, highlighting the crucial role of rainstorms in shifting lake dynamics. Piecewise structural equation modeling indicated that both abiotic factors (e.g., gas transfer velocity) and biotic factors (e.g., aquatic metabolism) influenced by rainstorms contributed to the elevated FCO2. These findings suggest that future reductions in lake FCO2 due to eutrophication, combined with more frequent rainstorms under climate change, could amplify the impact of extreme precipitation on CO2 emissions.

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.

The neglect of the change in inundation area leads to overestimation of carbon emission in cascade reservoirs

Reservoir carbon emissions reflect greenhouse gases emitted by flooded land post-construction. However, pre-construction flooded land has been overlooked in previous assessments. Utilizing annual land cover data from 1990 to 2022 and pertinent parameters of cascade reservoirs in the Lancang River (LCR), we calculated the actual flooded areas of these reservoirs. Subsequently, the Tier1 model was employed to estimate the carbon emissions during the reservoir's life cycle and the annual carbon emissions from newly flooded land during construction. Our findings indicate that the LCR cascade reservoir's carbon emission throughout its life cycle is 4.324Tg CO2eq (1.818-8.879Tg CO2eq). When compared with previous results, our estimated figures (0.496-2.106 g CO2eq/(kw·h)) fall below the global hydroelectric carbon footprint's average threshold range (IPCC: 4-14 g CO2eq/(kW·h)). This implies that the previously estimated carbon emissions from the reservoir may be inflated due to the flooded land prior to reservoir construction. Notably, the nutrient state of the water body predominantly governs reservoir carbon emissions. This research sheds light on the intricacies of carbon emissions from cascade reservoirs and underscores the importance of accurately delineating reservoir boundaries and managing nutrient inputs to mitigate carbon emissions.

Hydrogen generated by electrochemical water splitting as electron donor for nitrate removal from micro-polluted reservoir water

Extremely limited organic carbon sources and aerobic environment in micro-polluted reservoir water make conventional denitrification exceptionally challenging. As a result, total nitrogen (TN) concentration in most reservoir waters exceeds standard value year-round. In this study, for the first time, we constructed a mini water-lifting and aeration system (mini-WLAS) to remove nitrate in actual reservoir water. In the mini-WLAS, H2 was produced through electrolysis of reservoir water without adding any electrolyte, and the ascending water flow carried the generated H2 from lower layer to upper bacteria layer. The maximum denitrification rate reached 0.29 mg (L·d)-1 under dissolved oxygen (DO) concentration of 6-8 mg L-1, 6.04 times higher than that of the control group. There is almost no accumulation of NH4+-N, NO2--N, and N2O, and the concentration of CODMn decreased by 55.2 %. More importantly, the pH stayed near-neutral steadily throughout the whole process. Microbial community analysis showed that the abundances of hydrogenotrophic denitrifying bacteria (HDB) were 2 orders higher than those in the control system. Some HDB could work under aerobic conditions, providing an explanation for the excellent denitrification performance under high DO. This study provides a novel perspective for TN removal from reservoir water.

Seasonal and diurnal changes of pCO2 in the lower Brahmaputra River, Bangladesh

Monthly 24-h real-time high-resolution monitoring was conducted for 1 year to investigate the carbon dioxide (CO2) dynamics in the lower Brahmaputra River. The partial pressure of CO2 (pCO2) was observed between 136 and 1213 µatm. The pCO2 was almost 1.5 times higher during the wet season (May-October) than during the dry season (November-April). Diurnal variation in pCO2 was pronounced during the dry season. In contrast, no clear diurnal pattern was observed during the wet season. The combined measurements of O2-CO2 in river water provide insights into net chemical and microbial activity. In January and March, there was a bidirectional exchange of CO2, occurring from air to water (sink) and from water to air (source). However, in April, the CO2 exchange was unidirectional, taking place solely from air to water, whereas in all other months, it occurred from water to air. The carbon dioxide flux (FCO2) in the river ranged from - 31.12 to 137.74 mmol m-2 day-1, with an average of 25.26 mmol m-2 day-1 during the dry season and 78.56 mmol m-2 day-1 in the wet season. This flux significantly contributes to the regional net carbon budget of the world's largest delta.

