Water Dissolved Organic Carbon Research Articles

Controls on the Respiration of Ancient Carbon Draining From Permafrost Soils Into Sunlit Arctic Surface Waters

AbstractThe thawing of ancient organic carbon stored in arctic permafrost soils, and its oxidation to carbon dioxide (CO2, a greenhouse gas), is predicted to amplify global warming. However, the extent to which organic carbon in thawing permafrost soils will be released as CO2 is uncertain. A critical unknown is the extent to which dissolved organic carbon (DOC) from thawing permafrost soils is respired to CO2 by microbes upon export of freshly thawed DOC to both dark bottom waters and sunlit surface waters. In this study, we quantified the radiocarbon age and 13C composition of CO2 produced by microbial respiration of DOC that was leached from permafrost soils and either kept in the dark or exposed to ultraviolet and visible wavelengths of light. We show that permafrost DOC most labile to microbial respiration was as old or older (ages 4,000–11,000 a BP) and more 13C‐depleted than the bulk DOC in both dark and light‐exposed treatments, likely indicating respiration of old, 13C‐depleted lignin and lipid fractions of the permafrost DOC pool. Light exposure either increased, decreased, or had no effect on the magnitude of microbial respiration of old permafrost DOC relative to respiration in the dark, depending on both the extent of DOC oxidation during exposure to light and the wavelength of light. Together, these findings suggest that photochemical changes affecting the lability of permafrost DOC during sunlight exposure are an important control on the magnitude of microbial respiration of permafrost DOC in arctic surface waters.

Bioavailability and remineralization rates of sediment-derived dissolved organic carbon from a Baltic Sea depositional area

This pilot study investigated the bioavailability and remineralization kinetics of the sediment-derived dissolved organic carbon (DOC) from the Gdańsk Deep, a depositional area in the Baltic Sea. This was assessed in the long-lasting (126 d) incubation experiment, in which the mixture of DOC from sediment pore water and bottom water was exposed to oxic microbial respiration with incubation of bottom water as a control run. The obtained decay curves allowed us to distinguish three DOC fractions: labile (DOCL), semi-labile (DOCSL), and refractory (DOCR). In bottom water, the refractory fraction was predominant and amounted to almost 85% of total DOC, whereas about 15% of DOC was bioavailable: 6% labile and 9% semi-labile. In contrast, DOC from pore water was much more bioavailable DOC (~55% of total DOC) and contained 11% DOCL and 44% DOCSL. The remineralization rate constants recalculated to the in situ temperature of 6°C for labile and semi-labile DOC in pore water were 0.025 d−1 and 0.002 d−1, respectively, whereas, in bottom water, 0.026 d−1 and 0.004 d−1. The half-life times for DOCL were comparable for both bottom water and pore water and amounted to 26.2 d and 27.6 d, respectively. For DOCSL, the half-life time was shorter for bottom water (165.5 d) than for pore water (322.9 d).

A global database of dissolved organic matter (DOM) concentration measurements in coastal waters (CoastDOM v1)

Abstract. Measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) concentrations are used to characterize the dissolved organic matter (DOM) pool and are important components of biogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOM v1; available at https://doi.org/10.1594/PANGAEA.964012, Lønborg et al., 2023) compiling previously published and unpublished measurements of DOC, DON, and DOP in coastal waters. These data are complemented by hydrographic data such as temperature and salinity and, to the extent possible, other biogeochemical variables (e.g. chlorophyll a, inorganic nutrients) and the inorganic carbon system (e.g. dissolved inorganic carbon and total alkalinity). Overall, CoastDOM v1 includes observations of concentrations from all continents. However, most data were collected in the Northern Hemisphere, with a clear gap in DOM measurements from the Southern Hemisphere. The data included were collected from 1978 to 2022 and consist of 62 338 data points for DOC, 20 356 for DON, and 13 533 for DOP. The number of measurements decreases progressively in the sequence DOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focus on carbon cycling by the aquatic science community. The global database shows that the average DOC concentration in coastal waters (average ± standard deviation (SD): 182±314 µmol C L−1; median: 103 µmol C L−1) is 13-fold higher than the average coastal DON concentration (13.6±30.4 µmol N L−1; median: 8.0 µmol N L−1), which is itself 39-fold higher than the average coastal DOP concentration (0.34±1.11 µmol P L−1; median: 0.18 µmol P L−1). This dataset will be useful for identifying global spatial and temporal patterns in DOM and will help facilitate the reuse of DOC, DON, and DOP data in studies aimed at better characterizing local biogeochemical processes; closing nutrient budgets; estimating carbon, nitrogen, and phosphorous pools; and establishing a baseline for modelling future changes in coastal waters.

