Variability Of Dissolved Organic Carbon Research Articles

Modeling dissolved organic carbon export from water supply catchments in the northeastern United States.

Natural organic matter (NOM) in rivers is an important energy source to sustain aquatic ecosystem health. However, in surface water supply systems where chlorination is often used for disinfection, NOM is also a precursor for the carcinogenic and mutagenic disinfection byproducts such as trihalomethanes and haloacetic acids. Effective management of NOM in rivers to maintain both aquatic ecosystem functions and high-quality water supply requires better understanding of the NOM transport patterns. NOM is often operationally measured by dissolved organic carbon (DOC). Challenges of using DOC data for analysis on a catchment scale largely relate to the spatial and temporal variations in DOC, and low sampling frequency which fails to capture the multi-scale transport patterns. To help improve the understanding of DOC sources and transport, we analyzed its long-term patterns in six water supply catchments in the New York City Water Supply System using monitoring data and models. We tested six empirical models for DOC prediction including linear, nonlinear and time-series based model formulations. We found that generalized additive models (GAMs) produced the most robust results across catchments. Then, we applied the calibrated GAM to predict daily DOC concentrations to estimate fluxes and analyze for trends. Finally, we compared the relationships between temporal patterns in DOC and catchment features to investigate the regional differences, focusing on the catchment mechanistic processes associated with DOC by parsing out the hydrological signals. The results showed that hydrology plays a larger role on DOC temporal patterns in three catchments where the top 5% streamflow corresponded to nearly 50% of the annual DOC export, whereas nutrient associated production processes were more important in others. The study presents a robust approach for DOC prediction in streams and can inform targeted monitoring strategies for DOC management in water supply source waters.

Organic Matter Composition and Water Stoichiometry Are Main Drivers of Heterotrophic Nitrate Uptake in Mediterranean Headwater Streams

AbstractHeterotrophic bacteria can contribute to improve stream water quality by taking up nitrate (NO3−) from the water column, although microbial demand for this nutrient is usually lower than for other inorganic nitrogen (N) forms, such as ammonium. Heterotrophic NO3− uptake has been related to the availability of dissolved organic carbon (DOC) relative to nutrients (i.e., DOC: nutrients ratios). Yet, how dissolved organic matter (DOM) composition and specific microbial assemblages influence NO3− uptake remains poorly understood. We conducted laboratory incubations to investigate heterotrophic NO3− uptake kinetics in 9 Mediterranean freshwater ecosystems, primarily headwater streams, exhibiting wide variation in DOC:NO3 ratios (from 1.5 to 750). Moreover, we characterized DOM composition using spectroscopic indexes and its degradation via a reactivity continuum model approach. Microbial community composition and functioning were assessed by analyzing extracellular enzymatic activities and the potential abundance of N‐cycling genes. Our results revealed that NO3− uptake rates (kNO3) were positively related with DOC:NO3 ratios (r2 = 0.4) and to NO3:SRP ratios as well (r2 = 0.6). Furthermore, kNO3 was negatively correlated to the humification index (r2 = 0.7), suggesting that a higher proportion of humic‐like compounds slow down heterotrophic NO3− uptake. A partial least squares regression model (PLS) pinpointed that DOC and nutrient stoichiometry, DOM composition and reactivity, and microbial composition and activity collectively contributed to explain the variability in kNO3 observed across treatments. Our findings suggest that heterotrophic NO3− uptake may show significant responsiveness to shifts toward more labile DOM sources and nutrient imbalances induced by global change.

Has lake brownification ceased? Stabilization, re-browning, and other factors associated with dissolved organic matter trends in eastern Canadian lakes