Unraveling the role of anthropogenic and hydrologic drivers with respect to the optical and molecular properties of dissolved organic matter and organic phosphorus in a P-contaminated river

Anthropogenic and hydrological drivers are key factors influencing the fate of dissolved organic matter (DOM) and dissolved organic phosphorus (DOP) in river runoff. However, how anthropogenic disturbances and hydrological conditions jointly affect the composition and characteristics of DOM and DOP in river runoff remains unclear. This study used fluorescence spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and the stable water isotopes to interpret the chemical composition and properties of DOM and DOP as well as their linkages to anthropogenic disturbances and hydrological conditions in a typical P-contaminated tributary to the central Yangtze River. The results show in the wet season, the average abundance of humic-like components in DOM exceeded 60 %, while the average abundance of tryptophan-like components in DOM exceeded 50 % in the dry season. During the dry season, hydrological conditions had a greater impact on highly unsaturated DOM compounds compared to anthropogenic disturbances because a decrease in precipitation reduced the transport of terrestrial DOM into aquatic systems and increased water retention time in the river, promoting the production of unsaturated compounds from photochemistry. The effects of the two factors were similar in the wet season because active agricultural activities and intense precipitation jointly facilitated the entry of exogenous humics into the runoff, leading to the similar relative abundance of highly unsaturated DOM compounds associated with both factors. Anthropogenic disturbances had a greater impact on aliphatic DOM and DOP than hydrological conditions, which was associated with intense human activities in the watershed, such as phosphate mining, agricultural cultivation, and domestic sewage discharge. This study provides new knowledge about the composition, properties and underlying mechanisms of DOM and DOP in the P-contaminated watershed runoff.

Seasonal variation in greenhouse gas concentrations and diffusive fluxes in three river-reservoir systems in the Seine Basin (France)

River and reservoir ecosystems have been considered as hot spots for GHG (greenhouse gas) emissions while their specific hydrological and biogeochemical processes affect GHG concentrations; however, few studies integrated river-reservoir systems to identify the dominant drivers of GHG concentrations and flux changes associated with these systems. In the present study, we examined the seasonal variations in GHG concentrations in the surface water of three river-reservoir systems in the Seine Basin. The levels and seasonal variations of GHG concentrations exhibited distinct patterns among reservoirs, upstream, and downstream rivers. The concentrations of CH4 (methane) in the reservoirs were notably higher than those observed in both upstream and downstream rivers and showed higher values in summer and autumn, which contrasted with CO2 (carbon dioxide) concentrations, while N2O (nitrous oxide) concentrations did not show an obvious seasonal pattern. A high mole ratio of CH4/CO2 was found in these reservoirs, with a value of 0.03 and was more than 30 and 10 times higher than that in the upstream and downstream rivers, respectively. The three river-reservoir systems were oversaturated with GHG during the study period, with the average diffusive fluxes (expressed as CO2eq: CO2 equivalent) of 810 ± 1098 mg CO2eq m-2 d-1, 9920 ± 2413 mg CO2eq m-2 d-1, and 7065 ± 2704 mg CO2eq m-2 d-1 in the reservoirs, upstream and downstream rivers, respectively. CO2 and CH4-CO2 were respectively the dominant contributors to GHG diffusive fluxes in river and reservoir sections, while N2O contributed negligibly to GHG diffusive fluxes in the three river-reservoir systems. Our results showed that GHG concentrations and gas transfer coefficient have varying importance in driving GHG diffusive fluxes among different sections of the river-reservoir systems. In addition, our results also show the combined effect of reservoirs and upstream rivers on the water quality variables and hydrological characteristics of downstream rivers, highlighting the future need for additional investigations of GHG processes in the river-reservoir systems.

Using the Potential Transformation of Dissolved Organic Matter to Understand Carbon Emissions from Inland Rivers

Understanding the transformation of river dissolved organic matter (DOM) is important for assessing the emissions of greenhouse gases (GHGs) in inland waters. However, the relationships between the variations in DOM components and GHGs remain largely unknown. Here, parallel factor analysis (PARAFAC) was applied to investigate the DOM components in 46 inland rivers in China. We found that the GHG emissions in peri-urban rivers were 1.10-2.15 times greater than those in urban rivers. Microbial and environmental factors (e.g., living cell numbers, microbial activity and pH) explained more than 70% of the total variance in GHG emissions in rivers. DOM variations relationships between different components ware revealed based on compositional data principal component analysis (CoDA-PCA). Microbial-mediated DOM production and degradation were quantified, and the degradation levels in peri-urban rivers were 11.8-25.2% greater than those in urban rivers. Differences in carbon emission potential between urban and peri-urban rivers were related to DOM variances and transformations and were affected by water chemistry (e.g., NH4-N and As). This study clarifies the regulatory effects of DOM composition variations and transformations on GHG emissions, and enhances the understanding of the DOM biogeochemical cycle.