Interactions of ferrate(VI) and aquatic humic substances in water treatment

Aquatic humic substances, encompassing humic acid (HA) and fulvic acid (FA), can influence the treatment of ferrate(VI), an emerging water treatment agent, by scavenging Fe(VI) to accelerate its decomposition and hinder the elimination of target micro-pollutants. Meanwhile, HA and FA degrade the water quality through the transformation to disinfection byproducts over disinfection, contribution to water color, and enhanced mobility of toxic metals. However, the interplay with ferrate(VI) and humic substances is not well understood. This study aims to elucidate the interactions of ferrate(VI) with HA and FA for harnessing ferrate(VI) in water treatment. Laboratory investigations revealed distinctive biphasic kinetic profiles of ferrate(VI) decomposition in the presence of HA or FA, involving a 2nd order kinetic reaction followed by a 1st-order kinetic reaction. Both self-decay and reactions with the humic substances governed the ferrate(VI) decomposition in the initial phase. With increasing dissolved organic carbon (DOC), the contribution of self-decomposition to ferrate(VI) decay declined, while humic substance-induced ferrate(VI) consumption increased. To assess relative contributions of the two factors, DOC50% was first introduced to represent the level at which the two factors equally contribute to the ferrate(VI) loss. Notably, DOC50% (11.90 mg/L for HA and 13.10 mg/L for FA) exceeded typical DOC in raw water, implying that self-decay predominantly governs ferrate(VI) consumption. Meanwhile, ferrate(VI) could degrade and remove HA and FA across different molecular weight (MW) ranges, exhibiting treatment capabilities that are either better or, at least, equivalent to ozone. The ferrate(VI) treatment attacked high MW, hydrophobic organic molecules, accompanied by the production of low MW, more hydrophilic compounds. Particularly, FA was more effectively removed due to its smaller molecular sizes, higher solubility, and lower carbon contents. This study provides valuable insights into the effective utilization of ferrate(VI) in water treatment in presence of humic substances.

Comparison of Fe‐based and Al‐based coagulants in the removal of organic and disinfection by‐product precursors

AbstractSurface water is renowned for its natural organic matter, constituting approximately 45% of total dissolved organic carbon (DOC) which can be removed in water treatment plants. However, residual DOC in water can react with chlorine to form several carcinogenic disinfectant by‐products (DBPs). This study aimed to examine the molecular weight of organic fractions dissolved in three different water sources that act as precursors to the formation of DBPs species. The coagulants used were Al‐ and Fe‐based, frequently used in water treatment plants to remove organic fractions. Characterization of DOC in source water served as the first step in determining the performance of both coagulants in terms of organic properties. The results showed that the selected surface waters had similar DOC characteristics, including biopolymers, humic substances, building blocks, and a low molecular weight. These fractions contributed to the formation of trihalomethanes (THMs) and haloacetic acids (HAAs). The Fe‐based coagulant was more effective than the Al‐based coagulant in removing all organic fractions and reducing THMs compared to HAAs. Furthermore, one‐way ANOVA analysis showed a significant difference in the average removal of organic fractions and DBP species between the Fe‐based and Al‐based coagulants. The Fe‐based coagulant showed higher efficiency in removing biopolymers, dibromochloromethane, and chlorodibromoacetic acid than the Al‐based coagulant. In contrast, the Al‐based coagulant had better performance in reducing dibromo HAA and tribromo HAA. Both coagulants had no significant difference in extracting other organic fractions or DBPs species.