The increase in dissolved organic carbon (DOC) concentrations in freshwater systems has received considerable attention due to its implications for drinking water treatment and numerous limnological processes. While past studies have documented the influence of recovery from acidification and climate change on long-term DOC trends, the emerging importance of these explanatory factors remains less understood. In addition, few studies have followed up on recent trends in sites that have undergone increases in DOC. Using a dataset from 1980 to 2020, we investigated interannual variations in DOC and dissolved organic nitrogen (DON) in 49 lakes across four eastern Canadian regions with a history of increases in DOC. We identified recent shifts in DOC patterns using LOESS smoothing and piecewise regression. We observed a stabilizing pattern or even a decrease (p < 0.001) in high acidification regions (Dorset and Nova Scotia), where increases in DOC were previously documented. At the low acid deposition region, IISD-Experimental Lakes Area, an increasing pattern in DOC stabilized in the early 2000s; however, DOC appears to be increasing again in recent years (p = 0.03). Our analysis identified precipitation and SO4 deposition as the primary explanatory variables for DOC patterns (explaining 56–71% of variance). However, because acid deposition has declined substantially, climate and local watershed factors are becoming increasingly influential, leading to the emergence of new DOC patterns. Long-term changes in DOC and DON were not always synchronous, as these were often correlated with different factors (e.g., DON with ammonium deposition). This resulted in observable shifts in DOC:DON ratios, indicative of changes in dissolved organic matter (DOM) composition. We underscore the importance of ongoing monitoring in diverse regions because of the changing nature of environmental variables and new emerging trends.

Using physicochemical properties to predict the impact of natural dissolved organic carbon on transepithelial potential in the freshwater rainbow trout (Oncorhynchus mykiss) at neutral and acidic pH.

Dissolved organic carbon (DOC) is a complex mixture of molecules that varies in composition based on origin as well as spatial and temporal factors. DOC is an important water quality parameter as it regulates many biological processes in freshwater systems, including the physiological function of the gills in fish. These effects are often beneficial, especially at low pH where DOCs mitigate ion loss and protect active ion uptake. DOCs of different compositions and quality have varied ionoregulatory effects. The molecular variability of DOCs can be characterized using optical and chemical indices, but how these indices relate to the physiological effects exerted by DOCs is not well understood. We tested the effects of five naturally sourced DOCs, at both pH 7 and pH 4, on transepithelial potential (TEP) (a diffusion potential between the blood plasma and the external water) in rainbow trout. The five chosen DOCs have been well characterized and span large differences in physicochemical characteristics. Each of the DOCs significantly influenced TEP, although in a unique manner or magnitude which was likely due to their physicochemical characteristics. These TEP responses were also a function of pH. With the goal of determining which physicochemical indices are predictive of changes in TEP, we evaluated correlations between various indices and TEP at pH 7 and pH 4. The indices included: specific absorbance coefficient at 340nm, molecular weight index, fluorescence index, octanol-water partition coefficient, molecular charge, proton binding index, % humic acid-like, % fulvic acid-like, and % protein-like components by parallel factor analysis on fluorescence data (PARAFAC). Our results demonstrate the novel finding that there are three particularly important indices that are predictors of changes in TEP across pHs in rainbow trout: specific absorbance coefficient at 340nm, octanol-water partition coefficient; and proton binding index.

Large variations of dissolved carbon occurred in small ponds within an agricultural watershed

Dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) significantly affect the aquatic carbon budget and ecosystem functions. Small ponds are abundant globally and widely distributed especially in agricultural watersheds, however, the variability of DOC and DIC, along with their driving factors, remains poorly understood, which likely hampers the understanding of carbon cycle of inland waters. The presented study was designed to fill the knowledge gap based on a detailed year-long field investigation via examining DOC and DIC concentrations across ponds with differing functionalities (e.g. sewage ponds, irrigation ponds, and natural ponds) of a typical agricultural watershed in eastern China. Our results found a pronounced impact of human activities on pond DOC and DIC, with higher DOC occurring in sewage ponds (10.84 ± 2.83 mg L−1) and irrigation ponds (9.09 ± 2.57 mg L−1) and peak DIC in irrigation ponds (20.36 ± 2.49 mg L−1) compared to that at natural ponds (DOC: 7.54 ± 2.55 mg L−1; DIC: 11.16 ± 3.85 mg L−1) with less human activity. The positive correlations between DOC/DIC and key environmental variables (e.g. nutrients and chlorophyll-a) further demonstrated that human activity can either directly increase the carbon concentrations via pollutant discharge, or indirectly increase DOC concentration via stimulating primary production. Meanwhile, field measurements found precipitation and temperature play roles in determining the carbon variability. Specifically, precipitation increased the DOC of these ponds via enhancing land-based carbon inputs, and decrease the DIC of irrigation ponds via diluting. Temperature can influence the carbon dynamics through increasing primary productivity and metabolism. Our study underscores the roles of human and natural influences in determining the large variations of DOC and DIC in small ponds, which should be considered to better understand the carbon dynamic variability of human-impacted small aquatic systems.

