Lower Dissolved Organic Carbon Concentrations Research Articles

Can the heterogeneity in stream dissolved organic carbon be explained by contributing landscape elements?

Abstract. The controls on stream dissolved organic carbon (DOC) concentrations were investigated in a 68 km2 catchment by applying a landscape-mixing model to test if downstream concentrations could be predicted from contributing landscape elements. The landscape-mixing model reproduced the DOC concentration well throughout the stream network during times of high and intermediate discharge. The landscape-mixing model approach is conceptually simple and easy to apply, requiring relatively few field measurements and minimal parameterisation. Our interpretation is that the higher degree of hydrological connectivity during high flows, combined with shorter stream residence times, increased the predictive power of this whole watershed-based mixing model. The model was also useful for providing a baseline for residual analysis, which highlighted areas for further conceptual model development. The residual analysis indicated areas of the stream network that were not well represented by simple mixing of headwaters, as well as flow conditions during which simple mixing based on headwater watershed characteristics did not apply. Specifically, we found that during periods of baseflow the larger valley streams had much lower DOC concentrations than would be predicted by simple mixing. Longer stream residence times during baseflow and changing hydrological flow paths were suggested as potential reasons for this pattern. This study highlights how a simple landscape-mixing model can be used for predictions as well as providing a baseline for residual analysis, which suggest potential mechanisms to be further explored using more focused field and process-based modelling studies.

Dissolved organic carbon and apparent oxygen utilization in the Atlantic Ocean

Dissolved organic carbon (DOC) distributions along two Atlantic Meridional Transects conducted in 2005 in the region between 47°N and 34°S showed clear latitudinal patterns. The DOC concentrations in the epipelagic zone (0–100m) were the highest (70–90µM) in tropical and subtropical waters with stable mixed layers, and lowest (50–55µM) at the poleward extremities of the transects due to deep convective mixing supplying low DOC waters to the surface. A decrease in DOC occurred with depth, and lowest DOC concentrations (41–45µM) in the 100–300m depth range were observed in the equatorial region due to upwelling of low DOC waters. A strong relationship between DOC and AOU was observed in the σ–t 26–26.5 isopycnal layer which underlies the euphotic zone and outcrops at the poleward extremities of the North and South Atlantic Subtropical Gyres (NASG and SASG) in the region ventilating the thermocline waters. Our observations reveal significant north–south variability in the DOC–AOU relationship. The gradient of the relationship suggests that 52% of the AOU in the σ–t 26–26.5 density range was driven by DOC degradation in the NASG and 36% in the SASG, with the remainder due to the remineralisation of sinking particulate material. We assess possible causes for the greater contribution of DOC remineralisation in the NASG compared to the SASG.

Dissolved Organic Carbon in Headwater Streams and Riparian Soil Organic Carbon along an Altitudinal Gradient in the Wuyi Mountains, China

Stream water dissolved organic carbon (DOC) correlates positively with soil organic carbon (SOC) in many biomes. Does this relationship hold in a small geographic region when variations of temperature, precipitation and vegetation are driven by a significant altitudinal gradient? We examined the spatial connectivity between concentrations of DOC in headwater stream and contents of riparian SOC and water-soluble soil organic carbon (WSOC), riparian soil C:N ratio, and temperature in four vegetation types along an altitudinal gradient in the Wuyi Mountains, China. Our analyses showed that annual mean concentrations of headwater stream DOC were lower in alpine meadow (AM) than in subtropical evergreen broadleaf forest (EBF), coniferous forest (CF), and subalpine dwarf forest (SDF). Headwater stream DOC concentrations were negatively correlated with riparian SOC as well as WSOC contents, and were unrelated to riparian soil C:N ratio. Our findings suggest that DOC concentrations in headwater streams are affected by different factors at regional and local scales. The dilution effect of higher precipitation and adsorption of soil DOC to higher soil clay plus silt content at higher elevation may play an important role in causing lower DOC concentrations in AM stream of the Wuyi Mountains. Our results suggest that upscaling and downscaling of the drivers of DOC export from forested watersheds when exploring the response of carbon flux to climatic change or other drivers must done with caution.

