Fraction Of Dissolved Organic Carbon Research Articles

Experimental investigation and modeling of dissolved organic carbon removal by coagulation from seawater

Coagulation removes colloidal matters and dissolved organic carbon (DOC) which can cause irreversible membrane fouling. However, how DOC is removed by coagulant is not well-known. Jar test was used to study the removal of hydrophobic and hydrophilic DOC fractions at various doses (0.5–8.0mg-Fe+3L−1) of ferric chloride (FeCl3) and pH (5.0–9.0). Natural organic matter (NOM) in seawater and treated seawater were fractionated by liquid chromatography-organic carbon detector (LC-OCD). Compared to surface water, the removal of DOC in seawater by coagulation was remarkably different. Majority of DOC could be easily removed with very low coagulant dose (<5.0mg-Fe+3L−1) and the removal efficiency did not vary with pH, but the DOC composition in treated water had significantly changed. Hydrophobic fraction (HB) was better removed at high pH while hydrophilic fraction (HF) was better removed at low pH. A modified model of Kastl et al. (2004) which assumed that the removal occurred by adsorption of un-dissociated compounds onto ferric hydroxide was formulated and successfully validated against the jar test data.

Pyrolysis temperature-dependent release of dissolved organic carbon from plant, manure, and biorefinery wastes

Limited information is available to understand the chemical structure of biochar's labile dissolved organic carbon (DOC) fraction that will alter soil organic carbon (SOC) composition. This study utilized the high sensitivity of fluorescence excitation–emission (EEM) spectrophotometry to understand the structural changes in biochar-derived DOC as a function of (1) chemical property of feedstock and (2) pyrolysis temperature (350–800°C). Regardless of feedstock (almond shell, broiler litter, lignin, cottonseed hull, and pecan shell), low temperature (350–400°C) pyrolysis shifted EEM of hot water (80°C for 16h) extracts toward longer emission wavelengths, higher aromaticity regions. Five component parallel factor (PARAFAC) modeling of EEM afforded: (C1) (poly)phenolic pyrolysis products and other water-soluble aromatic structures similar to fulvic-like SOC; (C2–C3) aromatic structures similar to humic-like SOC having low water solubility that decomposed above 350°C; (C4) carboxyl and other transient thermochemical conversion intermediates that decomposed above 500°C; and (C5) thermally stable DOC fraction of lignin-rich biomass (pecan shell and lignin). Relative contribution of fulvic-like component exceeded that of humic-like components, and increased as a function of pyrolysis temperature. The C4 showed disproportionately high contributions to 500°C broiler litter and cottonseed hull biochars. The carboxyl-enriched DOC of biochars pyrolyzed at 350–500°C can have diverse environmental consequences, including the mobilization of heavy metals.

Dissolved organic carbon and trihalomethane precursor removal at a UK upland water treatment works

The removal of dissolved organic carbon (DOC) during potable water treatment is important for maintaining aesthetic water quality standards, minimising concentrations of micro-pollutants, controlling bacterial regrowth within distribution systems and, crucially, because it contains a sub-component that can act as trihalomethane (THM) precursors. In this study, the concentration and characteristics of raw water DOC and THM formation potential (THMFP) entering an upland potable water treatment works were analysed over twelve months. Correlations between raw water DOC characteristics, standardised THMFP (STHMFP) and % DOC removal were also investigated. DOC and THM precursor removal during a series of treatment stages was examined over this period, as well as potential selectivity in the removal of DOC fractions, to assess the importance of different treatment stages for DOC removal and THM amelioration. Though THMFP removal remained high and fairly stable throughout the study period (83–89%), the data suggest that this was mostly the result of high DOC removal rates rather than the selective removal of THM precursors. Whilst this chemical agnosticism makes DOC removal more robust, it may make the overall process more vulnerable to exceeding permissible THM concentrations under changing climatic conditions. The kinetics of the reaction between DOC and chlorine appeared to vary seasonally, indicating temporal changes in the proportions of fast- and slow-reacting precursors with implications for THM concentrations at the point of delivery to the consumer. The initial treatment stages, comprising coagulation–flocculation and dissolved air floatation (DAF) were by far the most important in terms of bulk DOC removal and the preferential removal of THM precursors, though, surprisingly, DOC quality was also modified following chlorination and secondary rapid gravity filtration (RGF). Though net THM concentration decreased following initial treatment stages, a doubling in the proportion of brominated THMs (BrTHMs), which are reported to be more carcinogenic, was also observed.