Spatial patterns and environmental functions of dissolved organic matter in grassland soils of China

Soil dissolved organic matter (DOM) is crucial to atmospheric, terrestrial and aquatic environments as well as human life. Here, by characterizing DOM from 89 grassland soils throughout China, we reveal the spatial association between DOM geochemistry in the dry season vs annual ecosystem exchange and cancer cases. The humic-like and high molecular weight (3.4-25 kDa) fractions with lower biodegradability, decline from the northern to the southern regions of China, and are correlated with lower soil respiration and net ecosystem productivity at the continental scale. The <1.2 kDa and proteinaceous fractions could serve as a geographical indicator of nasopharyngeal cancer incidence and mortality, while the 3.4-25 kDa and humified fractions are potentially associated with pancreatic cancer cases (P < 0.05). Our findings highlight that exploiting the environmental functions of soil DOM and mitigating the negative impacts are necessary, and require actions tailored to local soil DOM conditions.

A microcosm evaluation of metal cycling in an urbanized contaminated estuary varying with oxic-hypoxic-anoxic-reoxic transition: Behavior, fluxes, and mechanism

Water hypoxia and metal pollution are commonly co-existed in urbanized estuaries. This study focuses on the effect of an extended dissolved oxygen (DO) full-life dynamics (86 days) on metal behavior across the sediment-water interface through laboratory microcosms from two typical zones in Pearl River Estuary. Combining our time-series results of concentrations and fluxes, it showed that Co, Ni, and Zn consistently presented a release-precipitation-release trajectory with an oxic-hypoxic-anoxic-reoxic transition, characterized with highly variable behavior in the hypoxic-anoxic hotmoments. In parallel, changing DO dynamics significantly activated a repartitioning process of Co, Ni, and Zn among several species and elevated their risk in sediments, promoting the formation of more labile species in the 0-10 mm hotspots, where metals sensitively responded. Over DO transition, metal cycling was tightly co-related with Fe, Mn, and S elements. It was found that Mn was dominated in low oxygen-hypoxic period, but switched to S and Fe in anoxic stage, limiting sustained metal liberation to overlying water. Enlarging this experiment to practice, released Zn fluxes from sediments in hypoxic summer could contribute about ∼2.0% to their stocks in water column, while increase to 20% (1 m bottom water) in highly-stratified zones. This study has certain significance in understanding the long-term metal behavior and fate in estuarine regions, even lakes and reservoirs.

Optical properties and molecular compositions of dissolved organic matter in multiple runoff components during rainfalls on the karst hillslope

Understanding the chemical composition, origin, and molecular structure of dissolved organic matter (DOM) in multi-interface runoff is essential for comprehending the fate of laterally transported DOM in complex soil-epikarst systems of karst hillslopes. Limited information, however, is available for the optical properties and molecular compositions of the transported OM in multiple runoff components on the karst hillslope in relation to land-uses and soil thicknesses. In this study, we conducted a study to observe the changes in the quantity and quality of DOM in multiple interface flow (surface, subsurface, and epikarst) during natural rainfall events in 2022 in karst hillslopes that are covered by different land uses (cropland and shrubland) and soil thicknesses (with mean depths of 66.0 cm for deeper soil and 35.4 cm for shallower soil) in the karst region of southwest China. chemcial compositions of runoff DOM were determined by optical analysis and microbial compositions in runoff were inferred with high-throughput sequencing. The results showed that the soil-epikarst structure was controlling the runoff DOM quantity and quality during rainfall events. A decrease in the aromaticity, humification, unsaturation, and oxidation degree and an increase in carbohydrate, aminosugars, protein, and lipid compounds were found from surface to epikarst flow, indicating that plant-and soil-derived carbon decreased, while the microbially-derived carbon increased. The results were further comfirmed by the higher bacterial richness and diversity, along with fungal diversity in the epikarst flow compared to other runoff components. The bio-labile protein materials (C2) were the most important component of runoff DOM output in karst hillslopes. In surface and subsurface flow, rainfall amount, runoff rate, and discharge significantly affected the DOM concentration and quality during rainfalls, indicating that the dynamics of DOM in runoff from karst hillslopes were predominantly influenced by hydrological processes. Furthermore, the runoff DOM quality in cropland was dominated by lower unsaturation and oxidation degrees and higher protein component, compared to those in shrubland. The compositions of DOM in runoff from hillslope plots with thicker soils were primarily characterized by microbially-derived materials. Our findings were conducive to understanding the mechanism governing the migration of DOM quality and quantity in discharge during multi-interface hydrological processes on karst hillslopes.