Permafrost and fire as regulators of stream chemistry in basins of the Central Siberian Plateau

Stream chemistry in permafrost regions is regulated by a variety of drivers that affect hydrologic flowpaths and watershed carbon and nutrient dynamics. Here we examine the extent to which seasonal dynamics of soil active layer thickness and wildfires regulate solute concentration in streams of the continuous permafrost region of the Central Siberian Plateau. Samples were collected from 2006 to 2012 during the frost-free season (May–September) from sixteen watersheds with fire histories ranging from 3 to 120 years. The influence of permafrost was evident through significantly higher dissolved organic carbon (DOC) concentrations in the spring, when only the organic soil horizon was accessible to runoff. As the active layer deepened through the growing season, water was routed deeper through the underlying mineral horizon where DOC underwent adsorption and concentrations decreased. In contrast, mean concentrations of major cations (Ca2+ > Na+ > Mg2+ > K+) were significantly higher in the summer, when contact with mineral horizons in the active zone provided a source of cations. Wildfire caused significantly lower concentrations of DOC in more recently burned watersheds, due to removal of a source of DOC through combustion of the organic layer. An opposite trend was observed for dissolved inorganic carbon and major cations in more recently burned watersheds. There was also indication of talik presence in three of the larger watersheds evidenced by Cl− concentrations that were ten times higher than those of other watersheds. Because climate change affects both fire recurrence intervals as well as rates of permafrost degradation, delineating their combined effects on solute concentration allows forecasting of the evolution of biogeochemical cycles in this region in the future.

Factors affecting the movement and persistence of nitrate and pesticides in the surficial and upper Floridan aquifers in two agricultural areas in the southeastern United States

Differences in the degree of confinement, redox conditions, and dissolved organic carbon (DOC) are the main factors that control the persistence of nitrate and pesticides in the Upper Floridan aquifer (UFA) and overlying surficial aquifer beneath two agricultural areas in the southeastern US. Groundwater samples were collected multiple times from 66 wells during 1993–2007 in a study area in southwestern Georgia (ACFB) and from 48 wells in 1997–98 and 2007–08 in a study area in South Carolina (SANT) as part of the US Geological Survey National Water-Quality Assessment Program. In the ACFB study area, where karst features are prevalent, elevated nitrate-N concentrations in the oxic unconfined UFA (median 2.5 mg/L) were significantly (p = 0.03) higher than those in the overlying oxic surficial aquifer (median 1.5 mg/L). Concentrations of atrazine and deethylatrazine (DEA; the most frequently detected pesticide and degradate) were higher in more recent groundwater samples from the ACFB study area than in samples collected prior to 2000. Conversely, in the SANT study area, nitrate-N concentrations in the UFA were mostly <0.06 mg/L, resulting from anoxic conditions and elevated DOC concentrations that favored denitrification. Although most parts of the partially confined UFA in the SANT study area were anoxic or had mixed redox conditions, water from 28 % of the sampled wells was oxic and had low DOC concentrations. Based on the groundwater age information, nitrate concentrations reflect historic fertilizer N usage in both the study areas, but with a lag time of about 15–20 years. Simulated responses to future management scenarios of fertilizer N inputs indicated that elevated nitrate-N concentrations would likely persist in oxic parts of the surficial aquifer and UFA for decades even with substantial decreases in fertilizer N inputs over the next 40 years.