Concentration and characteristics of dissolved carbon in the Sanjiang Plain influenced by long-term land reclamation from marsh

Since the 1960s, the marshes in the Sanjiang Plain, Northeast China, which are an important reservoir for dissolved carbon, have undergone long-term reclamation to farmland, resulting in elevated marsh loss and degradation on a large scale. This study compared the concentrations of dissolved carbon, as well as the chemical characteristics of dissolved organic carbon (DOC), in natural marshes, a degraded marsh, and drainage ditches sampled during the growing seasons between 2008 and 2010 to clarify the temporal–spatial variability of the dissolved carbon in the fluvial system influenced by the long-term reclamation. The results show that the average concentrations of total dissolved carbon (TDC) and DOC are considerably greater in the natural marshes than in the degraded marsh and drainage ditches. The average DOC concentration for the natural marshes, approximately 35.53±5.15mgL−1, is approximately 2.39 times that in the degraded marsh (14.84±4.21mgL−1) and 2.77 times the average value in the ditches (12.84±4.49mgL−1). The dissolved inorganic carbon (DIC) exhibits increased trends in the drainage ditches compared with the natural marshes, whereas the hydrophobic fraction of DOC is present at lower concentrations in the degraded marsh and ditches. Fluorescence indices also indicate that the DOC in the degraded marsh and ditches has a simpler humification structure. In total, the long-term reclamation has led to great variability in the DOC concentration and chemical characteristics in the fluvial system. Changes in the DOC production potential and hydrological regimes due to sustained reclamation are deemed the predominant causes of this effect. The continuously decreased DOC concentration and high variability of DOC in the surface fluvial systems are inevitable if reclamation continues in the Sanjiang Plain. More importantly, the presence of tyrosine and tryptophan-like substances in the ditches indicates that there has been extensive agricultural organic pollution in the fluvial systems. This pollution could lead to more unexpected impacts on the downriver aquatic system. To avoid a destructive ecological crisis in the future, adjustment of the reclamation policy and agricultural management measures in the Sanjiang Plain is urgently needed.

Dissolved organic carbon (DOC) and select aldehydes in cloud and fog water: the role of the aqueous phase in impacting trace gas budgets

