Measured Dissolved Organic Carbon Concentrations Research Articles

Seasonally variable interactions between dissolved organic matter and mineral particles in an agricultural river

Streams and rivers form an important link in the global carbon cycle by transporting and transforming large amounts of carbon imported from terrestrial ecosystems to the oceans. Since streams in agricultural areas often experience increased concentrations of suspended mineral particles from soil erosion, they are important sites where dissolved organic carbon (DOC) may be adsorbed to particles and retained in the sediment. As the extent of adsorption varies with the molecular composition of dissolved organic matter (DOM), which is seasonally variable, we expect also the fraction of organic material that adsorbs to mineral particles to fluctuate over time. We sampled the agriculturally impacted River Fyrisån (Sweden) monthly during 1 year, and measured DOC concentration and DOM composition based on several optical properties. At each sampling occasion, we estimated the potential for adsorption by exposing the samples to a reference clay. The potential for adsorption was greatest when riverine DOM had the most terrestrial character, as this fraction of the DOM pool was selectively adsorbed to clay surfaces. The extent of adsorption was negatively related to the concentration of base cations, most notably calcium. We suggest that the observed relationships between the potential for adsorption, DOM composition and base cations are linked by discharge. A bioavailability test at one sampling occasion suggested that DOM remaining after exposure to clay particles was more biodegradable. This implies that adsorption may alter the degradation potential of DOM remaining in solution, which could have far reaching effects on the fate of organic carbon.

Determination of Dissolved Organic Carbon Concentrations Using UV-Visible Absorbance for Water Samples in a Rural Watershed, the Republic of Korea

Dissolved organic carbon (DOC) in water discharged from agricultural land is one of the indicators of water pollution. As aromatic compounds, which is the main component of DOC, absorb light in the UV-visible region, the intensity of UV-visible absorbance is correlated with DOC concentrations in water. In this study, a regression equation between DOC concentration of reference samples (n = 12, extracts of manure composts) and UV-visible absorbance was established. The applicability of the established equation was tested using water samples (n = 44) collected from a rural watershed in Jeonnam, South Korea. The peak absorbance of DOC in the compost extracts was detected at 275 nm, and was positively correlated (R2 = 0.99, P < 0.001) with DOC concentrations. Although the DOC concentrations of water samples determined using the equation were lower than those measured by TOC analysis by about 15%, the estimated DOC concentrations were strongly (R2 = 0.96, RMSE = 0.97, P < 0.001) correlated with measured DOC concentrations. Our study suggests that the DOC concentrations of watershed can be estimated by measuring the UV-visible absorbance method. The peak of UV-visible absorbance of dissolved organic carbon (DOC) in the compost extracts with different DOC concentrations was detected at 275 nm among the range of 200 - 500 nm.

Influence of hydro-morphologic variables of forested catchments on the increase in DOC concentration in 36 temperate lakes of eastern Canada

In the last decades, a worldwide increase in dissolved organic carbon (DOC) concentrations has been observed in temperate and boreal lakes. This phenomenon has several detrimental effects on the aquatic life and affect local C geochemical cycles. In this study, we measured DOC concentration in the water column of 36 lakes located in eastern Canada over a period of 35 years (1983–2017) and assessed the influence of climatic, hydrologic and morphometric variables on both DOC concentrations and on the rate of DOC changes (∆DOC). Our data show that morphometric and hydrologic variables have a stronger direct influence on lake water DOC concentrations than vegetation and climatic variables. DOC concentration strongly increased with the drainage ratio and the surface covered by organic deposits, which together explained 59% of the variance. As expected, we observed a significant increase in lake water DOC concentration in 72% of the surveyed lakes, which averaged 20% over the study period. Meanwhile, lake water SO42− concentration decreased by 60%. ∆DOC was poorly influenced by the rate of changes in lake water SO42− as well as by the rate of changes in mean annual air temperature and precipitation. ∆DOC was more related to the vegetation type and the morphometry of the catchment: a model including the percentage of conifers, terrestrial catchment area and ∆Cl yielded a variance explanation of 39%. This shows that the rate of increase was primarily driven by morphometric variables which did not change over the study period.