Influence of habitat on the quantity and composition of leachable carbon in the O2 horizon: Potential implications for potable water treatment

Organic material leached from the organic (O) horizon of soils is a major source of natural organic matter (NOM) in surface waters. Dissolved organic carbon (DOC) is a known precursor for the formation of disinfection by-products (DBPs), including trihalomethanes (THMs), formed during chlorination. In this study the concentration and composition of leachable O2 horizon DOC from 5 habitats within a United Kingdom upland reservoir catchment (beech, spruce, larch, and pine forests and blanket peat) were compared with an emphasis on potential treatment implications using XAD fractionation and THM formation potential (THMFP) tests. Statistically significant differences were found between habitats, with pine and larch leachates yielding particularly high DOC concentrations (mean 19.3 and 13.4 mg/L, respectively) and THMFP7d values (mean 1306 and 1527 μg/L, respectively). The interspecies variation observed suggests that the typical distinction made between deciduous and coniferous species in previous studies is overly simplistic. Interestingly, peat leachate exhibited a surprisingly low DOC concentration (mean 9.0 mg/L), suggesting that the high DOC flux associated with these habitats may be the result of other factors such as depth of organic matter and mineral content. Averaged across all habitats, mean standardized THMFP (STHMFP) was highest in the hydrophobic acid (HPOA) fraction, although substantial differences in the relative reactivities of fractions were found between habitats. Synergistic effects are also likely to complicate the relationship between fractional character and STHMFP.

Inorganic chemistry, gas compositions and dissolved organic carbon in fluids from sedimented young basaltic crust on the Juan de Fuca Ridge flanks

The permeable upper oceanic basement serves as a plausible habitat for a variety of microbial communities. There is growing evidence suggesting a substantial subseafloor biosphere. Here new time series data are presented on key inorganic species, methane, hydrogen and dissolved organic carbon (DOC) in ridge flank fluids obtained from subseafloor observatory CORKs (Circulation Obviation Retrofit Kits) at Integrated Ocean Drilling Program (IODP) boreholes 1301A and 1026B. These data show that the new sampling methods (Cowen et al., 2012) employed at 1301A result in lower contamination than earlier studies. Furthermore, sample collection methods permitted most chemical analyses to be performed from aliquots of single large volume samples, thereby allowing more direct comparison of the data. The low phosphate concentrations (0.06–0.2μM) suggest that relative to carbon and nitrogen, phosphorus could be a limiting nutrient in the basement biosphere. Coexisting sulfate (17–18mM), hydrogen sulfide (∼0.1μM), hydrogen (0.3–0.7μM) and methane (1.5–2μM) indicates that the basement aquifer at 1301A either draws fluids from multiple flow paths with different redox histories or is a complex environment that is not thermodynamically controlled and may allow co-occurring metabolic pathways including sulfate reduction and methanogenesis. The low DOC concentrations (11–18μM) confirm that ridge flank basement is a net DOC sink and ultimately a net carbon sink. Based on the net amounts of DOC, oxygen, nitrate and sulfate removed (∼30μM, ∼80μM, ∼40μM and ∼10mM, respectively) from entrained bottom seawater, organic carbon may be aerobically or anaerobically oxidized in biotic and/or abiotic processes.

Does quinone or phenol enrichment of humic substances alter the primary compound from a non-algicidal to an algicidal preparation?

Dissolved organic matter (DOM) has been shown to affect phytoplankton species directly. These interactions largely depend on the origin and molecular size of DOM and are different in prokaryotes and eukaryotes. In a preceding study, however, two humic substance preparations did not adversely affect coccal green algae or cyanobacterial growth even at high concentrations of dissolved organic carbon (DOC). These results contradicted previous findings, showing a clear, negative response of different phototrophs to much lower DOC concentrations. To test whether or not at least defined building blocks of humic substances (HSs) are effective algicidal structures, we enriched two humic preparations with hydroquinone and p-benzoquinone, respectively, and exposed two different green algae, Pseudokirchneriella subcapitata and Monoraphidium braunii, and two cyanobacterial species, Synechocystis sp. and Microcystis aeruginosa, to the unmodified and enriched HSs. As response variables, growth rates in terms of biomass increase, chlorophyll-a content, and photosynthetic yield were measured. The highest concentration (4.17mM DOC) of the modified HSs clearly inhibited growth; the cyanobacterial species were much more sensitive than the green algal species. However, realistic ecological concentrations did not adversely affect growth. Aerating the exposure solution for 24h strongly reduced the inhibitory effect of the modified HSs. The algicidal effect was obviously caused by monomers and not by polymerised high molecular weight HSs themselves. Furthermore, the maximum quantum yield (Φ PSII max) was stimulated in the green algal species by low and medium DOC concentrations, but reduced in the cyanobacterial species upon exposure to higher HS concentrations. The quinone- and phenol-enriched HSs only showed algicidal activity at high concentrations of 4.17mM DOC and lost their effects over time, presumably by oxidation and subsequent polymerisation. This study confirms that the applied humic substances themselves are not effective algicides even if enriched in effective structures.

