Geochemical characteristics and fluxes of organic carbon in a human-disturbed mountainous river (the Luodingjiang River) of the Zhujiang (Pearl River), China

This study aims to investigate the state of the riverine organic carbon in the Luodingjiang River under human impacts, such as reforestation, construction of reservoirs and in-stream damming. Seasonal and spatial characteristics of total suspended sediment (TSS), dissolved organic carbon (DOC) and particulate organic carbon (POC), as well as C/N ratios and the stable carbon isotopic signatures of POC (δ¹³C_POC) were examined based on a one-year study (2005) in the basin-wide scale. More frequent sampling was conducted in the outlet of the river basin at Guanliang hydrological station. DOC and POC concentrations showed flush effects with increasing water discharge and sediment load in the basin-wide scale. Atomic C/N ratio of POC had a positive relationship with TSS in the outlet of the basin, indicating the reduced aquatic sources and enhanced terrestrial sources during the high flood season. However, the similar relationship was not observed in the basin-wide scale mainly due to the spatial distributions of soil organic carbon and TSS. δ¹³C_POC showed obvious seasonal variations with enriched values in the period with high TSS concentration, reflecting the increased contribution from C₄ plants with enhanced soil erosion.The specific flux of the total organic carbon (2.30 t km^− 2 year^− 1) was smaller than the global average level. The ratio of DOC to POC was 1.17, which is higher than most rivers under Asian monsoon climate regime. The organic carbon flux was estimated to decline with decreasing sediment load as a result of reforestation, reservoir construction and in-stream damming, which demonstrates the impacts of human disturbances on the global carbon cycle.     

Carbon budget for a British upland peat catchment

This study describes the analysis of fluvial carbon flux from an upland peat catchment in the North Pennines. Dissolved organic carbon (DOC), pH, alkalinity and calcium were measured in weekly samples, with particulate organic carbon (POC) measured from the suspended sediment load from the stream outlet of an 11.4-km(2) catchment. For calendar year 1999, regular monitoring of the catchment was supplemented with detailed quasi-continuous measurements of flow and stream temperature, and DOC for the months September through November. The measurements were used to calculate the annual flux of dissolved CO(2), dissolved inorganic carbon, DOC and POC from the catchment and were combined with CO(2) and CH(4) gaseous exchanges calculated from previously published values and the observations of water table height within the peat. The study catchment represents a net sink of 15.4+/-11.9 gC/m(2)/yr. Carbon flows calculated for the study catchment are combined with values in the literature, using a Monte Carlo method, to estimate the carbon budget for British upland peat. For all British upland peat the calculation suggests a net carbon sink of between 0.15 and 0.29 MtC/yr. This is the first study to include a comprehensive study of the fluvial export of carbon within carbon budgets and shows the size of the peat carbon sink to be smaller than previous estimates, although sensitivity analysis shows that the primary productivity rather than fluvial carbon flux is a more important element in estimating the carbon budget in this regard.     

Ditch blocking, water chemistry and organic carbon flux: evidence that blanket bog restoration reduces erosion and fluvial carbon loss

The potential for restoration of peatlands to deliver benefits beyond habitat restoration is poorly understood. There may be impacts on discharge water quality, peat erosion, flow rates and flood risk, and nutrient fluxes. This study aimed to assess the impact of drain blocking, as a form of peatland restoration, on an upland blanket bog, by measuring water chemistry and colour, and loss of both dissolved (DOC) and particulate organic carbon (POC). The restoration work was designed to permit the collection of a robust experimental dataset over a landscape scale, with data covering up to 3 years pre-restoration and up to 3 years post-restoration. An information theoretic approach to data analyses provided evidence of a recovery of water chemistry towards more ‘natural’ conditions, and showed strong declines in the production of water colour. Drain blocking led to increases in the E4:E6 ratio, and declines in specific absorbance, suggesting that DOC released from blocked drains consisted of lighter, less humic and less decomposed carbon. Whilst concentrations of DOC showed slight increases in drains and streams after blocking, instantaneous yields of both DOC and POC declined markedly in streams over the first year post-restoration. Attempts were made to estimate total annual fluvial organic carbon fluxes for the study site, and although errors around these estimates remain considerable, there is strong evidence of a large reduction in aquatic organic carbon flux from the peatland following drain-blocking. Potential mechanisms for the observed changes in water chemistry and organic carbon release are discussed, and we highlight the need for more detailed information, from more sites, to better understand the full impacts of peatland restoration on carbon storage and release.     