Dissolved organic carbon lability increases with water residence time in the alluvial aquifer of a river floodplain ecosystem

AbstractWe assessed spatial and temporal patterns of dissolved organic carbon (DOC) lability and composition throughout the alluvial aquifer of the 16 km2 Nyack Floodplain in northwest Montana, USA. Water influx to the aquifer derives almost exclusively from the Middle Fork of the Flathead River, and water residence times within the aquifer range from days to months. Across seasons and channel discharge conditions, we measured DOC concentration, lability, and optical properties of aquifer water sampled from 12 wells, both near and ~3 m below the water table. Concentrations of DOC were typically low (542 ± 22.7 µg L−1; mean ± se), and the percentage of labile DOC averaged 18 ± 12% during 3 day laboratory assays. Parallel factor analysis of fluorescence excitation‐emission matrices revealed two humic‐like and two amino acid‐like fluorescence groups. Total DOC, humic‐like components, and specific UV absorbance decreased with water residence time, consistent with sorption to aquifer sediments. However, labile DOC (both concentration and fraction) increased with water residence time, suggesting a concurrent influx or production of labile DOC. Thus, although the carbon‐poor, oxygen‐rich aquifer is a net sink for DOC, recalcitrant DOC appears to be replaced with more labile DOC along aquifer flow paths. Our observation of DOC production in long flow paths contrasts with studies of hyporheic DOC consumption along short (centimeters to meters) flow paths and highlights the importance of understanding the role of labile organic matter production and/or influx in alluvial aquifer carbon cycling.

Production of Dissolved Organic Carbon in Canadian Forest Soils

To identify the controls on dissolved organic carbon (DOC) production, we incubated soils from 18 sites, a mixture of 52 forest floor and peats and 41 upper mineral soil samples, at three temperatures (3, 10, and 22°C) for over a year and measured DOC concentration in the leachate and carbon dioxide (CO2) production from the samples. Concentrations of DOC in the leachate were in the range encountered in field soils ( 50 mg l−1). There was a decline in DOC production during the incubation, with initial rates averaging 0.03–0.06 mg DOC g−1 soil C day−1, falling to averages of 0.01 mg g−1 soil C day−1; the rate of decline was not strongly related to temperature. Cumulative DOC production rates over the 395 days ranged from less than 0.01 to 0.12 mg g−1 soil C day−1 (0.5–47.6 mg g−1 soil C), with an average of 0.021 mg g−1 soil C day−1 (8.2 mg g−1 soil C). DOC production rate was weakly related to temperature, equivalent to Q10 values of 0.9 to 1.2 for mineral samples and 1.2 to 1.9 for organic samples. Rates of DOC production in the organic samples were correlated with cellulose (positively) and lignin (negatively) proportion in the organic matter, whereas in the mineral samples C and nitrogen (N) provided positive correlations. The partitioning of C released into CO2–C and DOC showed a quotient (CO2–C:DOC) that varied widely among the samples, from 1 to 146. The regression coefficient of CO2–C:DOC production (log10 transformed) ranged from 0.3 to 0.7, all significantly less than 1. At high rates of DOC production, a smaller proportion of CO2 is produced. The CO2–C:DOC quotient was dependent on incubation temperature: in the organic soil samples, the CO2–C:DOC quotient rose from an average of 6 at 3 to 16 at 22°C and in the mineral samples the rise was from 7 to 27. The CO2–C:DOC quotient was related to soil pH in the organic samples and C and N forms in the mineral samples.

Land Management Impacts on Dairy‐Derived Dissolved Organic Carbon in Ground Water