Biodegradation of dissolved organic carbon in soil extracts and leachates from a temperate forest stand and its relationship to ultraviolet absorbance

The amount and biodegradability of dissolved organic carbon (DOC) in forest floors can contribute to carbon sequestration in soils and the release of CO2-C from soil to the atmosphere. There is only limited knowledge about the biodegradation of DOC in soil extracts and leachates due to the limitations inherent in degradation experiments. Differences in the biodegradation of DOC were studied in forest soil extracts using cold and hot water and 4 mmol/L CaCl2 solution and in soil leachates sampled under different conditions over a wide range of DOC concentrations. From these results, we developed a simple and rapid method for determining the biodegradable organic C in forest floors. The hot water extracts and CaCl2 extracts after CH3Cl fumigation contained higher concentrations of biodegradable organic C than the cold water extracts and CaCl2 extracts before fumigation, with rapid DOC degradation occurring 24–48 h after incubation with an inoculum, followed by slow DOC degradation till 120–168 h into the incubation. During a 7-d incubation with an inoculum, the variation in DOC degradation in the different soil extracts was consistent with the change in special UV absorbance at 254 nm. Relatively higher levels of biodegradable organic C were detected in soil leachates from the forest canopy than in forest gaps between April and October 2008 (P <0.05). Relatively lower concentrations of DOC and biodegradable organic C were observed in soil leachates from N-fertilized plots during the growing season compared with the control, with the exception of the plot treated with KNO3 at a rate of 45 kg N ha−1 a−1. Around 77.4% to 96.3% of the variability in the biodegradable organic C concentrations in the forest floors could be accounted for by the initial DOC concentration and UV absorbance at 254 nm. Compared with the conventional inoculum incubation method, the method of analyzing UV absorbance at 254 nm is less time consuming and requires a much smaller sample volume. The results suggest that the regression models obtained using the initial DOC concentration and UV absorbance can provide a rapid, simple and reliable method for determining the biodegradable organic C content, especially in field studies involving relatively large numbers of samples.

Spatio-temporal variability of the dissolved organic carbon and nitrogen in a coastal area affected by river input: The north eastern shelf of the Gulf of Cádiz (SW Iberian Peninsula)

Four surveys (June and November 2006; February and May 2007) were carried out in the north eastern shelf of the Gulf of Cádiz (southwest Iberian Peninsula) to investigate the dynamics of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in a coastal area affected mainly by river input, but also by primary production/respiration, resuspension from the sediments, and mixing. In the period of the present study, DOC values ranged from 42 to 198 μM, while DON ranged from 0 to 20 μM. The seasonal variations showed high mean surface concentrations of DOC (106.2 ± 25.6 μM-C) and DON (8.6 ± 3.1 μM-N) in May, shifting to low DOC (89.1 ± 26.4 μM-C) and DON (4.2 ± 2.8 μM-N) in February. In spring, DOC and DON released by phytoplankton are likely to be the most significant source of organic matter. Low DOC and DON concentrations during winter are probably due to the uptake of DOC and DON by bacteria and to the strong mixing of the water column. The spatial variations showed the highest mean concentrations of dissolved organic matter (DOM) in the lower part of the Guadalquivir Estuary and the lowest mean concentrations of DOM in the bottom water of the oceanic zone. The very high mean DOC and DON concentrations measured off the Guadalquivir Estuary may be due to river input and/or re-suspension of the organic matter from the bottom sediments. The very low mean DOC and DON concentrations measured in the bottom water of the continental shelf may be due to mineralization and a surplus of more refractory organic carbon.