Response of soil water, salt, carbon and bacteria community to terrain and plant in coastal salt marsh

Terrain and plants play a crucial role in influencing or regulating the variations of water, salt, carbon, and bacteria community in the soil of coastal salt marshes. Investigating these variations and their interconnections under different conditions is essential to comprehensively understand the carbon sequestration function and reveal the underlying mechanisms of carbon sinks in coastal salt marshes. In this study, the various physicochemical properties of rhizosphere and non-rhizosphere soil under different terrain of coastal salt marsh in Jiaozhou Bay were determined. Additionally, the contents of soil carbon components including soil inorganic carbon (SIC), soil organic carbon (SOC), dissolved inorganic carbon (DIC), and dissolved organic carbon (DOC), and constituents of soil dissolved organic matter (DOM) and microbial community were analyzed. The results showed that the contents of SIC and SOC in high-terrain soil were significantly lower than those in low-terrain soil (P < 0.05). Plants could regulate the physicochemical properties, and carbon components of soil in different terrain, resulting in no significant difference in SOC contents of rhizosphere soil under different terrain. Soil DOM composition and bacterial community showed that high-terrain soil had higher bacterial diversity and activity, and plants significantly affected the bacterial community structure in soil through rhizosphere effects. The structural equation model demonstrated that terrain has an indirect impact on SOC through influencing on the fluorescence of soil DOM and bacterial diversity, and SIC through altering soil water content (SWC), electrical conductivity and DOC. Plants directly or indirectly affected SOC content, and indirectly affected SIC content by changing SWC. This study contributes to a better understanding of the complex interactions between wetland ecosystems and their soil water, salt, carbon, and bacteria community components, offering valuable information for wetland management and conservation efforts.

Investigating the influence of integrated landscape connectivity on water dissolved organic carbon variations in farm pond catchments of subtropical southern China

The landscape connectivity in farm pond catchments leads to complex hydrologic and biogeochemical processes, which is important to the carbon cycle of the watershed. However, little attention has been paid to analyzing the influence of landscape connectivity on water dissolved organic carbon (DOC) content in farm pond catchments. In this study, we developed an integrated landscape connectivity framework that encompasses landscape connectivity metrics, graph theory connectivity indices, and spatial distance. And the graph theory connectivity indices were constructed based on the modified resistance surface using a function that integrated landscape development intensity, slope, and livestock density. Using DOC content data from 86 farm ponds monitored between June 2017 and May 2018. The results demonstrated that the average water DOC content in farm ponds was higher during the dry season (5.35 ± 2.03 mg/L) compared to the wet season (4.27 ± 1.40 mg/L). The individual effects of landscape connectivity metrics and graph theory connectivity indices on DOC content were more prominent during the dry season, while the spatial distance had a more significant influence in the wet season. The interactive effects of determinant variables had a more substantial influence on pond water DOC content than their individual effects during both wet and dry seasons. The interactive effect variation ratio (IEVR) of all variables was positive in both seasons (0.143–0.777). Combining landscape connectivity metrics, graph theory connectivity indices, and spatial distance (22.0–24.3%) improved the interpretation of DOC content in comparison to using separate (0.4–12.1%) or paired variables (0.3–19.3%). Our findings suggest that integrated landscape connectivity is necessary to further understand the role of ponds in the organic carbon cycling of surface water. These strategies should not only focus on landscape connectivity metrics, but also consider the influences of graph theory connectivity indices and spatial distance in farm pond catchments.

Insight into the interaction of tetrabromobisphenol A with sediment-derived dissolved organic carbon in a multiphase system by direct immersion solid phase microextraction

Tetrabromobisphenol A (TBBPA), a ubiquitously used commercial brominated flame retardant (BFR), has been widely detected in aquatic environments, and has aroused much attention due to its potential adverse effects on aquatic organisms. However, current research on the environmental fate and transport of TBBPA in the sediment–dissolved organic carbon (DOC)–water polyphase system is lacking. In this study, the sorption behavior of TBBPA in a water–DOC–sediment system was investigated using the direct-immersion solid-phase microextraction (DI-SPME) method, and the free dissolved concentration (Cw-SPME) and DOC adsorption concentration (CDOC) of TBBPA in water were measured by applying this DI-SPME approach. In addition, the effects of pH, ionic strength, and soluble organic concentration on the adsorption of TBBPA in the multiphase system were evaluated. The adsorption kinetics experimental results show that the adsorption behavior of TBBPA on sediments conforms to a linear model, suggesting that it could be mainly absorbed by sediments. The solid–water partition coefficient (Kd) of TBBPA was artificially reduced 1.54 times using the traditional liquid–liquid extraction method because the sorption behavior of the DOC was ignored, which could be accurately corrected using the DI-SPME method. The logKd and logKOC of TBBPA in the multiphase system were 4.12 ± 0.25 and 6.48 ± 0.25, respectively. Finally, the interference experiment revealed that the sorption behavior of TBBPA was affected by the pH, ionic strength (calcium ion), and humic acid concentration, apart from the lead ion concentration itself.