Dairy operations have the potential to elevate dissolved organic carbon (DOC) levels in ground water, where it may interact with organic and inorganic contaminants, fuel denitrification, and may present problems for drinking water treatment. Total and percent bioavailable DOC and total and carbon-specific trihalomethane (THM) formation potential (TTHMFP and STHMFP, respectively) were determined for shallow ground water samples from beneath a dairy farm in the San Joaquin Valley, California. Sixteen wells influenced by specific land management areas were sampled over 3 yr. Measured DOC concentrations were significantly elevated over the background as measured at an upgradient monitoring well, ranging from 13 to 55 mg L(-1) in wells downgradient from wastewater ponds, 8 to 30 mg L(-1) in corral wells, 5 to 12 mg L(-1) in tile drains, and 4 to 15 mg L(-1) in wells associated with manured fields. These DOC concentrations were at the upper range or greatly exceeded concentrations in most surface water bodies used as drinking water sources in California. DOC concentrations in individual wells varied by up to a factor of two over the duration of this study, indicating a dynamic system of sources and degradation. DOC bioavailability over 21 d ranged from 3 to 10%, comparable to surface water systems and demonstrating the potential for dairy-derived DOC to influence dissolved oxygen concentrations (nearly all wells were hypoxic to anoxic) and denitrification. TTHMFP measurements across all management units ranged from 141 to 1731 microg L(-1), well in excess of the maximum contaminant level of 80 microg L(-1) established by the Environmental Protection Agency. STHMFP measurements demonstrated over twofold variation ( approximately 4 to approximately 8 mmol total THM/mol DOC) across the management areas, indicating the dependence of reactivity on DOC composition. The results indicate that land management strongly controls the quantity and quality of DOC to reach shallow ground water and hence should be considered when managing ground water resources and in any efforts to mitigate contamination of ground water with carbon-based contaminants, such as pesticides and pharmaceuticals.

Long-term trends in dissolved organic carbon concentration: a cautionary note

Dissolved organic carbon (DOC) plays an important role in surface water chemistry and ecology and trends in DOC concentration have been also associated with shifts in terrestrial carbon pools. Numerous studies have reported long-term trends in DOC concentration; however, some studies consider changes in average measured DOC whereas other compute discharge weighted concentrations. Because of differences in reporting methods and variable record lengths it is difficult to compare results among studies and make regional generalizations. Furthermore, changes in stream discharge may impact long-term trends in DOC concentration and the potentially subtle effect of shifts in stream flow may be missed if only measured DOC concentrations are considered. In this study we compare trends in volume-weighted vs. average measured DOC concentration between 1980 and 2001 at seven headwater streams in south-central Ontario, Canada that vary in wetland coverage and DOC (22-year mean vol. wt.) from 3.4 to 10.6 mg l−1. On average, annual measured DOC concentrations were 13–34% higher than volume-weighted values, but differences of up to 290% occurred in certain years. Estimates of DOC flux were correspondingly higher using measured concentration values. Both measured and volume-weighted DOC concentrations increased significantly between 1980 and 2001, but slopes were larger in measured data (0.04–0.35 mg l−1 year−1 compared with 0.05–0.15 mg l−1 year−1) and proportional increases at the most wetland-influenced sites ranged from 32 to 43% in volume-weighted DOC and from 52 to 75% in measured DOC. In contrast, DOC flux did not change with time when estimated using either method, because of the predominant influence of stream flow on DOC export. Our results indicate that changes in stream flow have an important impact on trends in DOC concentration, and extrapolation of trend results from one region to another should be made cautiously and consider methodological and reporting differences among sites.

Dissolved organic carbon in ridge-axis and ridge-flank hydrothermal systems

The circulation of hydrothermal fluid through the upper oceanic crustal reservoir has a large impact on the chemistry of seawater, yet the impact on dissolved organic carbon (DOC) in the ocean has received almost no attention. To determine whether hydrothermal circulation is a source or a sink for DOC in the oceans, we measured DOC concentrations in hydrothermal fluids from several environments. Hydrothermal fluids were collected from high-temperature vents and diffuse, low-temperature vents on the basalt-hosted Juan de Fuca Ridge axis and also from low-temperature vents on the sedimented eastern flanks. High-temperature fluids from Main Endeavour Field (MEF) and Axial Volcano (AV) contain very low DOC concentrations (average = 15 and 17 μM, respectively) compared to background seawater (36 μM). At MEF and AV, average DOC concentrations in diffuse fluids (47 and 48 μM, respectively) were elevated over background seawater, and high DOC is correlated with high microbial cell counts in diffuse fluids. Fluids from off-axis hydrothermal systems located on 3.5-Ma-old crust at Baby Bare Seamount and Ocean Drilling Program (ODP) Hole 1026B had average DOC concentrations of 11 and 13 μM, respectively, and lowered DOC was correlated with low cell counts. The relative importance of heterotrophic uptake, abiotic sorption to mineral surfaces, thermal decomposition, and microbial production in fixing the DOC concentration in vent fluids remains uncertain. We calculated the potential effect of hydrothermal circulation on the deep-sea DOC cycle using our concentration data and published water flux estimates. Maximum calculated fluxes of DOC are minor compared to most oceanic DOC source and sink terms.