Changes in the chemistry of acidified Adirondack streams from the early 1980s to 2008

Lakes in the Adirondack region of New York have partially recovered in response to declining deposition, but information on stream recovery is limited. Here we report results of Adirondack stream monitoring from the early 1980s to 2008. Despite a 50% reduction in atmospheric deposition of sulfur, overall increases in pH of only 0.28 and ANC of 13 μeq L−1 were observed in 12 streams over 23 years, although greater changes did occur in streams with lower initial ANC, as expected. In the North Tributary of Buck Creek with high dissolved organic carbon (DOC) concentrations, SO42− concentrations decreased from 1999 to 2008 at a rate of 2.0 μmol L−1 y−1, whereas in the neighboring South Tributary with low DOC concentrations, the decrease was only 0.73 μmol L−1 y−1. Ca2+ leaching decreased in the North Tributary due to the SO42− decrease, but this was partially offset by an increase in Ca2+ leaching from increased DOC concentrations.

How does storage affect the quality and quantity of organic carbon in sewage for use in the bioremediation of acidic mine waters?

Pit lakes (abandoned flooded mine pits) represent a potentially valuable water resource. However, acid mine drainage (AMD) generation due to mining activities often results in pit lake waters with low pH, high sulphate and dissolved metal concentrations. Sulphate reduction-based bioremediation offers tremendous scope for removal of acidity and metals from pit lake water. In this study, the effect of storing sewage on its carbon quality for bioremediation of acidic pit lake water was studied. In addition, the effectiveness of labile organic carbon (lactic acid and ethanol) on SRB activity was tested. Bioremediation experiments were performed in controlled and replicated microcosms with acidic (pH 2.2) water from a pit lake by addition of stored (3 years at 4°C) sewage for stimulation of sulphate reducing bacteria (SRB) activity. This sewage had been previously used successfully in remediating to pH 7 water from this pit lake. The initial aim was to test the sewage at lower doses (18 and 28g/L) and in a pulsed addition (over 5 weeks). Bioremediation efficacy was evaluated by measuring pit lake water pH increase, redox potential decrease, and acidity and sulphate removal. Though the stored sewage had retained a very similar high total organic carbon (TOC) equivalent to prior to storage, it failed to increase dissolved organic carbon (DOC) levels in pit lake water. Microcosms amended with doubled doses of sewage and an extended remediation time still failed to demonstrate any substantial improvement in water quality, other than a small amount of sulphate reduction and direct neutralisation by the sewage. In order to determine if low DOC concentrations in sewage were the cause of the bioremediation failure, labile organic carbon (LOC), consisting of 50:50 (w/w) lactic acid and ethanol, was added to all microcosm treatments at concentrations of 3000, 6000 and 9000mg/L. After LOC addition, water quality improved with effective removal of acidity, sulphate and metals in the lowest carbon concentration (3000mg/L). However, 6000 and 9000mg/L LOC concentrations showed a delay in response due to the increased acidity associated with the lactic acid addition. The experiments showed that pulsed dosing of carbon simply slowed the commencement of remediation but it was ultimately able to reach the same effectiveness as the equivalent quantity added all at once. Prolonged storage of sewage leads to loss of LOC. In situ pit lake remediations which aim to make use of sewage as the main carbon source will need to factor in the storage time required to obtain sufficient sewage for the treatment into the design. Pulsing may help reduce issues with storage or supplementation with LOC may need to be considered. Results highlight that LOC is a more useful indicator of material effectiveness compared to a simple measures of TOC.