Litter input promoted dissolved organic carbon migration in karst soil

Litter input is crucial for enhancing carbon sequestration in karst ecosystems. While previous studies have linked litter input to carbon storage in karst soils, the unique conditions of southwest China’s karst regions, characterized by high precipitation and rapid groundwater dynamics, pose challenges to understanding dissolved organic carbon (DOC) migration under litter cover. This study investigated how different litter types from karst-adapted vegetation affect DOC migration and loss in karst soils, addressing the gaps in carbon sequestration evaluations during vegetation restoration. Artificial soil columns were used to monitor the natural rainfall over one hydrological year. The results indicated that litter cover increased the soil carbon dioxide (CO2) concentration and enhanced DOC leaching and migration, with the highest DOC loss observed in the tree litter treatment (2614.50 mg) and the lowest in the shrub litter treatment (1844.13 mg). These differences were attributed to the synergistic interaction between rainfall and litter characteristics, which accounted for 29.94 % of the variation in soil DOC leaching after litter input. Regression analyses indicated that DOC leaching under litter cover was mainly affected by the soil CO2 concentration, temperature, humidity, rainfall, and litter decomposition duration. In summary, litter cover significantly intensified DOC migration and loss in karst soils. This study provides valuable insights into vegetation restoration and reconstruction, particularly in karst regions.

Understanding the environmental drivers of summer dissolved carbon in lakes on the Qinghai-Tibetan Plateau

How environmental factors affecting dissolved carbon remains unclear in lakes on the Qinghai-Tibetan Plateau (QTP), which limits the understanding of the lake carbon cycle. In this study, 60 lakes on the QTP in summer were investigated to clarify the variation in dissolved carbon, estimate dissolved carbon storage, and reveal how environmental factors affect the variation in dissolved carbon. The average dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) contents of 60 lakes on the QTP in summer were 12.78 mg/L and 103.66 mg/L, respectively. Salinity and total nitrogen were the important drivers of variations in DIC and DOC, respectively. Lake salinity and temperature were reduced only when precipitation was >50 mm, thus affecting the variations in lake dissolved carbon. Importantly, the elevation and area of the lake also significantly affected the variation in lake dissolved carbon. The total storage amounts of DOC and DIC in the 60 lakes on the QTP in summer were 58.94 Tg and 6.22 Tg, respectively. Lake area was the most direct factor influencing dissolved carbon storage in lakes on the QTP. Moreover, the TN and pH of the lake water also affected the DOC and DIC storage in the lakes, respectively. Interestingly, the lake pH at 9.1 was an important turning point that caused variations in lake DIC storage. Surprisingly, we found that rivers were able to transport 30 % of the DIC into QTP lakes and were the main source of DIC in the lakes in summer. The findings of this study clarify the sources of dissolved carbon and its drivers and improve our understanding of the carbon cycling processes in the lake system on the QTP.

Spatiotemporal variation in methylmercury and related water quality variables in a temperate river under highly dynamic hydrologic conditions

The understanding of the essential environmental factors affecting the spatiotemporal variation in methylmercury (MeHg) in river water is limited to date, despite its importance for predicting the effect of ongoing climate change on MeHg accumulation in freshwater ecosystems. This study aimed to explore the variation in MeHg concentration and related environmental factors in the downstream zone of the Yeongsan River under highly dynamic hydrologic conditions by using water quality and hydrologic data collected from 1997 to 2022, and Hg and MeHg data collected from 2017 to 2022. The mean concentration of unfiltered MeHg was 35.7 ± 13.7 pg L−1 (n = 24) in summer and 26.7 ± 7.43 pg L−1 (n = 24) in fall. Dissolved oxygen (DO), conductivity, nitrate, and dissolved organic carbon (DOC) were determined to be the most influential variables in terms of MeHg variation based on the partial least squares regression model, and their effects on the MeHg concentration were negative, except for DOC. Heatmaps representing the similarity distances between temporal trends of hydrologic and water quality variables were constructed to determine fundamental factors related to the time-based variations in DO, conductivity, nitrate, and DOC using a dynamic time warping algorithm. The heatmap cluster analysis showed that the temporal trends of these variables were closely related to rainfall variation rather than irradiance or water temperature. Overall, biogeochemical factors directly related to in situ methylation rate of Hg(II)—rather than transport of Hg(II) and MeHg from external sources—mainly control the spatiotemporal variation of MeHg in the downstream zone of the Yeongsan River.