Dissolved organic carbon (DOC) fluxes of deep-sea sediments from the Arabian Sea and NE Atlantic

Abstract Diffusive flux of dissolved organic carbon (DOC) from deep-sea sediments is a poorly constrained component of the global marine carbon cycle, but may play an important role for DOC concentrations in the oceans. During six cruises between the years 1995 and 2000 we measured DOC concentrations in pore-waters of surface sediments and the ambient bottom waters at five stations in the deep Arabian Sea (water depths: 3190–4420 m) and at two sites in the NE Atlantic (4500–4800 m). Pore-water DOC concentrations were usually one order of magnitude greater than in bottom waters, with maximum DOC contents between 509 and 1526 μmol/L in the uppermost sediment interval (0–0.5 cm). DOC values generally decrease towards ∼3 cm core depth with slightly increasing concentrations in deeper layers under sub-oxic to anoxic conditions. Calculated benthic DOC effluxes into the bottom water caused by concentration gradients ranged between 59.8 and 223.9 μmol m−2 d−1 in the Arabian Sea and 47.2 and 121.2 μmol m−2 d−1 in the NE Atlantic. DOC efflux rates were highly correlated with the annual vertical particle flux rates at the sites and thus appear to be a function of the regional productivity. Taking an average global oceanic DOC efflux rate between 50.5 and 78.1 μmol m−2 d−1, we calculated that the DOC release is in the order of 70–103×1012 g C yr−1 from sediments below 2000 m water depth. Thus, deep-sea sediments have the potential to represent a significant source of DOC in the oceans.

Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

Litter is one of the main sources of dissolved organic carbon (DOC) in forest soils and litter decomposition is an important control of carbon storage and DOC dynamics. The aim of our study was to evaluate (i) effects of tree species on DOC production and (ii) relationships between litter decomposition and the amount and quality of DOC. Five different types of leaves and needles were exposed in litterbags at two neighboring forest sites. Within 12 months we sampled the litterbags five times and leached aliquots of field moist litter in the laboratory. In the collected litter percolates we measured DOC concentrations and recorded UV and fluorescence spectra in order to estimate the aromaticity and complexity of the organic molecules. Furthermore, we investigated the biodegradability of DOC from fresh and decomposed litter during 6 weeks incubations. Fresh sycamore maple litter released the largest amounts of DOC reaching about 6.2% of litter C after applying precipitation of 94 mm. We leached 3.9, 1.6, 1.0 and 3.3% carbon from fresh mountain ash, beech, spruce and pine litter, respectively. In the initial phase of litter decomposition significantly decreasing DOC amounts were released with increasing litter mass loss. However, after mass loss exceeds 20% DOC production from needle litter tended to increase. UV and fluorescence spectra of percolates from pine and spruce litter indicated an increasing degree of aromaticity and complexity with increasing mass loss as often described for decomposing litter. However, for deciduous litter the relationship was less obvious. We assume that during litter decomposition the source of produced DOC in coniferous litter tended toward a larger contribution from lignin-derived compounds. Biodegradability of DOC from fresh litter was very high, ranging from 30 to 95% mineralized C. DOC from degraded litter was on average 34% less mineralizable than DOC from fresh litter. Taking into account the large DOC production from decomposed needles we can assume there is an important role for DOC in the accumulation of organic matter in soils during litter decomposition particularly in coniferous forests.