Land management as a factor controlling dissolved organic carbon release from upland peat soils 1: Spatial variation in DOC productivity

The importance of soil storage in global carbon cycling is well recognised and factors leading to increased losses from this pool may act as a positive feedback mechanism in global warming. Upland peat soils are usually assumed to serve as carbon sinks, there is however increasing evidence of carbon loss from upland peat soils, and DOC concentrations in UK rivers have increased markedly over the past three decades. A number of drivers for increasing DOC release from peat soils have been proposed although many of these would not explain fine-scale variations in DOC release observed in many catchments.We examined the effect of land use and management on DOC production in upland peat catchments at two spatial scales within the UK. DOC concentration was measured in streams draining 50 small-scale catchments (< 3 km²) in three discrete regions of the south Pennines and one area in the North Yorkshire Moors. Annual mean DOC concentration was also derived from water colour data recorded at water treatment works for seven larger scale catchments (1.5-20 km²) in the south Pennines. Soil type and land use/management in all catchments were characterised from NSRI digital soil data and ortho-corrected colour aerial imagery.Of the factors assessed, representing all combinations of soil type and land use together with catchment slope and area, the proportion of exposed peat surface resulting from new heather burning was consistently identified as the most significant predictor of variation in DOC concentration. This relationship held across all blanket peat catchments and scales.We propose that management activities are driving changes in edaphic conditions in upland peat to those more favourable for aerobic microbial activity and thus enhance peat decomposition leading to increased losses of carbon from these environments.     

Drain blocking: an effective treatment for reducing dissolved organic carbon loss and water discolouration in a drained peatland

Peatlands are an important terrestrial carbon store. However, heightened levels of degradation in response to environmental change have resulted in an increased loss of dissolved organic carbon (DOC) and an associated rise in the level of discolouration in catchment waters. A significant threat to peatland sustainability has been the installation of artificial drainage ditches. However, recent restoration schemes have pursued drain blocking as a possible strategy for reducing degradation, fluvial carbon loss and water discolouration. This paper investigates the effect of open cut drainage and the impact of drain blocking on DOC and colour dynamics in blanket peat soil-water solutions. Three treatments (intact peat, drained peat and drain-blocked peat) were monitored in an upland blanket peat catchment in the UK. DOC and colour values were significantly higher on the drained slopes compared with those of the intact peat, which in turn had greater DOC and colour values than the drain-blocked slopes. Consequently, drain blocking is shown to be a highly successful technique in reducing both the DOC concentration and level of discolouration in soil waters, even to values lower than those observed for the intact site, which suggests a process of store exhaustion and flushing may operate. The colour per carbon unit (C/C) ratio was significantly higher at the drain-blocked site than either the intact or the drained treatments, while the E4/E6 ratio (fulvic acid/humic acid) was significantly lower at the blocked site compared to the two other treatments. The high C/C and low E4/E6 ratios indicate that drain blocking also modifies the composition of DOC, such that darker-coloured humic substances become more dominant compared to the intact site. This implies disturbance to DOC production and/or transportation processes operating within the peat.     