Mollisol Erosion-Driven Efflux of Energetic Organic Carbon and Microflora Increases Greenhouse Gas Emissions from Cold-Region Rivers

Massive soil erosion occurs in the world's Mollisol regions due to land use change and climate warming. The migration of Mollisol organic matter to river systems and subsequent changes in carbon biogeochemical flow and greenhouse gas fluxes are of global importance but little understood. By employing comparative mesocosm experiments simulating varying erosion intensity in Mollisol regions of northeastern China, this research highlights that erosion-driven export and biomineralization of terrestrial organic matter facilitates CO2 and CH4 emission from receiving rivers. Stronger Mollisol erosion, as represented by a higher soil-to-water ratio in suspensions, increased CO2 efflux, particularly for the paddy Mollisols. This is mechanistically attributable to increased bioavailability of soluble organic carbon in river water that is sourced back to destabilized organic matter, especially from the cultivated Mollisols. Concurrent changes in microbial community structure have enhanced both aerobic and anaerobic processes as reflected by the coemission of CO2 and CH4. Higher greenhouse gas effluxes from paddy Mollisol suspensions suggest that agricultural land use by supplying more nitrogen-containing, higher-free-energy organic components may have enhanced microbial respiration. These new findings highlight that Mollisol erosion is a hidden significant contributor to greenhouse gas emissions from river water, given that the world's four major Mollisol belts are all experiencing intensive cultivation.

Control of carbon dioxide exchange fluxes by rainfall and biological carbon pump in karst river-lake systems

As an important component of inland water, the primary factors affecting the carbon cycle in karst river-lake systems require further investigation. In particular, the impacts of climatic factors and the biological carbon pump (BCP) on carbon dioxide (CO2) exchange fluxes in karst rivers and lakes deserve considerable attention. Using quarterly sampling, field monitoring, and meteorological data collection, the spatiotemporal characteristics of CO2 exchange fluxes in Erhai Lake (a typical karst lake in Yunnan, SW China) and its inflow rivers were investigated and the primary influencing factors were analyzed. The average river CO2 exchange flux reached 346.80 mg m-2 h-1, compared to -6.93 mg m-2 h-1 for the lake. The carbon cycle in rivers was strongly influenced by land use within the basin; cultivated and construction land were the main contributors to organic carbon (OC) in the river (r = 0.66, p < 0.01) and the mineralization of OC was a major factor in CO2 oversaturation in most rivers (r = 0.76, p < 0.01). In addition, the BCP effect of aquatic plants and the high pH in karst river-lake systems enhance the ability of water body to absorb CO2, resulting in undersaturated CO2 levels in the lake. Notably, under rainfall regulation, riverine OC and dissolved inorganic carbon (DIC) flux inputs controlled the level of CO2 exchange fluxes in the lake (rOC = 0.78, p < 0.05; rDIC = 0.97, p < 0.01). We speculate that under future climate and human activity scenarios, the DIC and OC input from rivers may alleviate the CO2 limitation of BCP effects in karst eutrophication lakes, possibly enabling aquatic plants to convert more CO2 into OC for burial. The results of this research can help advance our understanding of CO2 emissions and absorption mechanisms in karst river-lake systems.

Strong associations between dissolved organic matter and microbial communities in the sediments of Qinghai-Tibetan Plateau lakes depend on salinity

In aquatic ecosystems, dissolved organic matter (DOM) plays a vital role in microbial communities and the biogeochemical cycling of elements. However, little is known about the associations between DOM and microbial communities in lake sediments. This study investigated the composition of water-extractable organic matter and microbial communities in surface sediments of lakes with different salinities on the Qinghai-Tibet Plateau. Ultrahigh-resolution mass spectrometry and high-throughput microbial sequencing techniques were employed to assess the associations between molecular diversity and microbial diversity and the effects of salinity in 19 lakes spanning a salinity range from 0.22 ‰ to 341.87 ‰. Our results show that increasing salinity of lake water led to higher molecular diversity of DOM in surface sediments. High-salinity lakes exhibited distinct DOM characteristics, such as lower aromaticity, smaller molecular weight, and higher oxidation degree, compared to freshwater lakes. The complexity of the microbial network composition of sediments first increased and then decreased with the increase of salinity. Moreover, as salinity increases, the dominant species transitioned from Gammaproteobacteria to Bacteroidia, and this transition was accompanied by a decrease in microbial diversity and an increase in molecular diversity. Microbial factors accounted for 34.68 % of the variation in the molecular composition of DOM. Overall, this study emphasizes the significant effects of salinity on both molecular and microbial diversity in lake sediments. Furthermore, our findings underscore the importance of microbes in controlling the range of organic compounds present in lakes and deepen our knowledge of the biogeochemical cycling of DOM.

