Dissolved Organic Carbon Export Research Articles

Controls on dissolved organic carbon export through surface runoff from loamy agricultural soils

Dissolved organic carbon (DOC) is one of the most active and mobile carbon pools, and thus an important component of the global carbon cycle. Previous research on DOC transport in the soil and on factors controlling DOC export towards the river system focused mainly on forest and wetland areas, with only limited information available from agricultural soils. We carried out rainfall simulations on agricultural field sites to identify the effect of soil properties, field characteristics and hydrological conditions on DOC export by surface runoff from loamy agricultural soils. Furthermore, the temporal evolution of DOC concentrations and specific UV absorbance (SUVA) values in runoff water during a rainfall event was monitored. Additional rainfall simulations in the lab allowed to investigate the effects of drop impact, crop residue incorporation and drying–rewetting of the soil on DOC concentrations and SUVA values in both runoff and percolation water.DOC concentrations were the highest and SUVA values the lowest at the start of a rainfall event, both in runoff and percolation water. Afterwards, DOC concentrations diminished and SUVA values rose to steady values towards the end of the experiments. Overall, rainfall conditions prior to the experiment showed to be a major control on DOC concentrations and quality in runoff water from agricultural fields. Smaller rainfall depths before the experiment and lower initial soil moisture content led to high concentrations of low aromatic DOC in the runoff water. This drying–rewetting effect on DOC concentrations and quality was also observed in the lab for percolation water. For the range of considered soil types, only a limited effect of soil and field characteristics on DOC concentrations and quality in runoff was observed. The effect of reduced tillage on DOC concentrations in surface runoff was ambiguous, with effects differing between experimental field sites.

Environmental dynamics of dissolved black carbon in wetlands

Wetlands are ecosystems commonly characterized by elevated levels of dissolved organic carbon (DOC), and although they cover a surface area less than 2 % worldwide, they are an important carbon source representing an estimated 15 % of global annual DOC flux to the oceans. Because of their unique hydrological characteristics, fire can be an important ecological driver in pulsed wetland systems. Consequently, wetlands may be important sources not only of DOC but also of products derived from biomass burning, such as dissolved black carbon (DBC). However, the biogeochemistry of DBC in wetlands has not been studied in detail. The objective of this study is to determine the environmental dynamics of DBC in different fire-impacted wetlands. An intensive, 2-year spatial and temporal dynamics study of DBC in a coastal wetland, the Everglades (Florida) system, as well as one-time sampling surveys for the other two inland wetlands, Okavango Delta (Botswana) and the Pantanal (Brazil), were reported. Our data reveal that DBC dynamics are strongly coupled with the DOC dynamics regardless of location, season or recent fire history. The statistically significant linear regression between DOC and DBC was applied to estimate DBC fluxes to the coastal zone through two main riverine DOC export routes in the Everglades ecosystem. The presence of significant amounts of DBC in these three fire-impacted ecosystems suggests that sub-tropical wetlands could represent an important continental-ocean carrier of combustion products from biomass burning. The discrimination of DBC molecular structure (i.e. aromaticity) between coastal and terrestrial samples, and between samples collected in wet and dry season, suggests that spatially-significant variation in DBC source strength and/or degree of degradation may also influence DBC dynamics.

Peat porewater dissolved organic carbon concentration and lability increase with warming: a field temperature manipulation experiment in a poor-fen

Studies conducted across northern Europe and North America have shown increases in dissolved organic carbon (DOC) in aquatic systems in recent decades. While there is little consensus as to the exact mechanisms for the increases in DOC, hypotheses converge on such climate change factors as warming, increased precipitation variability, and changes in atmospheric deposition. In this study, we tested the effects of warming on peat porewater composition by actively warming a peatland with infrared lamps mounted 1.24 m above the peat surface for 3 years. Mean growing season peat temperatures in the warmed plots (n = 5) were 1.9 ± 0.4 °C warmer than the control plots at 5 cm depth (t statistic = 5.03, p = 0.007). Mean porewater DOC concentrations measured throughout the growing season were 15 % higher in the warmed plots (73.4 ± 3.2 mg L−1) than in the control plots (63.7 ± 2.1 mg L−1) at 25 cm (t = 4.69, p < 0.001). Furthermore, DOC from the warmed plots decayed nearly twice as fast as control plot DOC in laboratory incubations, and exhibited lower aromaticity than control plot porewater (reduction in SUVA254 in heated plots compared with control plots). Dissolved organic nitrogen (DON) concentrations tracked DOC patterns as expected, but the amount of dissolved N per unit C decreased with warming. Previous work has shown that warming increased net primary production at this site, and together with measured increases in the activities of chitinases and glucosidases we suggest that the increased DOC concentrations observed with warming were derived in part from microbial-plant interactions in the rhizosphere. We also detected more nitrogen containing compounds with higher double bond equivalents (DBE) unique to the warmed plots, within the pool of biomolecules able to deprotonate (16 % of all compounds identified using ultrahigh resolution ion electrospray mass spectrometry); we suggest these compounds could be the products of increased plant, microbial, and enzyme activity occurring with warming. With continued warming in peatlands, an increase in relatively labile DOC concentrations could contribute to dissolved exports of DOC in runoff, and would likely contribute to the pool of efficient electron donors (and acceptors) in the production of CO2 and CH4 in terrestrial and aquatic environments.