Abstract. Cloud and fog droplets efficiently scavenge and process water-soluble compounds and, thus, modify the chemical composition of the gas and particle phases. The concentrations of dissolved organic carbon (DOC) in the aqueous phase reach concentrations on the order of ~ 10 mgC L−1 which is typically on the same order of magnitude as the sum of inorganic anions. Aldehydes and carboxylic acids typically comprise a large fraction of DOC because of their high solubility. The dissolution of species in the aqueous phase can lead to (i) the removal of species from the gas phase preventing their processing by gas phase reactions (e.g., photolysis of aldehydes) and (ii) the formation of unique products that do not have any efficient gas phase sources (e.g., dicarboxylic acids). We present measurements of DOC and select aldehydes in fog water at high elevation and intercepted clouds at a biogenically-impacted location (Whistler, Canada) and in fog water in a more polluted area (Davis, CA). Concentrations of formaldehyde, glyoxal and methylglyoxal were in the micromolar range and comprised ≤ 2% each individually of the DOC. Comparison of the DOC and aldehyde concentrations to those at other locations shows good agreement and reveals highest levels for both in anthropogenically impacted regions. Based on this overview, we conclude that the fraction of organic carbon (dissolved and insoluble inclusions) in the aqueous phase of clouds or fogs, respectively, comprises 2–~ 40% of total organic carbon. Higher values are observed to be associated with aged air masses where organics are expected to be more highly oxidised and, thus, more soluble. Accordingly, the aqueous/gas partitioning ratio expressed here as an effective Henry's law constant for DOC (KH*DOC) increases by an order of magnitude from 7 × 103 M atm−1 to 7 × 104 M atm−1 during the ageing of air masses. The measurements are accompanied by photochemical box model simulations. These simulations are used to contrast two scenarios, i.e., an anthropogenically vs. a more biogenically impacted one as being representative for Davis and Whistler, respectively. Since the simplicity of the box model prevents a fully quantitative prediction of the observed aldehyde concentrations, we rather use the model results to compare trends in aldehyde partitioning and ratios. They suggest that the scavenging of aldehydes by the aqueous phase can reduce HO2 gas phase levels significantly by two orders of magnitude due to a weaker net source of HO2 production from aldehyde photolysis in the gas phase. Despite the high solubility of dicarbonyl compounds (glyoxal, methylglyoxal), their impact on the HO2 budget by scavenging is &lt; 10% of that of formaldehyde. The overview of DOC and aldehyde measurements presented here reveals that clouds and fogs can be efficient sinks for organics, with increasing importance in aged air masses. Even though aldehydes, specifically formaldehyde, only comprise ~ 1% of DOC, their scavenging and processing in the aqueous phase might translate into significant effects in the oxidation capacity of the atmosphere.

Variable Production by Different Pelagic Energy Mobilizers in Boreal Lakes

We studied production by three key pelagic energy mobilizer communities, phytoplankton (PP), heterotrophic bacteria (HB), and methanotrophic bacteria (MOB), in five boreal lakes of varying size and concentration of dissolved organic carbon (DOC). Production by PP was responsible for most (>55%) of the total pelagic energy mobilization in all five lakes. Production by HB and PP estimated for the whole water column during the ice-free period were positively correlated, but with the exception of the clearest and most eutrophic lake PP apparently could not support the total carbon demand of bacteria. However, the DOC concentration did not explain the variability of heterotrophic bacterial production (HBP) within or between the lakes. Thus, our results provide circumstantial evidence for the “priming effect” whereby labile organic matter from autochthonous production enhances decomposition of allochthonous DOC. However, HBP was only 10–23% of the total pelagic energy mobilization in the lakes, suggesting that only a minor fraction of allochthonous DOC became available for higher trophic levels. High MOB activity was detected in the water columns of the stratified lakes when the molar ratio of CH4:O2 varied between 0.5 and 12. In the small stratified lakes (area 70%) potential food source for zooplankton.

Major 20th century changes of the content and chemical speciation of organic carbon archived in Alpine ice cores: Implications for the long‐term change of organic aerosol over Europe

Dissolved organic carbon (DOC) and an extended array of organic compounds were investigated in an Alpine ice core covering the 1920–1988 time period. Based on this, a reconstruction was made of the long‐term trends of water‐soluble organic carbon (WSOC) aerosol in the European atmosphere. It is shown that light mono‐ and dicarboxylates, humic‐like substances, and formaldehyde account together for more than half of the DOC content of ice. This extended chemical speciation of DOC is used to estimate the DOC fraction present in ice that is related to WSOC aerosol and its change over the past. It is suggested that after World War II, the WSOC levels have been enhanced by a factor of 2 and 3 in winter and summer, respectively. In summer, the fossil fuel contribution to the enhancement is estimated to be rather small, suggesting that it arises mainly from an increase in biogenic sources of WSOC.