Rainstorm and strong wind weathers largely increase greenhouse gases flux in shallow ponds

Shallow-water ponds represent the hotspots of greenhouse gas (GHG) emissions. Most current studies focus on the temporal dynamics for GHGs in water, with little consideration given to the effects of weather changes. In this study, we measured and compared the concentrations and fluxes of CO2, CH4, and N2O from a pond in Northeast China under different meteorological conditions. Results showed that the rates of CO2, CH4, and N2O emissions from pond into the atmosphere during strong winds were 85.85 ± 7.55 mmol m-2 d-1, 22.05 ± 6.80 mmol m-2 d-1, and 10.87 ± 0.72 μmol m-2 d-1, respectively, significantly higher than those measured during non-rain weather. Among which, over 88 % of CH4 emissions were contributed by ebullition. Meanwhile, the CO2 and N2O flux were also significantly higher during heavy rainfall, reaching 100.05 ± 19.76 mmol m-2 d-1 and 5.90 ± 1.03 μmol m-2 d-1, respectively. Strong winds and precipitation induced sediment disturbances, high gas transport coefficients, reduced photosynthesis and oxygen greatly promoted the GHGs escape evasion. Wind speed, air pressure, solar radiation, and dissolved oxygen in water were important influencing factors. Our results emphasize the importance of capturing short-term weather disturbance events, especially rainstorm and strong winds, to accurately assess the annual GHG budget from these shallow water ecosystems.

High arsenic pollution of the eutrophic Lake Taihu and its relationship with iron, manganese, and dissolved organic matter: High-resolution synchronous analysis

Arsenic (As) is a metalloid that can accumulate in eutrophic lakes and cause adverse health effects to people worldwide. However, the seasonal process and dynamic mechanism for As mobilization in eutrophic lake remains effectively unknown. Here we innovatively used the planar optodes (PO), high-resolution dialysis (HR-Peeper) combined with fluorescence excitation-emission matrix coupled with parallel factor (EEM-PARAFAC) analysis technologies. We synchronously investigate monthly O2, As, iron (Fe), manganese (Mn), and naturally occurring dissolved organic matter (DOM) changes in sediments of Lake Taihu at high resolution in field conditions. We find high As contamination from sediments with 61.88-327.07 μg m-2 d-1 release As fluxes during the algal bloom seasons from May to October 2021. Our results show that an increase in DOM, mainly for humic-like components, resulting in high electron transfer capacity (ETC), promoted the reductive dissolution of Fe and Mn oxides to release As. Partial least square-path modeling (PLS-PM) and random forest modeling analysis identified that Mn oxide reductive dissolution directly accelerated sediments As contamination, which is the crucial factor. Understanding crucial factor controlling As release is especially essential in areas of eutrophic lakes developing effective strategies to manage As-rich eutrophic lake sediments worldwide.

Labile dissolved organic matter (DOM) and nitrogen inputs modified greenhouse gas dynamics: A source-to-estuary study of the Yangtze River

Although rivers are increasingly recognized as essential sources of greenhouse gases (GHG) to the atmosphere, few systematic efforts have been made to reveal the drivers of spatiotemporal variations of dissolved GHG (dGHG) in large rivers under increasing anthropogenic stress and intensified hydrological cycling. Here, through a source-to-estuary survey of the Yangtze River in March (spring) and October (autumn) of 2018, we revealed that labile dissolved organic matter (DOM) and nitrogen inputs remarkably modified the spatiotemporal distribution of dGHG. The average partial pressure of CO2 (pCO2), CH4 and N2O concentrations of all sampling sites in the Yangtze River were 1015 ± 225 μatm, and 87.5± 36.5 nmol L-1, and 20.3 ± 6.6 nmol L-1, respectively, significantly lower than the global average. In terms of longitudinal and seasonal variations, higher GHG concentrations were observed in the middle-lower reach in spring. The dominant drivers of spatiotemporal variations in dGHG were labile, protein-like DOM components and nitrogen level. Compared with the historical data of dGHG from published literature, we found a significant increase in N2O concentrations in the Yangtze River during 2004-2018, and the increasing trend was consistent with the rising riverine nitrogen concentrations. Our study emphasized the critical roles of labile DOM and nitrogen inputs in driving the spatial hotspots, seasonal variations and annual trends of dGHG. These findings can contribute to constraining the global GHG budget estimations and controls of GHG emission in large rivers in response to global change.

Quantitative modelling reservoir microalgae proliferation in response to water-soluble anions and cations influx

Atmospheric precipitation deposits acid-forming substances into surface water. However, the effects of water-soluble components on microalgae proliferation are poorly understood. This study analysed the growth characteristics of three microalgae bioindicators of water quality: Scenedesmus quadricauda, Chlorella vulgaris, and Scenedesmus obliquus, adopting on-site monitoring, culture experiments simulating 96 types of water by supplementing anions and cations, and predictive modelling. The result quantified pH > 3.0 rain with dominant Ca2+, Mg2+, and K+ cations, together with anions of NO3- and SO42-. The presence of Ca2+ of up to 0.1 mM and Mg2+ concentrations (>0.5 mM) suppressed Scenedesmus quadricauda growth. Soluble ions, luminosity, and pH had significant impacts (p ≤ 0.01) on increased microalgae proliferation. A newly proposed microalgae growth model predicted a 10.7-fold increase in cell density six days post-incubation in the case of rainfall. The modelling supports algal outbreaks and delays prediction during regional water cycles.