Contribution of marine macrophytes to pCO2 and DOC variations in human-impacted coastal waters

Carbon cycles in coastal waters are highly sensitive to human activities and play important roles in global carbon budgets. CO2 sink–source behavior is regulated by spatiotemporal variations in net biological productivity, but the contribution of macrophyte habitats including macroalgae aquaculture to atmospheric CO2 removal has not been well quantified. We investigated the variations in the carbonate system and dissolved organic carbon (DOC) in human-impacted macrophyte habitats and analyzed the biogeochemical drivers for the variations of these processes. Cultivated macroalgal metabolism (photosynthesis, respiration, calcification, and DOC release) was quantified by in situ field-bag experiments. Cultivated macroalgae took up dissolved inorganic carbon (DIC) (16.2–439 mmol-C m−2 day−1) and released DOC (1.2–146 mmol-C m−2 day−1). We estimated that seagrass beds and macroalgae farming contributed 0.8 and 0.4 mmol-C m−2 day−1 of the in situ total CO2 removal (5.7 and 6.7 mmol-C m−2 day−1, respectively) during their growing period in a semi-enclosed embayment but efficient water exchange (i.e., short residence time) in an open coastal area precluded detection of the contribution of macrophyte habitats to the CO2 removal. Although hydrological processes, biological metabolism, and organic carbon storage processes would contribute to the net CO2 sink–source behavior, our analyses distinguished the contribution of macrophytes from other factors. Our findings imply that macroalgae farming, in addition to restoring and creating macrophyte habitats, has potential for atmospheric CO2 removal.

Role of edaphic, hydrologic, and land cover variables in determining dissolved organic carbon in Missouri (USA) reservoirs and streams

Jones JR, Graham JL, Obrecht DV, Harlan JD, Knowlton MF, Pollard C, Parris J, Thorpe AP. 2024. Role of edaphic, hydrologic, and land cover variables in determining dissolved organic carbon in Missouri (USA) reservoirs and streams. Lake Reserv Manage. 40:177–195. In Missouri, distinct geophysical gradients influence statewide patterns in water quality. Here, we quantify the spatiotemporal variability of dissolved organic carbon (DOC) in reservoirs and streams and the edaphic, hydrologic, and land cover variables that account for cross-system variation. Datasets included statewide inventories collected over decades and studies with greater temporal resolution (n = >6350 DOC measurements). Among reservoirs, the smallest DOC concentration was measured in a spring-fed system within a forested watershed, and the largest was where agricultural biosolids were applied to the land (range 1.0–15.9 mg/L, overall mean 5.8 mg/L). Reservoir values increased from the southern forested Highlands (mean 4.7 mg/L) to the northern agricultural Plains (mean 7.0 mg/L). Stream DOC was similar to reservoir values (overall mean in streams 6.3 mg/L; Highlands mean 4.0 mg/L; Plains mean 6.6 mg/L), despite differences in study design and collection period. Reservoir DOC increased in spring, indicative of allochthonous loading, with small autochthonous additions during a broad summer peak. Temporal variability in DOC was low relative to macronutrients and chlorophyll in both reservoirs and streams, indicating DOC may be a sensitive and readily detected indicator of temporal change in these systems. In regression analyses, watershed features accounted for more than 60% of overall cross-system variability in DOC in both reservoirs and streams. Driver-response relations, however, differed between regions. This analysis extends our understanding of environmental influences on surface water chemistry in Missouri and indicates DOC is nearly as predictable as macronutrients using landscape-level features.

New insight into and characterization of DOC, DON and CDOM for urban waters in the lower reaches of the Yellow River, China