Removal of dissolved organic matter in municipal effluent with ozonation, slow sand filtration and nanofiltration as high quality pre-treatment option for artificial groundwater recharge

In the paper the combination process of ozonation, slow sand filtration (SSF) and nanofiltration (NF) was investigated with respect to dissolved organic matter (DOM) removal as high quality pre-treatment option for artificial groundwater recharge. With the help of ozonation leading to breakdown of the large organic molecules, SSF preferentially removes soluble microbial by-product-like substances and DOM with molecular weight (MW) less than 1.0 kDa. NF, however, removes aromatic, humic acid-like and fulvic acid-like substances efficiently and specially removes DOM with MW above 1.0 kDa. The residual DOM of the membrane permeate is dominated by small organics with MW 500 Da, which can be further reduced by the aquifer treatment, despite of the very low concentration. Consequently, the O 3/SSF/NF system offers a complementary process in DOM removal. Dissolved organic carbon (DOC) and trihalomethane formation potential (THMFP) can be reduced from 6.5 ± 1.1 to 0.7 ± 0.3 mg L −1 and from 267 ± 24 to 52 ± 6 μg L −1, respectively. The very low DOC concentration of 0.6 ± 0.2 mg L −1 and THMFP of 44 ± 4 μg L −1 can be reached after the aquifer treatment.

Effect of Dissolved Organic Carbon and Alkalinity on the Density of Arctic Ocean Waters

At constant temperature, the density of deep waters in the oceans is higher than that of surface waters due to the oxidation of plant material that adds NO3, PO4, and Si(OH)4, and the dissolution of CaCO3(s) that adds Ca2+ and HCO3. These increases in the density have been used to estimate the absolute salinity of seawater that is needed to determine its thermodynamic properties. Density (ρ), total alkalinity (TA), and dissolved organic carbon (DOC) measurements were taken on waters collected in the eastern Arctic Ocean. The results were examined relative to the properties of North Atlantic Waters. The excess densities (Δρ = ρMeas − ρCalc) in the surface Arctic waters were higher than expected (maximum of 0.008 kg m−3) when compared to Standard Seawater. This excess is due to the higher values of the normalized total alkalinity (NTA = TA * 35/S) (up to ~2,650 μmol kg−1) and DOC (up to ~130 μmol kg−1) resulting from river water input. New measurements are needed to determine how the DOC in the river waters contributes to the TA of the surface waters. The values of Δρ in deep waters are slightly lower (−0.004 ± 0.002 kg m−3) than that in Standard Seawater. The deep waters in the Arctic Ocean, unlike the Atlantic, Pacific, Indian, and Southern Oceans, do not have significant concentrations of silicate (maximum ~15 μmol kg−1) and that can affect the densities. Since the NTA of the deep Arctic waters (2,305 ± 6 μmol kg−1) is the same as Standard Seawater (2,306 ± 3 μmol kg−1), the decrease in the density may be caused by the lower concentrations of DOC in the deep waters (44–50 μmol kg−1 compared to the Standard Seawater value of 57 ± 2 μmol kg−1). The relative deficit of DOC (7–13 μmol kg−1) in the deep Arctic waters appears to cause the lower densities (−0.004 kg m−3) and Absolute Salinities (S A, −0.004 g kg−1). The effect of increases or decreases in Δρ and δS A due to DOC in other deep ocean waters may be hidden in the correlations of the changes with silicate. Further work is needed to separate the effects of SiO2 and DOC on the density of deep waters of the world oceans.