Removal of DOC and its fractions from surface waters of the Canadian Prairie containing high levels of DOC and hardness

In this paper removal of dissolved organic carbon (DOC) and its fractions by chemical coagulation was studied. Raw water was collected from the Red River (Manitoba, Canada). This source water has a DOC concentration ranging from 8 to 12 mg L−1 and total hardness of about 400 mg L−1 CaCO3, which represents a typical surface water quality of the Canadian Prairie. Four coagulants were tested at different pH levels: alum, ferric sulfate, ferric chloride and titanium sulfate. Coagulation effectiveness was evaluated by removal of DOC, DOC fractions, specific UV absorbance (SUVA), and trihalomethane formation potential (THMFP) of the coagulated water. The water DOC was separated into six fractions based on hydrophobicity and acid base functionality: hydrophobic acid (HPOA), hydrophobic base (HPOB), hydrophobic neutral (HPON), hydrophilic acid (HPIA), hydrophilic base (HPIB), and hydrophilic neutral (HPIN). Results showed that ferric sulfate had the highest total DOC removal of 66% while ferric chloride had the lowest DOC reduction of 54%. Although the THMFP was found to be lowered significantly with all four coagulants the ferric chloride showed the greatest THMFP reduction. Fractionation results found a significant reduction in the HPOA fraction for all coagulants with 91% for ferric chloride as the highest removal value. Poor removal of hydrophilic fractions was found for all coagulants. The result of this study showed that total DOC reduction cannot guarantee THMFP reduction and coagulation should be optimized to remove DOC fractions which form most THMs.

Labile soil organic matter fractions as influenced by non-flooded mulching cultivation and cropping season in rice–wheat rotation

Labile soil organic matter (SOM) fractions are especially important because they are more vulnerable to disturbance and play a crucial role for nutrient and carbon cycling. Although water conservation has become increasingly important in rice–wheat rotation, the effects of non-flooded mulching cultivation on labile SOM fractions remain unknown. Based on a long-term field experiment (10 years), we analyzed the impact of non-flooded mulching cultivation and cropping season on labile SOM fractions in a rice–wheat rotation in Chengdu Plain, southwest China. Compared with traditional flooding (TF), the plastic film mulching (PM) and wheat straw mulching (SM) treatments increased dissolved organic carbon (DOC) (42% after rice season and 41% after wheat season), but decreased microbial biomass carbon (MBC) (19% after rice season and 28% after wheat season) and nitrogen (MBN) (17% after rice season and 24% after wheat season) in the 0–5 cm depth. SM increased particulate organic carbon (POC) and KMnO4-oxidizable C (KMnO4-C) contents after both the rice and wheat seasons. Microbial biomass and DOC concentrations were higher for all three cultivations after the rice season than after the wheat season. In contrast, POC contents under PM and SM were higher after the wheat season than after the rice season. In general, results in this study indicate that non-flooded mulching and cropping season significantly influenced labile SOM fractions. The DOC fraction was the most sensitive fraction affected by non-flooded mulching, while POC and PON fractions respond fast within the two cropping seasons.

Dissolved organic carbon dynamics in anaerobic sediments of the Santa Monica Basin

Cycling of dissolved organic carbon (DOC) was investigated in anoxic sediments of the Santa Monica Basin, California Borderland, by analyzing the concentration and isotopic signatures (Δ14C and δ13C) of pore-water DOC and dissolved inorganic carbon (DIC), and organic compound classes extracted from the bulk sediments. DOC and DIC increased across the sediment–water interface, indicating net efflux of these solutes out of the sediments. Throughout the depth interval examined (0–30cm), the Δ14C value of DOC (Δ14CDOC) was similar to, or higher than, that of bulk sedimentary particulate organic carbon (POC), indicating degradation of relatively 14C-rich components of POC. There were prominent peaks in both Δ14CDOC and Δ14CDIC in the uppermost 2 cm of the sediment column, indicating degradation and remineralization of 14C-rich, labile organic matter in the near-surface sediments. However, below these sub-surface maxima, Δ14CDOC and Δ14CDIC decreased with depth by ∼200‰ and ∼50‰, respectively. Given the diffusive time scales, these decreases were too large to be explained by 14C loss due to radioactive decay. To help explain these observations, we constructed and implemented a selective degradation model that considers bulk pore-water DOC to be the sum of three kinetically- and isotopically-distinct sub-components. Based on this model, the most reactive DOC fraction, which supported ∼60% of the DIC production, had a Δ14C value indicating the presence of bomb-14C. The intermediate fraction had a Δ14C value of ∼−60‰ and accounted for most of the pore-water DOC standing stock. The least reactive fraction was virtually non-reactive in these sediments, and had a Δ14C value of ∼−500‰. The benthic DOC flux of this 14C-depleted, poorly-reactive DOC fraction may represent a source of pre-aged, refractory DOC to the oceans.