Dissolved organic matter (DOM), usually measured as concentrations of dissolved organic carbon (DOC) and nitrogen (DON) and chromophoric dissolved organic matter (CDOM), plays a crucial role in the global carbon and nitrogen cycles in inland waters. The aim of this study is to characterize spatial variations of DOC, DON, and C:N molar ratios and CDOM using absorption spectroscopy and further determine their relationships using pearson correlations (r) for urban waters along trophic gradients in the lower reaches of the Yellow River (YR) of China. A dataset of 122 urban water samples were collected from four typical cities during 2021–2022. Principal component analysis (PCA) was performed to assess the relative distribution of these urban water samples. These urban waters can be grouped into two categories: mesotrophic waters (n = 57) and eutrophic waters (n = 65) based on trophic state index (TSI) threshold value of 50. Results revealed that there were significantly higher DOC, CDOM absorption and SUVA254 along with lower S275–295 and M (t-test, p < 0.01) in eutrophic waters compared with mesotrophic waters. DOC was significantly higher than DON (p < 0.01). DOC was highly related to DON (R2 = 0.649) and COD (R2 = 0.840) suggesting they were derived from a common source. CDOM absorption a(254) was strongly correlated with DOC (R2 = 0.770) and COD (R2 = 0.800). DON can be calculated based on the strong correlations between DOC and DON and then C:N molar ratios can be further obtained for these urban waters. These results implied that CDOM optical absorption a(254) can be viewed as the best indicators of DOC and associated with water quality parameters. These findings can help us to better understand DOC, DON and CDOM of DOM coupled cycling processes for urban waters with similar climatic and hydrologic conditions on basin scale in China.

Spatial-temporal distribution and source apportionment of riverine dissolved organic carbon in the Miyun section of the Chaobai River in Beijing, China

Dissolved organic carbon (DOC) is indicative of water quality in drinking source water and has an impact on drinking water treatment. Exploring occurrence of DOC in drinking source water is of great significance for the drinking water safety in Beijing. This study elucidated spatial-temporal DOC dynamics in the Miyun section of Chaobai River in Beijing of China and identified sources of DOC based on DOC fluorescent spectral characteristics. Results showed that the average riverine DOC concentration was 30.60 mg/L. DOC concentrations in the Miyun section of the Chaobai River were 30.40, 30.24, and 31.09 mg/L in spring, summer and autumn, respectively. DOC concentrations at the three segments of the Miyun section of the Chaobai River (the Chaohe River, the Baihe River and the Chaobai River) were 29.93, 30.29 and 32.57 mg/L, respectively. Land use contributed to DOC variations rather than season and river segment. Significant increases in DOC concentration were observed in river water flowing through farmland and urban areas, and DOC concentration presented the highest after flowing through the farmland area. The results of FI, HIX and BIX showed that DOC mainly came from endogenous sources such as aquatic biological activities, and was less affected by terrestrial inputs. Special attentions ought to be paid on prevention and control of endogenous DOC inputs.

Dissolved Organic Radiocarbon in the West Indian Ocean

AbstractWe report marine dissolved organic carbon (DOC) concentrations, and DOC Δ14C and δ13C in seawater collected from the West Indian Ocean during the GO‐SHIP I07N cruise in 2018. We find bomb 14C in DOC from the upper 1,000 m of the water column. There is no significant change in ∆14C of DOC in deep water northward, unlike that of dissolved inorganic carbon (DIC), suggesting that transport of deep water northward is not controlling the 14C age of DOC. Variability of DOC ∆14C, including high values in the deep waters, is more pronounced than in other oceans, suggesting that dissolution of surface derived particulate organic carbon is a source of modern carbon to deep DOC in the West Indian Ocean. Low δ13C are present at two of the five stations studied, suggesting a source of low δ13C DOC, or additional microbial utilization of deep DOC.

Spatiotemporal variability of organic carbon in streams and rivers of the Northern Hemisphere cryosphere

The earth's cryosphere is the outpost of climatic warming, which leads to rapid changes of organic carbon (OC) transport from terrestrial to aquatic ecosystems. OC in the cryosphere rivers plays vital roles in the carbon cycle and river ecosystem health. Yet, we still lack a comprehensive assessment of the spatiotemporal patterns of riverine OC across the Northern Hemisphere cryosphere. Here, we compiled OC concentration, radiocarbon (14C), and the specific ultraviolet absorbance (SUVA) of dissolved OC (DOC) at 254 nm data from 1007 unique sites, extracted from 138 published literature between 1972 and 2022. Overall, the average DOC and particulate OC (POC) concentrations are 6.34 and 2.61 mg C L−1, respectively, with the average age of DOC and POC being ~1100 and ~4300 years BP, respectively, indicating the release of aged carbon pools. Seasonal variations in DOC and POC concentrations, Δ14C-DOC and SUVA254 were observed, with distinct spatial variations closely linked to specific watershed characteristics. We found permafrost-impacted watersheds displayed significantly higher DOC concentrations, younger OC ages but lower POC concentrations compared to glacier-impacted watersheds. Meanwhile, in boreal forest watersheds, DOC is the most concentrated and youngest in varied ecoregions. Additionally, in permafrost regions characterized by higher permafrost extent, ground ice content, or lowlands with thick overburden cover, riverine DOC is more concentrated and aromatic. We estimated that specific OC fluxes in glacier rivers are higher than that in permafrost rivers (4.77 and 1.86 g C m−2 yr−1, respectively). Our results highlight the complex and variable spatiotemporal patterns of riverine OC in the northern cryosphere, which are essential for assessing the impact of OC on the global carbon cycle and climate warming.