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.

Spatial gradients and molecular transformations of DOM, DON and DOS in human-impacted estuarine sediments

Dissolved organic matter (DOM) constitutes the most active fraction in global carbon pools, with estuarine sediments serving as significant repositories, where DOM is susceptible to dynamic transformations. Anthropogenic nitrogen (N) and sulfur (S) inputs further complicate DOM by creating N-bearing DOM (DON) and S-bearing DOM (DOS). This study delves into the spatial gradients and transformation mechanisms of DOM, DON, and DOS in Pearl River Estuary (PRE) sediments, China, using combined techniques of UV-visible spectroscopy, Excitation-emission matrix (EEM) fluorescence spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and microbial high-throughput sequencing. Results uncovered a distinct spatial gradient in DOM concentration, aromaticity (SUVA254), hydrophobicity (SUVA260), the content of substituent groups including carboxyl, carbonyl, hydroxyl and ester groups (A253/A203) of chromophoric DOM (CDOM), and the abundances of tyrosine/tryptophan-like protein and humic-like substances in fluorophoric DOM (FDOM). These all decreased from upper to lower PRE, accompanied by a decrease in O3S and O5S components, indicating seaward reduction in the contribution of terrestrial OM, especially anthropogenic inputs. Additionally, sediments exhibited a reduction in molecular diversity (number of formulas) of DOM, DON, and DOS from upper to lower PRE, with molecules tending towards a lower nominal oxidation state of carbon (NOSC) and higher bio-reactivity (MLBL), molecular weight (m/z) and saturation (H/C). While molecular composition of DOM remained similar in PRE sediments, the relative abundance of lignin-like substances decreased, with a concurrent increase in protein-like and lipid-like substances in DON and DOS from upper to lower PRE. Mechanistic analysis identified the joint influence of terrestrial OM, anthropogenic N/S inputs, and microbial processes in shaping the spatial gradients of DOM, DON, and DOS in PRE estuarine sediments. This study contributes valuable insights into the intricate spatial gradients and transformations of DOM, DON, and DOS within human-impacted estuarine sediments.

Terrestrial dissolved organic matter inputs accompanied by dissolved oxygen depletion and declining pH exacerbate CO2 emissions from a major Chinese reservoir

Terrestrial inputs and subsequent degradation of dissolved organic matter (DOM) in lake ecosystems can result in rapid depletion of dissolved oxygen (DO). Inputs of terrestrial DOM including organic acids can also lead to decreases in pH. However, to date, few studies have investigated the linkages between terrestrial DOM inputs, DO and pH levels in the water column, and carbon dioxide (CO2) emissions from lake ecosystems. Based on monthly field sampling campaigns across 100 sites in Lake Qiandao, a major man-made drinking water reservoir in China, from May 2020 to April 2021, we estimated an annual CO2 efflux (FCO2) of 37.2 ± 29.0 gC m-2 yr-1, corresponding to 0.02 ± 0.02 TgC yr-1 from this lake. FCO2 increased significantly with decreasing DO, chlorophyll-a (Chl-a) and δ2H-H2O, while FCO2 increased with increasing specific UV absorbance (SUVA254) and a terrestrial humic-like component (C2). We found that DO concentration and pH declined with increasing terrestrial DOM inputs, i.e. increased SUVA254 and terrestrial humic-like C2 levels. Vertical profile sampling revealed that the partial pressure of CO2 (pCO2) increased with increasing terrestrial DOM fluorescence (FDOM), while DO, pH, and δ13C-CO2 declined with increasing terrestrial FDOM. These results highlight the importance of terrestrial DOM inputs in altering physico-chemical environments and fueling CO2 emissions from this lake and potentially other aquatic ecosystems.