The biotic ligand model as a promising tool to predict Cu toxicity in amazon blackwaters

The Rio Negro basin of Amazonia (Brazil) is a hotspot of fish biodiversity that is under threat from copper (Cu) pollution. The very ion-poor blackwaters have a high dissolved organic carbon (DOC) concentration. We investigated the Cu sensitivity of nine Amazonian fish species in their natural blackwaters (Rio Negro). The acute lethal concentration of Cu (96 h LC50) was determined at different dilutions of Rio Negro water (RNW) in ion-poor well water (IPW), ranging from 0 to 100%. The IPW was similar to RNW in pH and ionic composition but deficient in DOC, allowing this parameter to vary 20-fold from 0.4 to 8.3 mg/L in tests. The Biotic Ligand Model (BLM; Windward version 3.41.2.45) was used to model Cu speciation and toxicity over the range of tested water compositions, and to estimate lethal Cu accumulations on the gills (LA50). The modeling predicted a high relative abundance of Cu complexes with DOC in test waters. As these complexes became more abundant with increasing RNW content, a concomitant decrease in free Cu2+ was observed. In agreement with this modeling, acute Cu toxicity decreased (i.e. 96 h LC50 values increase) with increasing RNW content. The three most sensitive species (Hemigrammus rhodostomus, Carnegiella strigatta and Hyphessobrycon socolofi) were Characiformes, whereas Corydoras schwartzi (Siluriformes) and Apistogramma agassizii (Cichliformes) were the most tolerant. These sensitivity differences were reflected in the BLM-predicted lethal gill copper accumulation (LA50), which were generally lower in Characiformes than in Cichliformes. Using these newly estimated LA50 values in the BLM allowed for accurate prediction of acute Cu toxicity in the nine Amazonian fish. Our data emphasize that the BLM approach is a promising tool for assessing Cu risk to Amazonian fish species in blackwater conditions characterized by very low concentrations of major ions but high concentrations of DOC.

Top-down control over dissolved organic carbon in the bottom water of the Weddell Sea and its implication for the continental shelf pump

Dense water out of the Antarctic shelves is expected to drive the transport of carbon into the deep Southern Ocean via the formation of Antarctic Bottom Water. However, bottom water formation’s capacity to sequester carbon into the deep ocean is poorly constrained. Here, dissolved organic carbon (DOC), dissolved black carbon (DBC), and particulate organic carbon (POC) were examined to reveal the influence of the Weddell Sea Deep Water (WSDW) on DOC transport during its flowing out of the Weddell Sea. High DOC concentrations (>60 μM-C) and low DBC/DOC ratios (<1.5%) were observed in surface water near the South Orkney Islands, ascribing to sea ice melt-induced phytoplankton blooms. Seawater at the mid-deep depths exhibited a higher DOC concentration (averaging 48.1 ± 3.7 μM-C) than the incoming water source, resulting from the release of DOC from sinking particles. Bottom water had higher DOC concentration compared to the mid-deep layer water (t-test, p < 0.005), while the DBC concentrations were comparable. In addition, the excess DOC (relative to WSDW) in bottom water showed a close relation with POC in surface water. These results reveal a top-down control over the DOC concentration in bottom water through a quick sinking of diatom detritus and subsequently solubilization in bottom water and/or sediment. With an estimate, the WSDW carries 5.1 ± 2.8 Tg-C/yr of excess DOC out of the Powell Basin, representing an important carbon source to the deep Southern Ocean. This study highlights the key role of the Antarctic continental shelf pump in carbon sequestration.