Eutrophication and salinization elevate the dissolved organic matter content in arid lakes

Dissolved organic matter (DOM) plays an essential role in the global lake carbon cycle. Understanding DOM composition and monitoring its spatiotemporal dynamics are of great significance for understanding the lake carbon cycle, controlling water pollution, and protecting water resources. However, previous studies have focused mainly on eutrophic freshwater lakes, with limited attention given to saline lakes. Based on in situ data collected in ten lakes in northwestern China, this study reported the changes in DOM components in different lake types. Parallel factor analysis (PARAFAC) was used to analyze the three-dimensional excitation emission matrix (EEMs) to obtain the DOM fluorescence components. The contributions of different environmental factors to the changes in DOM components were quantified by the generalized linear model (GLM). The results showed that the eutrophication index was significantly positively related to dissolved organic carbon (DOC) (R2 = 0.95, p < 0.01) and colored DOM (CDOM) (R2 = 0.96, p < 0.01) concentrations. Terrestrial humic-like and tryptophan-like components, which are highly correlated with human activities, explained 62% and 64% of the variations in DOC and CDOM, respectively. In sum, the contributions of human activities to the DOC and CDOM variations were 61% and 57%, respectively. Salinity also showed significant positive correlations with both DOC (R2 = 0.88, p < 0.01) and CDOM (R2 = 0.87, p < 0.01). Lake salinization led to increases in DOM concentration, and salinity contributed 20% and 16% to the DOC and CDOM variations, respectively. Therefore, human activities and salinity codetermined the DOM concentration and its composition in the western arid lakes.Based on these findings, this study proposed a feasible flowchart for remotely estimating DOM in saline lakes using satellite data. This study is significant for the long-term monitoring of the carbon cycle and the effective protection of lake water resources in saline lakes.

Dissolved organic carbon load variability in the rainfall season dominated its interannual variability in the snowmelt driven basin: Evidence from 41 years data of headwater streams

AbstractNumerous reports have observed variations of dissolved organic carbon (DOC) under the changing environment. From the DOC load variability perspective, however, how much of the variability in DOC load occurred among different seasons, and which season was most synchronized with the interannual variability remains unknown. The weekly DOC concentration and daily discharge records from four headwater streams with long‐term (1978–2018) records in the snowmelt driven basin of Harp Lake, south‐central Ontario, Canada, were used to answer these issues. We also examined the contributions of variabilities in discharge and DOC concentration to the variability in DOC load. We found that the variability of DOC load in the rainfall season (autumn) instead of snowmelt season (spring) was most closely synchronized with its interannual variability, suggesting that if the DOC variability in the rainfall season stabilized, the interannual variability would be more stable. The interannual variabilities for discharge, DOC concentration and DOC load showed an insignificant decreasing trend in the monitored 41 years, and the respective contributions of the variability trend of discharge and DOC concentration to DOC load in rainfall season increased from 28.82% to 144.58% and decreased from 71.18% to −44.58% because the change trend of discharge variability was greater than that in DOC concentration. This study provides a case information in determining which factors contribute most to the variability of DOC load in the watershed, and which season may dominate its variability throughout the year.