The influence of summer seasonal extremes on dissolved organic carbon export from a boreal peatland catchment: evidence from one dry and one wet growing season

A small peatland catchment in eastern Finland was monitored for runoff and export of dissolved organic carbon. To exemplify the response of a peatland under years of different meteorological conditions, data from one very dry and one wet growing season are presented. Runoff was 194 and 387 mm, and DOC export was 4.2 and 11.3 gC m^− 2 in the dry and wet year respectively. Analysis of the hydrographs showed that in both years most runoff and DOC export was generated during peak flow events, with low baseflow in the intermediate periods. Runoff response to rain events was strongest under a high water table in the spring and autumn, with a summer runoff minimum. The effect of drought on runoff occurred only after mid June when the effects of snowmelt had dissipated. Snowmelt therefore dominated DOC export in the dry year (61%) but contributed much less (29%) in the wet year. The relationship between runoff and the water table was highly similar in both years. Any variation that was observed herein was potentially related to a superficial subsidence of the peat surface under drier summer conditions and to raised spring water levels, causing restricted drainage due to flooding. The observed variation of DOC concentrations in both years generally had a limited impact on DOC export. During peak runoff events, concentrations dropped proportionally to the magnitude of the events. This decrease in available DOC is argued to be caused by a dominance of leaching over supply and production and by the specific hydraulic characteristics of the peatland surface. DOC concentrations were some 25% higher under drought conditions. These elevated concentrations were dispelled by spring snowmelt of the next year, demonstrating that a winter snowpack in boreal regions may be instrumental in limiting the longer term hydrological and biochemical effects of a drought.     

Runoff erosion and nutrient depletion in five Mediterranean soils of NE Spain under different land use

We investigated the influence of agricultural management and various plant covers related to the period of abandonment on soil properties, erosion and nutrient depletion in a typical Mediterranean area with sandy loam shallow soils. Cultivated soils (CS) with insufficient management, 5 year abandoned soils covered with meadow (A5), 25 year abandoned soils covered with dense scrubs (A25), 50 year abandoned soils covered with cork trees (A50) and soils in a 50 year pine reforested area (P50) were studied over a period of 6 months (May-October 1999). The soils were classified as Lithic Xerorthents. Both the differences in soil properties and response to rainfall events were mainly attributed to the different vegetation types and stages in land management. Principal components analysis (PCA) was performed on the results, by running the overall data determined after five rainfall events. The factors extracted by PCA of the samples by variables matrix represented the response of the environments to different rainfall intensities as a function of management or natural evolution after abandonment. CS environments showed the highest runoff and sediment yield as well as the highest amount of dissolved organic carbon (DOC) and nitrogen in runoff water. The sequence of abandonment (A5, A25 and A50) showed approximately the same runoff production, whereas eroded sediments (ES) and DOC were inversely correlated. Organic carbon in the ES and DOC in runoff water always increased with the period of abandonment, which accounted for consistent nutrient depletion. Nevertheless, the A50 environment (dominated by Quercus suber) showed the best soil properties, whilst the A25 environment with dense cover of Cistus monspeliensis and Calicotome espinosa seemed to cause a worsening effect on the soil's physical and chemical properties. This is probably because these environments are more severely damaged by wild fire occurrence. In terms of sediment yield, the P50 environment followed CS environment, indicating that reforestation followed by insufficient forest management may negatively affect both soil properties and response to the erosive action of rainfall.     

Temporal dynamics of dissolved and particulate organic carbon in the northern Adriatic Sea in relation to the mucilage events

The accumulation of dissolved and particulate organic matter may play an important role in mucilage formation in the northern Adriatic. Distributions of dissolved and particulate organic carbon were therefore investigated during the period June 1999-July 2002, when massive mucilage events occurred: in the summer of 2000 and, to a greater extent, of 2002. The seasonal variations in dissolved organic carbon (DOC) concentrations were significant, doubling in summer (up to 150 micromol L(-1)) with respect to winter. The particulate organic carbon (POC) variations were also very large, with a less pronounced seasonal pattern compared to DOC, because the POC changes are much more dependent both on river discharges and on phytoplankton blooms. The comparison of the concentrations between the period before (March-May) and after the onset of mucilage events (June-August) showed that DOC, but particularly POC, were higher in the period before the event of 2002, more markedly in the surface waters of low salinity. The POC increased, reaching mean concentrations of up to 36 micromol L(-1) in March 2002 before the outbreak of the massive mucilage formation in June. This suggests that POC may have a more important role in the mucilage formation than DOC. The highest seasonal variations of organic matter concentrations took place in the upper layer of lower salinity, stressing the importance of stratification and pycnoclines for accumulation and aggregation of the organic matter in the northern Adriatic. The POC contribution to the total organic carbon was low in the oligotrophic waters (DOC/POC ratio >15) and increased with the phytoplankton biomass in the productive waters (DOC/POC ratio <10). Particulate organic carbon predominated over the dissolved inside the mucilage aggregate (DOC/POC ratio <1), probably because aggregation processes, in which colloidal organic carbon is involved, are important. The organic carbon within the aggregates reached a concentration of 13.6 mmol L(-1) which was about 100 times more than in the surrounding waters or in the waters when the mucilages were absent. This indicates that distributions of organic carbon in the northern Adriatic can be extremely patchy during mucilage events.     