Mechanisms of 137Cs leaching based on long-term observations in forested headwater catchments in Yamakiya, Fukushima Prefecture, Japan

Dissolved radiocesium (mainly 134Cs and 137Cs) is thought to be leached mainly from suspended sediment in downstream rivers, while organic matter, such as leaf litter, contributes to catchments in forested headwater streams. It is also known that dissolved 137Cs in headwater streams exhibit seasonal variation with water temperature. Some mechanisms have been proposed as the causes: ionic competition of potassium ion (K+) and ammonium ion (NH4+), leaching associated with the decomposition of organic matter, and thermodynamic adsorption-desorption processes. We investigated the relationship between K+ and the dissolved organic carbon (DOC) concentrations and seasonal changes in dissolved 137Cs concentration using a large number of samples from a headwater's small catchments. We examined temporal trends in 137Cs concentrations in coarse organic matter, suspended sediments, and dissolved forms at four sites (one decontaminated site and three undecontaminated sites) in the Yamakiya area since 2011. The distribution coefficients (Kdcss and Kdorg) of dissolved 137Cs concentrations relative to suspended sediment and coarse organic matter 137Cs concentrations were calculated, and differences in temporal changes due to decontamination were investigated. In addition, we examined the relationship between water temperature and DOC, K+, NH4+ and 137Cs concentrations in the headwater catchments. The suspended sediment 137Cs concentrations at the decontaminated headwater site (IBO) decreased significantly after decontamination and remained low thereafter. In contrast, dissolved 137Cs concentrations decreased temporarily during the decontamination period, but returned to pre-decontamination levels. Almost no NH4+ has been detected in headwater streams in our catchments. In the SET and ISH watersheds, where the distance from the groundwater spring is short, a correlation was found between DOC concentration and dissolved 137Cs concentration. In contrast, in the IBO watershed, where the distance from the groundwater spring is long, temperature dependence and a good correlation between K+ and 137Cs were observed. Therefore, microbial decomposition of organic matter may have a significant effect on the seasonal variation of dissolved 137Cs in forested headwater streams at short distances from the spring, but the influence of competing ions is expected to increase gradually as the water flows downstream.

Effects of boundary hydraulics, dissolved oxygen, and dissolved organic carbon on growth and death dynamics of aerobic microbes in riverbed dune-induced hyporheic zones

Surface and groundwater interact in the hyporheic zone beneath and adjacent to rivers in the presence of a diverse microbial community. Heterotrophic bacteria mediate a range of environmentally important reactions, yet few studies have quantified bacterial growth and death dynamics in the hyporheic zone, and none have systematically analyzed their response to variations in hydraulic or chemical conditions. We used MODFLOW and SEAM3D to simulate hydraulics; dissolved oxygen (DO) and dissolved organic carbon (DOC) transport; and aerobic microbial metabolism, growth, and death in hyporheic zones induced by riverbed dunes. We ran simulations both with and without growth/death processes, and varied hydraulic parameters and DO/DOC boundary concentrations. Microbial biomass reached steady state (t = 3 days) in every simulation, at which time there was greater biomass and DOC biodegradation rates in the hyporheic flowcell (300 % and 85 % higher for the base case, respectively) when accounting for microbial growth dynamics. This occurred as microbial biomass tailored its spatial distribution to the availability of DO and DOC, demonstrating the importance of simulating growth/death processes. Biomass generally increased with hyporheic flow cell area as upwelling groundwater decreased. When varying surface water DO and DOC source concentrations relative to the base case, the greatest effect on biomass occurred when increasing DOC and decreasing DO. We determined minimum DO and DOC steady-state concentrations required for microbial growth, but the minimums were not absolute or related by stoichiometry. Increasing DOC created a smaller area of microbes with higher concentrations relative to the base case. Increasing DO slightly increased the area occupied by microbes while keeping the total biomass nearly constant. Overall, microbial growth and death dynamics depend on DO and DOC availability in the hyporheic zone, which is dependent on DOC/DO boundary concentrations and hyporheic flow paths, and in turn the hydraulic interaction between surface water and groundwater.

Seasonal variation characteristics and influencing factors of dissolved organic carbon of soil water in permafrost peatlands of the Great Hing'an Mountains in summer and autumn.