Benthic fluxes and porewater concentration profiles of dissolved organic carbon in sediments from the North Carolina continental slope

Numerous studies of marine environments show that dissolved organic carbon (DOC) concentrations in sediments are typically tenfold higher than in the overlying water. Large concentration gradients near the sediment–water interface suggest that there may be a significant flux of organic carbon from sediments to the water column. Furthermore, accumulation of DOC in the porewater may influence the burial and preservation of organic matter by promoting geopolymerization and/or adsorption reactions. We measured DOC concentration profiles (for porewater collected by centrifugation and “sipping”) and benthic fluxes (with in situ and shipboard chambers) at two sites on the North Carolina continental slope to better understand the controls on porewater DOC concentrations and quantify sediment–water exchange rates. We also measured a suite of sediment properties (e.g., sediment accumulation and bioturbation rates, organic carbon content, and mineral surface area) that allow us to examine the relationship between porewater DOC concentrations and organic carbon preservation. Sediment depth-distributions of DOC from a downslope transect (300–1000 m water depth) follow a trend consistent with other porewater constituents (ΣCO 2 and SO 4 2−) and a tracer of modern, fine-grained sediment (fallout Pu), suggesting that DOC levels are regulated by organic matter remineralization. However, remineralization rates appear to be relatively uniform across the sediment transect. A simple diagenetic model illustrates that variations in DOC profiles at this site may be due to differences in the depth of the active remineralization zone, which in turn is largely controlled by the intensity of bioturbation. Comparison of porewater DOC concentrations, organic carbon burial efficiency, and organic matter sorption suggest that DOC levels are not a major factor in promoting organic matter preservation or loading on grain surfaces. The DOC benthic fluxes are difficult to detect, but suggest that only 2% of the dissolved organic carbon escapes remineralization in the sediments by transport across the sediment-water interface.

Analysis of marine DOC using a dry combustion method

Abstract As part of a continuing effort to verify and improve measurements of marine dissolved organic carbon (DOC), we combusted dried sea salts + adhered organic matter to assay DOC concentrations in representative samples from the Atlantic and Pacific oceans. Combustions were performed overnight at 580 °C in sealed tubes, and oxidation of organic materials occurred via a novel mechanism, thermal sulfate reduction: 2H2SO4 + CH2O → 2SO2 + 3H2O + CO2 Measured DOC concentrations ranged from 43 to 114 μM C, with highest values observed in inshore surface samples from Woods Hole Harbor, and lowest values observed in twelve deep offshore Atlantic and Pacific waters. Stable carbon isotope values determined for all samples were near − 22%., consistent with a predominantly marine phytoplankton origin for DOC. A seasonal study in Woods Hole Harbor showed no significant temporal trend in nearshore DOC concentrations. Problems associated with sample storage and contamination during drying steps prevented highly precise (± 1 μM) DOC concentration determinations; however, an improved drying and measurement system is outlined (Appendix A) for possible future dry-combustion studies of DOC concentrations.

Comparative analyses of DOC and DON in natural waters

Abstract Extensive comparisons of the measured dissolved organic carbon (DOC) and nitrogen (DON) contents of four different natural waters were carried out prior to the Seattle DOC/DON Workshop in July 1991 to assess the current variability in these measurements among aquatic scientists. This study included 34 DOC analyses with over 20 different instruments and 13 DON analyses of the same samples using nine different instruments. In spite of good precision for individual instruments (5.9% average sample mean deviation), independently measured DOC concentrations of the same water ranged over an order of magnitude and varied on average approximately ±40% of the mean value. Variability among similar instruments operated in different labs ( ± 15–42% of the mean) was almost as large. This observation plus evidence for constant offsets among DOC analyses made in different labs suggest that unaccounted for blanks contribute more toward the observed variability than other instrument characteristics such as oxidation method, catalyst or oven temperature. The corresponding relative analytical precisions for total dissolved nitrogen and DON were 6.8% and 27%, respectively. Independent measurements of DON varied on average ±30% of the mean value and could not be related to instrument characteristics within this highly variable data set. During the Workshop, manufacturers of four different DOC analyzers measured the DOC content of untreated and spiked lake and seawater samples using common blank and calibration procedures. Excellent agreement was obtained, supporting indications from the pre-workshop exercise that operator-dependent factors, such as blank determination and treatment, are more important sources of measurement variability than instrument design.