Spatial and temporal variability in the relationship between water colour and dissolved organic carbon in blanket peat pore waters

The transfer of carbon from terrestrial peat to the fluvial environment forms an important component of the peatland carbon cycle, and has major implications for water quality. Dissolved organic carbon (DOC) is generally considered the largest constituent of aquatic carbon and tends to be the most intensively monitored, particularly in peatland catchments. However, many long-term records for DOC are based on proxy studies that use water colour as a surrogate. This paper tests the robustness of using spectrophotometric techniques to monitor water colour, based on absorbance from a single wavelength at 400 nm, as a surrogate for true DOC determination. The general ability of spectrophotometric analysis to measure low DOC concentrations depends on the calibration used; thus, the minimum mass of DOC detectable varies considerably and in this study was found to be as high as 10.32 mg C L − 1 . While there is often a significant correlation between water colour and DOC, it was found that the use of single or even “pooled” regressions to predict DOC concentrations could result in miscalculations of more than 50%. Further, the water colour–DOC relationship in blanket peat pore waters was found to vary significantly between peat layers, land management treatments and through time. Thus, studies using long-term water colour records as a proxy for long-term DOC concentrations in peatlands must be treated with a certain degree of caution, especially in cases where changes may have taken place to DOC production, such as those caused by land management change, during the course of investigation.

Dynamics of dissolved organic 14C in throughfall and soil solution of a Norway spruce forest

Dissolved organic carbon (DOC) is an important component of the C cycle in forest ecosystems, but dynamics and origin of DOC in throughfall and soil solution are yet poorly understood. In a 2-year study, we analyzed the radiocarbon signature of DOC in throughfall and soil solution beneath the Oa horizon and at 90 cm depth in a Norway spruce forest on a Podzol soil. A two-pool mixing model revealed that throughfall DOC comprised mainly biogenic C, i.e. recently fixed C, from canopy leaching and possibly other sources. The contribution of fossil DOC from atmospheric deposition to throughfall DOC was on average 6% with maxima of 8–11% during the dormant season. In soil solution from the Oa horizon, DO14C signature was highly dynamic (range from −8‰ to +103‰), but not correlated with DOC concentration. Radiocarbon signatures suggest that DOC beneath the Oa horizon originated mainly from occluded and mineral associated organic matter fractions of the Oa horizon rather than from the Oi or Oe horizon. Relatively old C was released in the rewetting phase following a drought period in the late summer of 2006. In contrast, the DO14C signature indicated the release of younger C throughout the humid year 2007. In soil solutions from 90 cm depth, DO14C signatures were also highly dynamic (−127‰ to +3‰) despite constantly low DOC concentrations. Similar to the Oa horizon, the lowest DO14C signature at 90 cm depth was found after the rewetting phase in the late summer of 2006. Because of the variation in the DO14C signatures at this depth, we conclude that DOC was not equilibrated with the surrounding soil, but also originated from overlaying soil horizons. The dynamics of DO14C in throughfall and soil solution suggest that the sources of DOC are highly variable in time. Extended drought periods likely have a strong influence on release and translocation of DOC from relatively old and possibly stabilized soil organic matter fractions. Temporal variations as well as the input of fossil DOC needs to be considered when calibrating DOC models based on DO14C signatures.

Modeling the effect of water chemistry on the bioaccumulation of waterborne cadmium in zebra mussels

The present study aims at investigating the effects of Zn, Ca, and dissolved organic carbon (DOC) on the waterborne Cd bioaccumulation of a freshwater bivalve (Dreissena polymorpha). Mussels were exposed for 48 h at 3 µg/L of Cd in different media. Their physiological activities were assessed by separately measuring the filtration rate in the same exposure water. Increased Zn (from 3 to 89 µg/L) and Ca (from 37 to 131 mg/L) concentrations in water led to a threefold and sevenfold reduction of Cd bioaccumulation, whereas the effect of DOC varied greatly depending on its concentration. At low DOC concentrations (from 0.2 to 1.1 mg/L), the uptake of Cd increased, whereas at higher concentrations (from 1.1 to 17.1 mg/L), the uptake decreased. The filtration activity was not strongly influenced by either Zn or Ca concentration, whereas it was modified in enriched DOC media in the same manner as Cd uptake. A competitive model was built to predict the waterborne uptake rate constant of Cd (k (u)) as a function of Zn and Ca concentrations in the water. Over the range of DOC concentrations we tested, organic matter was shown to influence Cd bioaccumulation in two ways: by modifying Cd speciation and thus its bioavailability and its interaction with the biological membrane, and by affecting the mussel's physiology and therefore its sensitivity to metal. The present study provides a useful means of adjusting the toxicokinetic constant to the water's physicochemical characteristics and proposes a unifying model that takes into account the different geochemical and biological influences on bioaccumulation.