Dissolved organic carbon (DOC) plays a crucial role in the assessment of greenhouse gas emission and carbon balance in peatlands. However, limited research has been conducted on the seasonal variations and properties of soil water DOC content at different depths in the permafrost peatlands of the Great Hing'an Mountains. In this study, we analyzed the seasonal patterns of soil water DOC contents (surface, 10 cm, 20 cm, 30 cm, 40 cm, and permafrost layer) the permafrost peatlands of the Great Hing'an Mountains (Tuqiang Forestry Bureau), and investigated the influencing factors, such as electrical conductivity, dissolved oxygen, HCO3- concentration, pH value, oxidation-reduction potential, and CO2 content. The stability of DOC was assessed by using UV-Vis spectrum. There were significant seasonal dynamics of DOC content in soil water, with higher contents in autumn and lower content in summer, ranging from 55.7 to 188.1 mg·L-1. There were significant differences in DOC content among different soil depths, with the highest levels detected in the permafrost layer. The DOC content showed a significantly positive correlation with pH value and electrical conductivity, while showed a significantly negative correlation with redox potential, HCO3- concentration, and dissolved oxygen content. Additionally, there was a significantly positive correlation between DOC and CO2 contents. The dissolved CO2 content in soil water increased with soil depth, with the highest content observed in the permafrost layer. Results of spectral analysis showed higher aromaticity in autumn compared to summer, indicating greater stability of DOC during the autumn season. Our results clarified the seasonal variations of soil water DOC in permafrost peatlands of the Great Hing'an Mountains and could provide important data to understand the carbon cycling in the region.

Watershed scale modeling of Dissolved organic carbon export from variable source areas

Dissolved organic carbon (DOC) in surface water influences the global carbon cycle, ecosystem productivity, and water quality in potable water supply systems. Few physically based watershed models can simulate carbon cycling and predict DOC in surface waters under the influence of natural and anthropogenic drivers. In this work we transform the SWAT-Carbon (C) model to simulate DOC from variable source runoff areas in a humid forested watershed in the northeastern United States. Remotely sensed data were used to parameterize and simulate forest growth and evapotranspiration. The calibrated model accurately simulated streamflow and DOC flux at six sites across the watershed compared to measured data. The DOC predictions across sites showed model sensitivity to soil properties, particularly soil depth and available water capacity. The spatial distribution of DOC export across the watershed followed the pattern of surface runoff from variable source areas. Model sensitivity of DOC flux showed greater sensitivity to changes in precipitation than temperature. The overall good performance of the model makes it a valuable tool for watershed-scale modeling of DOC to understand the influence of climate and watershed management towards developing mitigation strategies. The methods presented in this study can be used in forested watersheds in regions where runoff from variable source areas is important for water quality predictions.

Impact of nanomaterials on microbial communities in the carbon sequestration processes of urban river ecosystems

Herein, zero-valent iron nanoadsorbents were synthesized by hydrothermal and coprecipitation methods. The structure and morphology of the materials were characterized through X-ray diffraction, energy-dispersive X-ray spectroscopy (EDS), and scanning electron microscopy with EDS. The adsorption performance of nanoscale Fe2O3 nanomaterials was evaluated. Additionally, adsorption experiments were conducted with varying amounts of nanomaterials and initial concentrations of dissolved organic carbon (DOC) in water. The results showed that as the initial concentration of the DOC in water increased, the efficiency of organic carbon removal gradually decreased. In particular, when the initial concentration of the DOC increased from 20 to 35 mg/L, the removal efficiency of the nanomaterials decreased from 85% to 55%. Furthermore, a considerable decrease in the removal efficiency was observed between 30 to 60 min. After 60 min, the efficiency of organic carbon removal was almost constant. The increase in the initial concentration of DOC in water resulted in a gradual decrease in the efficiency of its removal. The decrease in the organic carbon content in water lead to a scarcity of carbon sources for microbial reproduction, thereby reducing the microbial population considerably. This study provides a clear evidence of the pronounced carbon fixation effect of nanomaterials and their ability to control microbial population in aquatic ecosystems.