Land management as a factor controlling dissolved organic carbon release from upland peat soils 2: Changes in DOC productivity over four decades

Increasing DOC concentrations in surface waters have been observed across parts of Europe and North America over the past few decades. Most proposed explanations for these widespread trends invoke climate change or reductions in sulphate deposition. However, these factors do not seem apposite to explain either the fine-scale (within kilometres) or regional-scale spatial variation in DOC concentrations observed across the UK.We have reconstructed DOC concentrations and land use for one North Pennine and five South Pennine catchments (UK), located in three discrete areas, over the last four decades. Rainfall, temperature and sulphate deposition data, where available, were also collated and the potential influence of these factors on surface water DOC concentrations was assessed.Four of the six catchments examined showed highly significant (p < 0.001) increases (53-92%) in humic coloured DOC (hDOC) concentrations in drainage waters over the period 1990-2005. Changes in temperature and sulphate deposition may explain 20-30% of this trend in these four catchments. However, the rapid expansion of new moorland burn on blanket peat can explain a far greater degree (> 80%) of the change in hDOC. Far smaller increases in hDOC (10-18%) were identified for the two remaining catchments. These two sites experienced similar changes in sulphur deposition and temperature to those that had seen largest increases in DOC, but contained little or no moorland burn management on blanket peat.This study shows that regional-scale factors undoubtedly underlie some of the recent observed increases in drainage humic coloured DOC. However, changes in land management, in this case the extensive use of fire management on blanket peat, are a far more important driver of increased hDOC release from upland catchments in some parts of the UK. It suggests that the recent rapid increase in the use of burning on blanket peat moorland has implications for ecosystem services and carbon budgets.     

The effect of induced anoxia and reoxygenation on benthic fluxes of organic carbon, phosphate, iron, and manganese

Eutrophication causes seasonally anoxic bottom waters in coastal environments, but we lack information on effects of onset of anoxia and subsequent reoxygenation on benthic fluxes of redox-sensitive minerals and associated organic carbon (OC). As the first study, we determined the effect of inducing anoxia and subsequently restoring oxic conditions in mesocosms with surface sediment and water from a coastal environment. These concentration changes were compared with those in an oxygenated control. We determined water column concentrations of dissolved organic carbon (DOC), particulate organic carbon (POC), iron, manganese, and phosphate.Benthic fluxes of DOC, POC, and iron increased at the onset of anoxia in oxygen-depleted treatments. DOC and iron concentrations increased concomitantly towards maxima, which may have indicated reductive dissolution of FeOOH and release of associated OC. The subsequent concomitant concentration decreases may have been the result of coprecipitation of OC with iron-containing minerals. In contrast, the phosphate-concentration increase occurred several days after the onset of anoxia and the manganese concentration was not affected by the onset of anoxia. Restoring oxic conditions resulted in a decrease in DOC, POC, and phosphate concentrations, which may indicate coprecipitation of OC with phosphate-containing minerals. The high DOC fluxes at the onset of anoxia indicate that redox oscillations may be important in OC degradation. Further, our results indicate a close coupling between OC cycling and dissolution/precipitation of iron-containing minerals in intermittently anoxic sediments.     

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.     