Trihalomethane formation potential of DOC fractions isolated from two Canadian Prairie surface water sources

This study evaluated the dissolved organic carbon (DOC) composition of two surface waters in Manitoba, Canada, the Red and Assiniboine Rivers, to identify DOC fractions with the greatest potential to form trihalomethanes (THMs). The DOC was fractionated into six fractions based on hydrophobicity and acid base functionality: hydrophobic acid (HPOA), hydrophobic base (HPOB), hydrophobic neutral (HPON), hydrophilic acid (HPIA), hydrophilic base (HPIB), and hydrophilic neutral (HPIN). The trihalomethane formation potential (THMFP) of the isolated fractions was measured. Fractions collected from the Red River showed significant variation in the specific THMFP, normalized to DOC concentration, ranging from 23 to 1,372 μg THM/mg DOC, while the Assiniboine River THMFP ranged from 11 to 575 μg THM/mg DOC. Although the DOC composition of the two rivers was similar, the THMFP for the fractions differed significantly. The Red River HPOB and HPIB fractions had the highest normalized THMFP of 1,002 μg THM/mg DOC and 1,372 μg THM/mg DOC, respectively, while the HPOA fraction showed the lowest with 23 μg THM/mg DOC. Comparatively, the HPOA fraction from the Assiniboine River was found to have a higher specific THMFP of 478 μg THM/mg DOC. These findings suggest that THM formation is dependent on unique organic composition of the local water and that caution should be taken when estimating THMFP from DOC fractions isolated from different water sources.

Dissolved organic carbon in the precipitation of Seoul, Korea: Implications for global wet depositional flux of fossil-fuel derived organic carbon

Precipitation was sampled in Seoul over a one-year period from 2009 to 2010 to investigate the sources and fluxes of atmospheric dissolved organic carbon (DOC). The concentrations of DOC varied from 15 μM to 780 μM, with a volume-weighted average of 94 μM. On the basis of correlation analysis using the commonly acknowledged tracers, such as vanadium, the combustion of fossil-fuels was recognized to be the dominant source. With the aid of air mass backward trajectory analyses, we concluded that the primary fraction of DOC in our precipitation samples originated locally in Korea, albeit the frequent long-range transport from eastern and northeastern China might contribute substantially. In light of the relatively invariant organic carbon to sulfur mass ratios in precipitation over Seoul and other urban regions around the world, the global magnitude of wet depositional DOC originating from fossil-fuels was calculated to be 36 ± 10 Tg C yr−1. Our study further underscores the potentially significant environmental impacts that might be brought about by this anthropogenically derived component of organic carbon in the atmosphere.