Controlling factors on patterns of dissolved organic carbon and volatile fatty acids in a submarine mud volcano offshore southwestern Taiwan

Dissolved organic carbon (DOC) and volatile fatty acids (VFAs) play key roles in the carbon cycling of marine sediment. Both microbially or thermally activated cracking of organic matter often produces high quantities of DOC and VFAs. To uncover the distribution pattern of DOC and VFAs in sediments under both impacts, a submarine mud volcano (SMV), was chosen to denote a model system that could witness how microbial activities react under the mixing of seawater and deeply-sourced fluids in a subsurface environment. We examined the concentration profiles of DOC and several VFAs (lactate, formate, acetate, propionate, and butyrate) in pore water, covering both sulfate reduction and methanogenesis zones, and further numerically modeled six porewater species (DOC, bromide, calcium, magnesium, ammonium, and total alkalinity) to quantify their fluxes from depth as well as the rates of in-situ microbial processes. Apparently, bulk DOC concentrations fluctuated with depths, probably primarily controlled by in situ microbial processes. Lactate was detectable in some samples, while propionate and butyrate were under detection limit. Acetate and formate concentrations were consistently and uniformly low throughout all biogeochemical zones, with a slightly increasing trend with depth at the center of the SMV, suggesting active utilization and turnover by the terminal steps of organic matter mineralization. The numerical modeling suggests that most DOC patterns were primarily influenced by in-situ organic matter degradation, while the impact of upward migrating fluid become more significant at center sites. The calculation of the Gibbs energy of metabolic redox reactions reveals that acetoclastic sulfate reduction yields the highest energy throughout sediment columns and may co-exist with methanogenesis below sulfate reduction zone. In contrast, acetoclastic methanogenesis yields higher energy within sulfate reduction zone than below that region, suggesting it is thermodynamically feasible to co-occur with sulfate reduction in dynamic SMV environments.

The hidden role of dissolved organic carbon in the biogeochemical cycle of carbon in modern redox-stratified lakes

Abstract. The dissolved organic carbon (DOC) reservoir plays a critical role in the C cycle of marine and freshwater environments because of its size and implication in many biogeochemical reactions. Although it is poorly constrained, its importance in ancient Earth's C cycles is also commonly invoked. Yet DOC is rarely quantified and characterized in modern stratified analogues. In this study, we investigated the DOC reservoirs of four redox-stratified alkaline crater lakes in Mexico. We analyzed the concentrations and isotopic compositions of DOC throughout the four water columns and compared them with existing data on dissolved inorganic and particulate organic C reservoirs (DIC and POC). The four lakes have high DOC concentrations with great variability between and within the lakes (averaging 2 ± 4 mM; 1 SD, n=28; i.e., from ∼ 15 to 160 times the amount of POC). The δ13CDOC signatures also span a broad range of values from −29.3 ‰ to −8.7 ‰ (with as much as 12.5 ‰ variation within a single lake). The prominent DOC peaks (up to 21 mM), together with their associated isotopic variability, are interpreted as reflecting oxygenic and/or anoxygenic primary productivity through the release of excess fixed carbon in three of the lakes (Alberca de los Espinos, La Preciosa, and Atexcac). By contrast, the variability of [DOC] and δ13CDOC in the case of Lake Alchichica is mainly explained by the partial degradation of organic matter and the accumulation of DOC in anoxic waters. The DOC records detailed metabolic functions such as active DIC-uptake and DIC-concentrating mechanisms, which cannot be inferred from DIC and POC analyses alone but which are critical to the understanding of carbon fluxes from the environment to the biomass. Extrapolating our results to the geological record, we suggest that anaerobic oxidation of DOC may have caused the very negative C isotope excursions in the Neoproterozoic. It is, however, unlikely that a large oceanic DOC reservoir could overweigh the entire oceanic DIC reservoir. This study demonstrates how the analysis of DOC in modern systems deepens our understanding of the C cycle in stratified environments and helps to set boundary conditions for the Earth's past oceans.

Fates of selected pharmaceuticals in a large recirculated mesocosm with a pond and bank filtration