Production of dissolved organic carbon (DOC) and trihalomethane (THM) precursor from peat soils

Water passing through the Sacramento-San Joaquin Delta contains elevated concentrations of dissolved organic carbon (DOC) and trihalomethane (THM) precursor relative to upstream waters from the Sacramento River and the San Joaquin River. Drainage from agricultural peat soils has been identified as one of the major sources of DOC and THM precursor. A series of controlled laboratory experiments were conducted to evaluate abiotic and biotic effects on the quantity and the nature of DOC and THM precursors produced from oxidized surface and reduced subsurface soils in the Delta. For abiotic effects, DOC was extracted from both soils with synthetic solutions containing a range of salinity (0-4 dS/m) and sodicity (0 to infinity ). The results showed that an increase in salinity significantly decreased the concentration of DOC in the soil-water from both soils but increased its aromaticity, as indicated by specific ultraviolet absorbance at 254 nm (SUVA). For biotic effects, peat soils were incubated over a range of temperatures (10 degrees C, 20 degrees C and 30 degrees C) and soil moisture contents (0.3-10 g water/g soil). After 8 weeks of incubation, only extracted DOC from flooded conditions and flooded and non-flooded cycles showed an increase in DOC. These findings indicate that neither salinity nor sodicity is the major factor for DOC production, but both can affect the solubility and mobility of DOC in the Delta soils. We believe wetting processes in oxidized peat soils produce significant amounts of DOC found in agricultural drainage discharged into the Delta waters.     

The multi-annual carbon budget of a peat-covered catchment

This study estimates the complete carbon budget of an 11.4 km² peat-covered catchment in Northern England. The budget considers both fluvial and gaseous carbon fluxes and includes estimates of particulate organic carbon (POC); dissolved organic carbon (DOC); excess dissolved CO₂; release of methane (CH₄); net ecosystem respiration of CO₂; and uptake of CO₂ by primary productivity. All components except CH₄ were measured directly in the catchment and annual carbon budgets were calculated for the catchment between 1993 and 2005 using both extrapolation and interpolation methods. The study shows that: Over the 13 year study period the total carbon balance varied between a net sink of − 20 to − 91 Mg C/km²/yr. The biggest component of this budget is the uptake of carbon by primary productivity (− 178 Mg C/km²/yr) and in most years the second largest component is the loss of DOC from the peat profile (+ 39 Mg C/km²/yr). Direct exchanges of C with the atmosphere average − 89 Mg C/km²/yr in the catchment. Extrapolating the general findings of the carbon budget across all UK peatlands results in an approximate carbon balance of − 1.2 Tg C/yr (± 0.4 Pg C/yr) which is larger than previously reported values. Carbon budgets should always be reported with a clear statement of the techniques used and errors involved as this is significant when comparing results across studies.     

Link between DOC in near surface peat and stream water in an upland catchment

Hydrologic transport of dissolved organic carbon (DOC) from peat soils may differ to organo-mineral soils in how they responded to changes in flow, because of differences in soil profile and hydrology. In well-drained organo-mineral soils, low flow is through the lower mineral layer where DOC is absorbed and high flow is through the upper organic layer where DOC is produced. DOC concentrations in streams draining organo-mineral soils typically increase with flow. In saturated peat soils, both high and low flows are through an organic layer where DOC is produced. Therefore, DOC in stream water draining peat may not increase in response to changes in flow as there is no switch in flow path between a mineral and organic layer. To verify this, we conducted a high-resolution monitoring study of soil and stream water at an upland peat catchment in northern England. Our data showed a strong positive correlation between DOC concentrations at -1 and -5 cm depth and stream water, and weaker correlations between concentrations at -20 to -50 cm depth and stream water. Although near surface organic material appears to be the key source of stream water DOC in both peat and organo-mineral soils, we observed a negative correlation between stream flow and DOC concentrations instead of a positive correlation as DOC released from organic layers during low and high flow was diluted by rainfall. The differences in DOC transport processes between peat and organo-mineral soils have different implications for our understanding of long-term changes in DOC exports. While increased rainfall may cause an increase in DOC flux from peat due to an increase in water volume, it may cause a decrease in concentrations. This response is contrary to expected changes in DOC exports from organo-mineral soils, where increase rainfall is likely to result in an increase in flux and concentration.     