Dissolved Organic Carbon Dynamics Variability from Ponds Draining Different Landscapes in a Typical Agricultural Watershed

Dissolved organic carbon (DOC) in inland waters (rivers, reservoirs, lakes, and small ponds) plays a significant role in the global carbon cycle and affects global climate change. In addition, DOC is also a vital indicator of the water environment due to its multiple physical, chemical, and ecological roles. Lakes and ponds of small sizes are abundant on a global and regional scale, and a large increase in ponds is expected with global agricultural land expansion. However, the DOC characteristics of ponds in agricultural watersheds are still unclear, posing a challenge to better understanding the carbon cycle of inland waters. In this study, we explored the DOC variability and their influencing factors in ponds draining different landscapes in a typical agricultural watershed to address the issue. The field measurements over a year showed the DOC concentration varied among ponds draining different landscapes. Specifically, the mean DOC concentrations in the natural pond, sewage pond, aquaculture pond, and irrigation pond were (6.17 ± 1.49) mg/L, (12.08 ± 2.92) mg/L, (9.36 ± 2.92) mg/L, and (8.91 ± 2.71) mg/L, respectively. Meanwhile, monthly measurements found the DOC varied across sampling dates. The DOC variability was positively correlated with nutrients, primary production, and precipitation, suggesting anthropogenic loadings, an internal production rate, and hydrological regime that regulated the substantial variability of DOC in these ponds at the watershed scale. Further, large pollutant discharge and high primary production led to peak DOC occurring in the sewage pond. Our results implied that more attention should be paid to ponds in agricultural watersheds to better understand the roles of inland waters in the global carbon cycle.

Fluxes of dissolved organic matter and nitrate and their contribution to soil acidification across changing permafrost landscapes in northwestern Canada

Climate warming is predicted to change the fluxes of dissolved organic matter and nitrate by increasing active layer thickness, plant productivity, and organic matter decomposition. These changes are hypothesized to increase mineral weathering and soil acidification rates. We investigated whether acidification rates and solute leaching fluxes are variable between permafrost-affected soils with different active layer thicknesses. We compared the fluxes of dissolved organic carbon (DOC) and total dissolved N and the ion fluxes associated with solute leaching and plant uptake to calculate proton budgets in the soils that differed in slope positions (upper slope and lower slope) and soil texture (clayey and sandy soils) in NW Canada. We found the wide variation in DOC and nitrate-N fluxes, depending on slope positions and soil texture. The nitrate-N fluxes were higher at the lower slope position of the sandy soil, compared to the upper slope position. Compared to sandy soils, the DOC fluxes from the organic horizons were higher in the clayey soils on shallower permafrost table. Organic acids were major proton sources in the organic horizons at all sites, but acidity was also contributed by nitrate (sandy and clayey soils at lower slope position) and carbonic acid (clayey soil at upper slope position). The weathering by carbonic acid lead to accumulation of short-range-order minerals in the clayey soils, while incipient podzolization of the sandy soils included the weathering of illite to vermiculite and the dissolution of short-range-order minerals. The shallower active layer thickness at the lower slope position resulted in lower plant productivity and acidification rates. In the permafrost-affected soils, the active layer thickness, the DOC and nitrate-N fluxes, and their contribution to acidification are dependent on the local variation in slope position and soil texture, as well as climate change.

Constraints on the spatial variations of soil carbon fractions in a mangrove forest in Southeast China

Mangroves, carbon-rich ecosystems, store significant amounts of carbon in the soil to mitigate climate change. For better mangrove carbon management, the spatial distributions and driving factors of distinct soil carbon fractions, notably soil inorganic carbon (IC) and labile carbon, require clarification. Here, we quantified variations in the soil carbon fractions (total organic carbon (TOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), permanganate-oxidizable carbon (POC), and IC) with landward distance and soil depth, and identified the means by which edaphic and biotic factors affect soil carbon fractions in a mangrove forest in Southeast China. Average TOC, IC, POC, DOC, and MBC densities were 15.22 kg/m3, 1.39 kg/m3, 4.23 kg/m3, 43.49 g/m3, and 63.25 g/m3, respectively. Soil total nitrogen was the most important factor affecting TOC, POC, and MBC. Soil IC was found to be controlled by pH, and plant biomass was the main factor explaining soil DOC variation. Except for MBC, landward distance was negatively correlated with organic carbon fractions and positively correlated with IC via soil nutrients, pH, bulk density, and plant biomass. Soil depth directly or indirectly altered soil carbon fractions via soil nutrients, pH, clay, and salinity, resulting in negative relationships with TOC, POC, and MBC and a positive relationship with IC. Collectively, our results indicate that future changes in soil total nitrogen content and pH are likely to impact carbon storage in mangroves, and that the conservation of landward mangroves is important with respect to their high labile carbon contents.

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.