Pharmaceuticals such as carbamazepine (CBZ), sulfamethoxazole (SMX) and diclofenac (DCF) are detected at a high frequency in the aquatic environment. The fates of these compounds in bank filtration (BF), a nature-based water treatment system, have been extensively studied, mainly in batch and laboratory column studies. This study, for the first time, investigated the fates of CBZ, SMX and DCF in a large recirculated mesocosm with a pond and subsequent BF. Changes in dissolved organic carbon (DOC) in the pond and bank filtrate were also observed. The averaged spiking concentration of CBZ, SMX and DCF was 1 μg/L at the pond influent and the hydraulic retention time of surface water in the pond was 15 days to reach at the bank. The infiltrated surface water travelled through two parallel subsurface layers and a combined effluent (from both layers) was collected (35 m from the bank) and recirculated as an influent of the pond. The redox conditions in both layers were significantly different (p < 0.05) and correlated with temperature (R2 = 0.91, p < 0.05). The results revealed that CBZ was persistent in the surface water and groundwater passage, SMX was persistent in surface water and removed entirely through BF within 50 days of operation. DCF was completely removed after infiltration and groundwater passage (within 2 m). The DOC in the surface water showed insignificant variations between the influent and the bank. A significant reduction of DOC was observed within the first 5 m after infiltration, and the reduction was associated with the removal of biopolymers. This work shows that the selected organic micropollutants in surface water were not affected by sunlight intensity, water chemistry and water depth. In addition, recirculation mesocosm BF confirms the potential environmental risk and predictive concentrations of organic micropollutants in the aquatic environment.

A high-throughput atomic emission analyzer for simultaneous field detection of dissolved inorganic and organic carbon in seawater and lake water

Dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) are two important indicators of global carbon cycle. However, there are no portable analyzers available to simultaneously accomplish high-throughput field detection of them in the same sample. Herein, a simple analyzer comprising a dual-mode reactor to accomplish both chemical vapor generation and headspace sampling, and a miniature point discharge optical emission spectrometer (μPD-OES) was developed for simultaneous and high-throughput detection of DIC and DOC in seawater and lake water. Phosphoric acid and persulfate were successively injected into sample solutions to convert DIC and DOC to CO2 under the conditions of magnetic stirring and UV irradiation, respectively. Subsequently, the generated CO2 was swept into the μPD-OES for quantitation of DIC and DOC via monitoring carbon atomic emission at 193.0 nm. Under optimal conditions, limits of detection for DIC and DOC (as C) were both 0.01 mg L−1 with relative standard deviations (n = 20) better than 5% and sample throughput of 80 samples per hour. Compared to conventional analyzers, the proposed instrument provides the advantages of high throughput, compactness, low energy consumption and eliminates expensive instruments. The accuracy of the system was validated by simultaneous determination of DIC and DOC in various water samples in laboratory and field environments.

Estimation of dissolved organic carbon from inland waters at a large scale using satellite data and machine learning methods

Dissolved Organic Carbon (DOC) in inland waters plays an essential role in the global carbon cycle and has significant public health effects. Machine learning (ML) together with remote sensing has emerged as a powerful and promising combination to quantify water quality parameters from space. However, inland water sample data for DOC is limited. Hence, little is known about the potential to quantify DOC content in inland waters, especially over large-scale areas. This study presents the first attempt to estimate DOC in inland waters over a large-scale area using satellite data and ML methods with the newly published open-source dataset AquaSat. Four ML approaches, namely Random Forest Regression (RFR), Support Vector Regression (SVR), Gaussian Process Regression (GPR), and a Multilayer Backpropagation Neural Network (MBPNN) were trained using more than 16 thousand samples across the continental United States matched with satellite data from Landsat 5, 7 and 8 missions. Satellite data from the Landsat missions were further extended with environmental data from the ERA5-Land product and used as input to train the ML algorithms. Our results show that including environmental data as inputs considerably improved the prediction of DOC for all ML algorithms, with GPR showing the most promising performance results with moderate estimation errors (RMSE: 4.08 mg/L). Permutation feature importance analysis showed that the wavelength range in the visible Green band (from Landsat) and the monthly average air temperature (from ERA5-Land) were the most important variables for the ML approaches. The results demonstrate the predictive strength of GPR and its useful feature to derive per pixel standard deviations for detailed analysis. Our results further highlight the important role of considering environmental processes to explain DOC variations over large scales. The application and performance of the GPR in mapping spatiotemporal variations of DOC in an entire water body were discussed by taking Lake Okeechobee (the 8th largest freshwater lake in the U.S.) as an illustrative example. While performance evaluation showed that DOC concentrations can be retrieved with adequate accuracy, algorithm development was challenged by the heterogenous nature of large-scale open source in situ data, issues related to atmospheric correction, and the low spatial and temporal resolution of the environmental predictors. This research demonstrates how open source, large-scale datasets like AquaSat in combination with ML and satellite remote sensing can make research toward large-scale estimation of inland water DOC more realistic while highlighting its remaining limitations and challenges.