Effect of moisture and temperature variation on DOC release from a peatland: conflicting results from laboratory, field and historical data analysis

Peatlands are large repositories of atmospheric carbon and concern has been raised over the stability of this carbon store because increasing dissolved organic carbon (DOC) concentrations have been observed in peatland drainage waters. A number of potential causes have been proposed in the literature, and conflicting results among studies conducted at different spatial and temporal scales suggest that the methodological approach may be an important confounding factor. The objective of this study was to determine the influence of moisture and temperature on DOC release from a south-central Ontario peatland during the fall (a major export period) following three commonly used approaches: laboratory microcosms, an intensive field study and analysis of long-term data (1980-2008). The effect of variations in temperature and moisture differed among microcosm, field study and analysis of the long-term record. Water content was important at the microcosm scale as DOC concentration and aromaticity increased with peat water-saturation. Drought caused a decrease in DOC concentration and pH, and an increase in sulphate and base cation concentrations. In contrast, the field study indicated that DOC concentration was strongly associated with temperature, and weakly correlated (negatively) with stream discharge. Average fall DOC concentration (but not export) increased over the 29 year record, and was correlated with fall discharge and precipitation (negative) and summer precipitation and fall stream pH (positive). As no common strong predictor of fall DOC was found at three scales of investigation at a single, well-studied site, it may be unreasonable to expect to identify a universal driver behind the widespread increase in DOC concentration.     

Production of dissolved organic carbon in aquatic sediment suspensions

In many water quality models production of dissolved organic carbon (DOC) is modelled as mineralisation from particulate organic matter (POM). In this paper it is argued that the DOC production from dessicated sediments by water turbulence may be of similar importance. The DOC production from sediments was measured under sterile conditions, as a function of shaking time, sediment to water ratio and OC content. Furthermore, the maximum fraction of particulate organic carbon (POC) that can be mobilised by agitation (excluding the contribution of mineralisation) was assessed. The experiments can be considered as laboratory simulations of resuspension of aquatic sediments due to turbulence caused by wind, benthivorous fish or dredging. It was found that aquatic resuspension of dessicated sediment may cause mobilisation of up to 100% of indigenous particulate carbon in a few weeks. An empirical equation was derived that expresses mobilised carbon (DOC) as a function of sediment concentration (POC), OC fraction (f(OC)) and time. At a typical value of 25mg/L suspended sediment, the halflife for mobilisation is 0.5 day, which is much faster than bacterial degradation of POM in aquatic systems.     

Biomarker signature in tropical wetland: lignin phenol vegetation index (LPVI) and its implications for reconstructing the paleoenvironment

Organic matter of a peat core (3.60 m, 7428 years BP) collected from Rawa Danau, west Java, Indonesia, was analyzed to evaluate the early diagenetic fates of lignin in a tropical wetland and to reconstruct past vegetation and climate changes. Vertical profiles of (Ad/Al)v, (Ad/Al)s, and lambda(8) show that the lignin composition is well preserved in a sub-aqueous environment under reducing conditions. The sedimentary terrigenous plant material at Rawa Danau is comprised predominantly of angiosperm wood. For this kind of tropical, diverse, and dynamic ecosystem, a new vegetation change index called lignin phenol vegetation index (LPVI): LPVI is defined using the lignin phenol composition. This index can sensitively detect terrestrial vegetation changes as well as environmental conditions forcing such changes. The LPVI of the Rawa Danau peat core provides better resolution than other lignin parameters used previously, and reveals four major vegetation change events since the mid-late Holocene. In comparison to other geochemical data (i.e. elemental carbon, isotopes, and hydrocarbons), the LPVI is more sensitive and is able to trace even minor vegetation and climate changes and thus could improve biogeochemical interpretations of peat records.     