Influences of dissolved organic carbon on stream water chemistry in two forested catchments in central Sweden

Stream water chemistry in two headwater streams draining two small, adjacent catchments in Bispgarden, central Sweden was studied during the 2003 and 2004 growing seasons. The two catchments closely resemble each other in regard to size, shape and drainage density, with the major difference found in the area of wetland lining the streambeds. The emphasis of the study was to investigate the stream water chemistry of these closely resembling catchments, regarding the quality and quantity of dissolved organic carbon (DOC) and its influence on the concentrations of di- and trivalent cations. The streams showed significant differences in the content and size distribution of DOC and in the distribution of cations between the different size fractions. For both streams the high flow events induced by precipitation influenced the chemistry of the streams through increase of organic matter and its associated cations. Fanbergsbacken, with relatively low pH and high DOC concentration, had a greater amount of high molecular mass (HMM) DOC to which approximately 75% of Al and Fe and about 50% of Ca and Mg were associated. Grabergsbacken, with a higher pH and lower DOC level, had approximately 65% of Al, 40% of Fe and 30% of Ca and Mg associated to its HMM DOC fraction. Sixteen different low molecular mass organic acids were found in the stream water, of which oxalic and lactic acid were present in the highest concentrations.

Organic matter as a potential complementary tool for δ18O data interpretation in heterogeneous aquifers†

Evaluating transit time by using δ18O seasonal variation is often difficult in a Mediterranean context due to the erratic rainfall signature added to the complexity of flow mixing. The present study aims to show that using organic matter can improve interpretations of the δ18O signal. The natural fluorescence of organic compounds and dissolved organic carbon (DOC) data were recorded in the underground low-noise laboratory, located within the catchment area of the Fontaine de Vaucluse karstic system, over a four-year period. By combining both total fluorescence and DOC, a systematic seasonal variation is observed only due to soil–water interaction. Therefore, when the recharge rate is enough at the time of the season concerned, a new specific seasonal tracer, independent of rainfall signature, is available. Besides, the DOC concentration is a tracer of rapid infiltration with short transit time associated with high DOC concentration, while long transit time waters are characterised by a low DOC concentration. Then this can also shed light on such a mixing, recent/old waters. A more sensitive interpretation of δ18O variations is then possible: if the rainfall amount and δ18O follow a seasonal trend, both tracers may be used in the same way; if the recharge is discontinuous due to discontinuous rainfall regime, δ18O tracing alone is usable; in case of erratic or smooth rainfall signature during the homogeneous rainy period, fluorescent organic matter tracing alone is then usable.

Diets of crustacean zooplankton, inferred from stable carbon and nitrogen isotope analyses, in lakes with varying allochthonous dissolved organic carbon content

We used stable carbon and nitrogen isotope analyses to estimate the relative proportions of three putative food sources (1) algae, (2) allochthonous organic matter (but including also heterotrophic bacteria and green-sulphur bacteria having similar isotopic values) and (3) methane-oxidizing bacteria (MOB) in the diets of crustacean zooplankton in five small boreal lakes representing a gradient of dissolved organic carbon (DOC) concentration from ca. 5 to 40 mg C l−1. The lakes were sampled in May, after establishment of stratification, and again in October during autumnal mixing of the water column. IsoSource mixing model outputs indicated that the proportion of algae in the diets of zooplankton was generally higher in May than in October, and that bacteria contributed to the diets of both cladocerans and copepods. Our results indicate that bacteria, especially MOB, can make an appreciable contribution to zooplankton diets in these small lakes, even in those with relatively low DOC concentrations.