Carbonaceous species and humic like substances (HULIS) in Arctic snowpack during OASIS field campaign in Barrow

Snowpacks contain many carbonaceous species that can potentially impact on snow albedo and arctic atmospheric chemistry. During the OASIS field campaign, in March and April 2009, Elemental Carbon (EC), Water insoluble Organic Carbon (WinOC) and Dissolved Organic Carbon (DOC) were investigated in various types of snow: precipitating snows, remobilized snows, wind slabs and depth hoars. EC was found to represent less than 5% of the Total Carbon Content (TCC = EC + WinOC + DOC), whereas WinOC was found to represent an unusual 28 to 42% of TCC. Snow type was used to infer physical processes influencing the evolution of different fractions of DOC. DOC is highest in soil influenced indurated depth hoar layers due to specific wind related formation mechanisms in the early season. Apart from this specific snow type, DOC is found to decrease from precipitating snow to remobilized snow to regular depth hoar. This decrease is interpreted as due to cleaving photochemistry and physical equilibration of the most volatile fraction of DOC. Depending on the relative proportions of diamond dust and fresh snow in the deposition of the seasonal snowpack, we estimate that 31 to 76% of DOC deposited to the snowpack is reemitted back to the boundary layer. Under the assumption that this reemission is purely photochemical, we estimate an average flux of VOC out of the snowpack of 20 to 170 μgC m−2 h−1. Humic like substances (HULIS), short chain diacids and aldehydes are quantified, and showed to represent altogether a modest (&lt;20%) proportion of DOC, and less than 10% of DOC + WinOC. HULIS optical properties are measured and could be consistent with aged biomass burning or a possible marine source.

Migration of dissolved organic carbon in biochars and biochar-mineral complexes

The objective of this work was to determine the contribution of dissolved organic carbon (DOC) from a biochar mineral complex (BMC), so as to better understand the interactions between DOC, biochar, clay, and minerals during thermal treatment, and the effects of BMC on amended soils. The BMC was prepared by heating a mixture of a H3PO4-treated saligna biochar from Acacia saligna, clays, other minerals, and chicken manure. The BMC was applied to a sandy loam soil in Western Australia, where wheat was grown. Liquid chromatography-organic carbon detection (LC-OCD) tests were carried out on water extracts from the untreated biochar, the BMC, the BMC-amended soil, and on a control soil to measure the DOC concentration. LC-OCD tests provide a fingerprint of the DOC, which allows the fractions of DOC to be determined. Thermal processing enhanced the reaction of the A. saligna biochar with manure, clays and minerals, and affected the distribution of the DOC fractions. Notably, the process leads to immobilization of hydrophobic DOC and to an increase in the concentration of low-molecular-weight neutrals in the BMC. The application of the BMC to soil increases the DOC in the amended soil, especially the biopolymer fraction.

TRANSFORMATION AND ALLELOPATHY OF NATURAL DISSOLVED ORGANIC CARBON AND TANNIC ACID ARE AFFECTED BY SOLAR RADIATION AND BACTERIA(1).

The aim of this study was to test whether abiotic and biotic factors may affect allelopathic properties. Therefore, we investigated how solar radiation and bacteria influence allelopathic effects of the plant-derived, polyphenolic tannic acid (TA) on microalgae. Using a block design, lake water samples with and without TA were exposed to solar radiation or kept in darkness with or without bacteria for 3 weeks. Dissolved organic carbon (DOC), specific size fractions of DOC analyzed by chromatography-organic carbon detection (LC-OCD), and concentrations of total phenolic compounds (TPC) were measured to follow the fate of TA in lake water with natural DOC exposed to photolytic and microbial degradation. DOC and TPC decreased in dark-incubated lake water with TA and bacteria indicating microbial degradation. In contrast, exposure to solar radiation of lake water with TA and bacteria did not decrease DOC. Chromatographic analyses documented an accumulation of DOC mean size fraction designated as humic substances (HS) in sunlit water samples with TA. The recalcitrance of the humic fraction indicates that photolytic degradation may contribute to a DOC less available for bacterial degradation. Subsequent growth tests with Desmodesmus armatus (Chodat) E. Hegewald showed low but reproducible difference in algal growth with lower algal growth rate cultured in photolytically and microbially degraded TA in lake water than cultured in respective dark treatments. This finding highlights the importance of photolytic processes and microbial degradation influencing allelopathic effects and may explain the high potential of allelochemicals for structuring the phytoplankton community composition in naturally illuminated surface waters.