Impacts of rainfall events on runoff water quality in an agricultural environment in temperate areas

Since a rise in dissolved organic carbon (DOC) concentrations has been observed for surface waters at least over the last two decades, a change in weather conditions (temperature and precipitations) has been proposed to partly explain this increase. While the majority of DOC delivery from soils to stream occurs during rainfall events, a better understanding of the rainfall influence on DOC release is needed. This study has been conducted in Brittany, western France, on agricultural experimental plots receiving either cattle manure (CM) or pig slurry (PS) as fertilizers in accordance with local practices. Each plot was instrumented with a flow meter and an auto sampler for runoff measurements. The results show that export of DOC during high intensity events is higher than during lower intensity rainfalls. Fertilization has a noticeable impact on total organic carbon (TOC) fluxes with an increase of five to seven folds for PS and CM respectively. If TOC shock load occurs shortly after the rainfall peak, DOC maximum appears with the first flush of the event. Organic carbon (OC) is mainly under colloidal (41.2%) and soluble (23.9%) forms during the first stage of a rainfall event and a control of rainfall intensity on OC colloidal transport is suggested. These findings highlight the potential risk of receiving water quality degradation due to the increase of heavier rainfall events with climate change in temperate areas.     

Application of ultrafiltration and stable isotopic amendments to field studies of mercury partitioning to filterable carbon in lake water and overland runoff

Results from pilot studies on colloidal phase transport of newly deposited mercury in lake water and overland runoff demonstrate that the combination of ultrafiltration, and stable isotope amendment techniques is a viable tool for the study of mercury partitioning to filterable carbon. Ultrafiltration mass balance calculations were generally excellent, averaging 97.3, 96.1 and 99.8% for dissolved organic carbon (DOC), total mercury (Hg(T)), and methylmercury (MeHg), respectively. Sub nanogram per liter quantities of isotope were measurable, and the observed phase distribution from replicate ultrafiltration separations on lake water agreed within 20%. We believe the data presented here are the first published colloidal phase mercury data on lake water and overland runoff from uncontaminated sites. Initial results from pilot-scale lake amendment experiments indicate that the choice of matrix used to dissolve the isotope did not affect the initial phase distribution of the added mercury in the lake. In addition there was anecdotal evidence that native MeHg was either recently produced in the system, or at a minimum, that this 'old' MeHg partitions to the same subset of DOC that binds the amended mercury. Initial results from pilot-scale overland runoff experiments indicate that less than 20% of newly deposited mercury was transported in the filterable fraction (<0.7 microm). There is some indication of colloidal phase enrichment of mercury in runoff compared to the phase distribution of organic carbon, but the mechanism of this enrichment is unclear. The phase distribution of newly deposited mercury can differ from that of organic carbon and native mercury, suggesting that the quality of the carbon (available ligands), not the quantity of carbon, regulates partitioning. Further characterization of DOC is needed to clarify the underlying mechanisms.     

Can climate change explain increases in DOC flux from upland peat catchments?

Long-term increases in DOC concentration in rivers draining areas of upland peat are a ubiquitous phenomenon in the UK. Several hypotheses have been proposed to explain these increases, but one compelling explanation is the observed long-term increase in temperature in UK uplands causing increases in peat decomposition rates, and increasing the depth of oxidation as evaporation increases depth to the water table. The study constructed an empirical model for water table depth and decomposition rate calibrated against observations from the Environmental Change Network monitoring site at Moor House in the North Pennines, UK. The study shows: (i) Depth of the water table has not changed significantly over a 30-year period, reflecting the fact that blanket peat is well buffered against climate change. (ii) Increases in temperature are responsible for a 12% increase in DOC production while an approximate 78% increase in DOC production has been observed. (iii) Overall DOC production is predicted to rise by 6% but observation suggests increases on the scale of 97%. (iv) The model inadequately represents changes in production and supply of DOC during periods of severe drought. The study shows that temperature change alone is insufficient to explain observed increases in DOC production. Alternative explanations for large increases in DOC production could include changes in land management, but an enzymic latch mechanism, i.e. derepression of anaerobic degradation, causing increased decomposition rates in response to severe drought is preferred.