Divergent Fate and Roles of Dissolved Organic Matter from Spatially Varied Grassland Soils in China During Long-Term Biogeochemical Processes

Terrestrial dissolved organic matter (DOM) is critical to global carbon and nutrient cycling, climate change, and human health. However, how the spatial and compositional differences of soil DOM affect its dynamics and fate in water during the carbon cycle is largely unclear. Herein, the biodegradation of DOM from 14 spatially distributed grassland soils in China with diverse organic composition was investigated by 165 days of incubation experiments. The results showed that although the high humified fraction (high-HS) regions were featured by high humic-like fractions of 4-25 kDa molecular weight, especially the abundant condensed aromatics and tannins, they unexpectedly displayed greater DOM degradation during 45-165 days. In contrast, the unique proteinaceous and 25-100 kDa fractions enriched in the low humified fraction (low-HS) regions were drastically depleted and improved the decay of bulk DOM but only during 0-45 days. Together, DOM from the high-HS regions would cause lower CO2 outgassing to the atmosphere but higher organic loads for drinking water production in the short term than that from the low-HS regions. However, this would be reversed for the two regions during the long-term transformation processes. These findings highlight the importance of spatial and temporal variability of DOM biogeochemistry to mitigate the negative impacts of grassland soil DOM on climate, waters, and humans.

Intense methane diffusive emissions in eutrophic urban lakes, Central China

Urban lakes are hotspots of methane (CH4) emissions. Yet, actual field measurements of CH4 in these lakes are rather limited and our understanding of CH4 response to urban lake eutrophication is still incomplete. In this study, we measured dissolved CH4 concentrations and quantified CH4 diffusion from four urban lakes in subtropical China during wet and dry seasons. We found that these lakes were constantly CH4-saturated, contributing the greenhouse gas (GHG) to the atmosphere. Nutrient enrichment significantly increased CH4 concentrations and diffusive fluxes. Average CH4 flux rate in the highly-eutrophic lake zones (4.18 ± 7.68 mmol m-2 d-1) was significantly higher than those in the mesotrophic (0.19 ± 0.18 mmol m-2 d-1) and lightly/moderately-eutrophic zones (0.72 ± 2.22 mmol m-2 d-1). Seasonally, CH4 concentrations and fluxes were significantly higher in the wet season than in the dry season in the mesotrophic and the lightly/moderately-eutrophic lake zones, but an inverse pattern existed in the highly-eutrophic lake zones. CH4 concentrations and fluxes increased with elevated levels of nitrogen, phosphorus and dissolved organic carbon (DOC). The accumulation of nutrients provided autochthonous substrate for methanogenesis, indicated by a negative correlation between CH4 and the C:N ratio. Ammonium-nitrogen (NH4+-N) was the best predictor for spatial fluctuation of CH4 concentrations and diffusive fluxes in the mesotrophic and the lightly/moderately-eutrophic lake zones, while total nitrogen (TN) and total phosphorus (TP) levels showed the highest predictability in the highly-eutrophic lake zones. Based on the findings, we conclude that nutrient enrichment in urban lakes can largely increase CH4 diffusion, and that urban sewage inflow is a key concern for eutrophication boosting CH4 production and diffusive emission. Furthermore, our study reveals that small urban lakes may be an important missing source of GHG emissions in the global C accounting, and that the ratio of littoral-to-pelagic zones can be important for predicting lake-scale estimation of CH4 emission.

Persistent response of the bottom free-living bacteria to typhoon events in a subtropical reservoir

Typhoon-induced perturbations can result in long-lasting effects on aquatic communities in subtropical lakes or reservoirs. However, the responses of bacterial communities and their related nutrient cycling to episodic typhoon events throughout the water column in deep waters remain largely unknown. Here, we conducted a four-year field study to reveal the depth-specific responses of both free-living (FL) and particle-attached (PA) bacteria to typhoon events in a subtropical deep reservoir from 2015 to 2018. By comparing the depth-specific responses of FL and PA bacteria, we found that typhoon-induced inputs of organic matter and microorganisms significantly increased FL bacterial diversity and changed FL bacterial community composition in bottom waters perhaps through the density current or undercurrent. Typhoon events had a more persistent effect on FL than PA bacterial communities, especially in bottom waters of the reservoir. Free-living bacteria were more associated with nutrient cycling in bottom waters than particle-attached bacteria. These findings provide deep understanding of how FL and PA bacteria respond to typhoon events at community level in subtropical deep reservoir and thus help us to improve reservoir management in a rapidly changing world.

The joint effects of atmospheric dry and wet deposition on organic carbon cycling in a mariculture area in North China