Reactivity continuum of dissolved organic carbon decomposition in lake water

We determined microbial decomposition of dissolved organic carbon (DOC) over 3.7 year long dark bioassays of six Swedish lake waters. The overall lost DOC fraction was similar in clearwater lakes (34.8 ± 2.4%) and in brownwater lakes (37.8 ± 1.9%). Reactivity continuum modeling revealed that the most labile DOC fraction, degrading at rates &gt;0.01 d−1, was larger in the clearwater lakes (11.1 ± 1.2%) than in the brownwater lakes (0.8 ± 0.1%). The initial apparent first‐order decay coefficientk was fivefold larger in the clearwater lakes (0.0043 ± 0.0012 d−1) than in the brownwater lakes (0.0009 ± 0.0003 d−1). Over time, k decreased more steeply in the clearwater lakes than in the brownwater lakes, reaching the k of the brownwater lakes within 5 months. Finally, k averaged 0.0001 d−1 in both lake categories. In the brownwater lakes, colored dissolved organic matter (CDOM) absorption decayed with an initial k twice as large (0.0018 ± 0.0008 d−1) as that of DOC. The initial kwas inversely correlated with initial specific UV absorption and CDOM absorption and positively correlated with initial tryptophan‐like fluorescence as proxy for autochthonous DOC. Exposure to simulated sunlight at the end of the incubations caused loss of color in the clearwater lakes and loss of DOC in the brownwater lakes, where subsequent mineralization was also stimulated. The DOC lost in the absence of photochemical processes fell below previously reported watershed‐scale losses in Sweden by 25% at most. This suggests that a major part of the in situ DOC loss could potentially be attributed to dark reactions alone.

Dissolved Organic Carbon as Major Environmental Factor Affecting Bacterioplankton Communities in Mountain Lakes of Eastern Japan

Relationships between environmental factors and bacterial communities were investigated in 41 freshwater lakes located in mountainous regions of eastern Japan. Bacterioplankton community composition (BCC) was determined by polymerase chain reaction-denaturing gradient gel electrophoresis of the 16S rRNA gene and then evaluated on the basis of physicochemical and biological variables of the lakes. Canonical correspondence analysis revealed that BCC of oligotrophic lakes was significantly influenced by dissolved organic carbon (DOC) content, but its effect was not apparent in the analysis covering all lakes including mesotrophic and eutrophic ones. The generalized linear model showed the negative association of DOC on the taxon richness of bacterioplankton communities. DOC was positively correlated with the catchment area per lake volume, suggesting that a large fraction of DOC supplied to the lake was derived from terrestrial sources. These results suggest that allochthonous DOC has a significant effect on bacterioplankton communities especially in oligotrophic lakes. The genus Polynucleobacter was detected most frequently. The occurrence of Polynucleobacter species was positively associated with DOC and negatively associated with total phosphorus (TP) levels. In addition, TP had a stronger effect than DOC, suggesting that oligotrophy is the most important factor on the occurrence of this genus.

Dynamics of dissolved organic carbon after a cyanobacterial bloom in hypereutrophic Lake Taihu (China)

To establish the influence of the cyanobacterial bloom collapse on the characteristics of dissolved organic carbon (DOC) in Lake Taihu, high-molecular-weight dissolved organic matter (HMW-DOM), with sizes between 1 kDa and 0.5 μm, were collected using cross-flow ultrafiltration, from three different eutrophic regions. Isolated HMW-DOM was further characterized by atomic carbon to nitrogen ratio and neutral sugars composition by gas chromatography and mass spectrometry. The results indicated that the cyanobacterial cell lysis induced by nitrate depletion is the likely mechanism for DOC release. The relatively high DOC level was associated with the high chlorophyll a concentration in Meiliang Bay, one of the most eutrophic bays in the northern part of the lake. However, no significant correlations were observed between chlorophyll a concentration and HMW-DOC concentration during the demise of the cyanobacterial bloom in Lake Taihu. No significant differences were found in the HMW-DOC concentration among the three sampling sites, which were selected to represent different eutrophic status. However, a significant difference in the HMW-DOC concentration was found between October 2009 and January 2010 in all three sampling sites ( p = 0.02). The HMW-DOC release may be attributed to the cyanobacterial cell lysis after the peak of summer bloom. The similarity in neutral sugar composition between the HMW-DOM and cyanobacterial exopolysaccharides suggests that the cyanobacterial bloom is the source of HMW-DOM. However, the significant correlation between the carbon to nitrogen ratio in HMW-DOM and chlorophyll a concentration was only observed in Meiliang Bay, which implies that apart from the cyanobacteria-derived DOC, a fraction of DOC was from riverine input. The decline of the cyanobacterial bloom also changed the overall DOM pool, leading to a shift in the component of HMW-DOM from a C-enriched material to an N-enriched material, as revealed by the variation in the carbon to nitrogen ratios. Overall, these results demonstrate that the quantitative and qualitative DOM is affected by the post-cyanobacterial bloom in Lake Taihu.

Speciation of Se and DOC in Soil Solution and Their Relation to Se Bioavailability

A 0.01 M CaCl(2) extraction is often used to asses the bioavailability of plant nutrients in soils. However, almost no correlation was found between selenium (Se) in the soil extraction and Se content in grass. The recently developed anion Donnan membrane technique was used to analyze chemical speciation of Se in the 0.01 M CaCl(2) extractions of grassland soils and fractionation of DOC (dissolved organic carbon). The results show that most of Se (67-86%) in the extractions (15 samples) are colloidal-sized Se. Only 13-34% of extractable Se are selenate, selenite and small organic Se (<1 nm). Colloidal Se is, most likely, Se bound to or incorporated in colloidal-sized organic matter. The dominant form of small Se compounds (selenate, selenite/small organic compounds) depends on soil. A total of 47-85% of DOC is colloidal-sized and 15-53% are small organic molecules (<1 nm). In combination with soluble S (sulfur) and/or P (phosphor), concentration of small DOC can explain most of the variability of Se content in grass. The results indicate that mineralization of organic Se is the most important factor that controls Se availability in soils. Competition with sulfate and phosphate needs to be taken into account. Further research is needed to verify if concentration of small DOC is a good indicator of mineralization of soil organic matter.

Source water controls on the character and origin of dissolved organic matter in streams of the Yukon River basin, Alaska

Climate warming and permafrost degradation at high latitudes will likely impact watershed hydrology, and consequently, alter the concentration and character of dissolved organic carbon (DOC) in northern rivers. We examined seasonal variation of DOC chemistry in 16 streams of the Yukon River basin, Alaska. Our primary objective was to evaluate the relationship between source water (shallow versus deep groundwater flow paths) and DOC chemical composition. Using base cation chemistry and principal component analysis, we observed high contributions of deep groundwater to glacial and clearwater streams, whereas blackwater streams received larger contributions from shallow groundwater sources. DOC concentration and specific ultraviolet absorbance peaked during spring snowmelt in all streams, and were consistently higher in blackwater streams than in glacial and clearwater streams. The hydrophobic acid fraction of DOC dominated across all streams and seasons, comprising between 35% and 56% of total DOC. The hydrophilic acid fraction of DOC was more prominent in glacial (23% ± 3%) and clearwater streams (19% ± 1%) than in blackwater streams (16% ± 1%), and was enriched during winter base flow (29% ± 1%) relative to snowmelt and summer base flow. We observed that an increase in the contribution of deep groundwater to streamflow resulted in decreased DOC concentration, aromaticity, and DOC‐to‐dissolved organic nitrogen ratio, and an increase in the proportion of hydrophilic acids relative to hydrophobic acids. Our findings suggest that future permafrost degradation and higher contributions of groundwater to streamflow may result in a higher fraction of labile DOM in streams of the Yukon basin.