In this research, the atmospheric dry and wet deposition fluxes of particulate organic carbon (POC) over the coastal waters around the Yangma Island in North Yellow Sea were investigated. Combining the results of this research and previous reports about the wet deposition fluxes of dissolved organic carbon (DOC) in precipitation (F_DOC-wet) and dry deposition fluxes of water-dissolvable organic carbon in atmospheric total suspended particles (F_DOC-dry) in this area, a synthetic assessment of the influence of atmospheric deposition on the eco-environment was conducted. It was found that the annual dry deposition flux of POC was 1097.9 mg C m^-2 a^-1, which was approximately 4.1 times that of F_DOC-dry (266.2 mg C m^-2 a^-1). For wet deposition, the annual flux of POC was 445.4 mg C m^-2 a^-1, accounting for 46.7 % that of F_DOC-wet (954.3 mg C m^-2 a^-1). Therefore, atmospheric POC was mainly deposited through dry process with the contribution of 71.1 %, which was contrary to the deposition of DOC. Considering the indirect input of organic carbon (OC) from atmospheric deposition, that is, the new productivity supported by nutrient input from dry and wet deposition, the total OC input from atmospheric deposition to the study area could be up to 12.0 g C m^-2 a^-1, highlighting the important role of atmospheric deposition in the carbon cycling of coastal ecosystems. The contribution of direct and indirect input of OC through atmospheric deposition to the dissolved oxygen consumption in total seawater column was assessed to be lower than 5.2 % in summer, suggesting a relatively smaller contribution to the deoxygenation in summer in this region.

Changes of dissolved organic matter following salinity invasion in different seasons in a nitrogen rich tidal reach

Dissolved organic matter (DOM) is a heterogeneous mixture of dissolved material found ubiquitously in aquatic systems and dissolved organic nitrogen is one of its most important components. We hypothesised nitrogen species and salinity intrusions affect the DOM changes. Here, using the nitrogen rich Minjiang River as an easily accessible natural laboratory 3 field surveys with 9 sampling sites (S1-S9) were conducted in November 2018, April and August 2019. The excitation emission matrices (EEMs) of DOM were explored with parallel factor (PARAFAC) and cosine-histogram similarity analysis. Four indices including fluorescence index (FI), biological index (BIX), humification index (HIX) and the fluorescent DOM (FDOM) were calculated and the impact of physicochemical properties was assessed. The results suggested that the highest salinities of 6.15, 2.98 and 10.10, during each campaign corresponded to DTN concentrations of 119.29-240.71, 149.12-262.42 and 88.27-155.29 μmol·L^-1, respectively. PARAFAC analysis revealed the presence of tyrosine-like proteins (C1), tryptophan-like proteins or a combination of the peak N and tryptophan-like fluorophore (C2) and the humic-like material (C3). The EEMs in the upstream reach (i.e. S1-S3) were complex with larger spectra ranges, higher intensities and similar similarity. Subsequently, the fluorescence intensity of three components decreased significantly with low similarity of EEMs (i.e. S4-S7). At the downstream, the fluorescence levels dispersed significantly and no obvious peaks were seen except in August. In addition, FI and HIX increased, while BIX and FDOM decreased from upstream to downstream. The salinity positively correlated with FI and HIX, and negatively related to BIX and FDOM. Besides, the elevated DTN had a significant effect on the DOM fluorescence indices. Altogether, salinity intrusion and elevated nitrogen are relevant for the distribution of the DOM, which is helpful for the water management tracing the DOM source according to the on-line monitoring of salinity and nitrogen in estuaries.

Variation of glomalin-metal binding capacity in 1 m soil profiles from mangrove forests to mudflat and affected factor analysis

Glomalin-related soil protein (GRSP) plays an important role in soil metal sequestration in coastal wetlands. Additionally, it can release dissolved organic matter (GDOM) in water-soaked condition. The purpose of this study was to clarify the variation of GRSP's heavy metal immobilisation capacity at soil profiles of coastal wetland, and explore the compositional characteristics of GDOM and its influence on the heavy metals' environmental behaviour. The results indicated that the metal immobilisation capacity of GRSP decreased with increasing burial depth. The contributions of GRSP to soil Cr, As, and Pb were higher in both mangrove soils (K. obovata and A. marina forests) than in the mudflat. Oxygen-containing functional groups of GRSP (CO, -COO^-, etc.) played a positive role in heavy metals accumulation. Redundancy analysis (RDA) showed that high soil pH was not conducive to the enrichment of heavy metals by GRSP. Besides, the concentrations of GRSP-Fe showed a significant positive correlation with the concentrations of other metals (Cu, As, and Pb) in GRSP. It is speculated that the Fe minerals in GRSP contributed the enrichment of heavy metals. Based on PARAFAC modelling, four fluorescent components of GDOM were identified, including three humic-like fluorescent components and one tyrosine-like fluorescent component. The contributions of GDOM to GRSP-bound heavy metals fluctuated between 4.05 % and 88.80 %, which could enhance the fluidity of heavy metals in water and weaken the soil heavy metal immobilisation capacity of GRSP. High salinity exerted an inhibitory effect on the heavy metal content of the GDOM. This study comprehensively explored the potential of GRSP to immobilise heavy metals in wetland soils and highlighted the potential heavy metal risks associated with the GDOM component in water, which could contribute to the multidimensional assessment and control of heavy metal pollution in coastal wetlands.

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.