Catchment-scale dissolved carbon concentrations and export estimates across six subarctic streams in northern Sweden

Abstract. Climatic change is currently enhancing permafrost thawing and the flow of water through the landscape in subarctic and arctic catchments, with major consequences for the carbon export to aquatic ecosystems. We studied stream water carbon export in several tundra-dominated catchments in northern Sweden. There were clear seasonal differences in both dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations. The highest DOC concentrations occurred during the spring freshet while the highest DIC concentrations were always observed during winter baseflow conditions for the six catchments considered in this study. Long-term trends for the period 1982 to 2010 for one of the streams showed that DIC concentrations has increased by 9% during the 28 yr of measurement while no clear trend was found for DOC. Similar increasing trends were also found for conductivity, Ca and Mg. When trends were discretized into individual months, we found a significant linear increase in DIC concentrations with time for September, November and December. In these subarctic catchments, the annual mass of C exported as DIC was in the same order of magnitude as DOC; the average proportion of DIC to the total dissolved C exported was 61% for the six streams. Furthermore, there was a direct relationship between total runoff and annual dissolved carbon fluxes for these six catchments. These relationships were more prevalent for annual DIC exports than annual DOC exports in this region. Our results also highlight that both DOC and DIC can be important in high-latitude ecosystems. This is particularly relevant in environments where thawing permafrost and changes to subsurface ice due to global warming can influence stream water fluxes of C. The large proportion of stream water DIC flux also has implications on regional C budgets and needs to be considered in order to understand climate-induced feedback mechanisms across the landscape.

Biogeochemical effects of simulated sea level rise on carbon loss in an Everglades mangrove peat soil

Saltwater intrusion and inundation can affect soil microbial activity, which regulates the carbon (C) balance in mangroves and helps to determine if these coastal forests can keep pace with sea level rise (SLR). This study evaluated the effects of increased salinity (+15 ppt), increased inundation (−8 cm), and their combination, on soil organic C loss from a mangrove peat soil (Everglades, Florida, USA) under simulated tides. Soil respiration (CO2 flux), methane (CH4) flux, dissolved organic carbon (DOC) production, and porewater nutrient concentrations were quantified. Soil respiration was the major pathway of soil organic C loss (94–98%) and was approximately 90% higher in the control water level than the inundated treatment under elevated salinity. Respiration rate increased with water temperature, but depended upon salinity and tidal range. CH4 flux was minimal, while porewater DOC increased with a concomitant, significant decline in soil bulk density under increased inundation. Porewater ammonium increased (73%) with inundation and soluble reactive phosphorus increased (32%) with salinity. Overall, the decline in soil organic C mineralization from combined saltwater intrusion and prolonged inundation was not significant, but results suggest SLR could increase this soil’s susceptibility to peat collapse and accelerate nutrient and DOC export to adjacent Florida Bay.

Interannual drought length governs dissolved organic carbon dynamics in blackwater rivers of the western upper Suwannee River basin

AbstractThe Little River (LR) in southern Georgia, U.S., has experienced lengthening droughts since monitoring began in 1972. We evaluated the impacts of drought on riverine carbon cycling using a 9 year data set of dissolved organic carbon (DOC) coupled with laboratory experiments in the LR, as well as long‐term data sets in three additional rivers within the Suwannee River basin. Longer drought periods reduced downstream DOC export but also led to higher DOC concentrations in the following hydroperiod. Within a hydroperiod, DOC concentration was positively correlated with temperature and negatively correlated with river discharge but also negatively correlated with total discharge during the previous hydroperiod. Among hydroperiods, DOC concentration was more strongly correlated with antecedent hydrological conditions than with current hydrological conditions across broad spatial scales: in three additional rivers within the Suwannee River basin (Alapaha, Withlacoochee, and Okapilco) and in headwater and downstream reaches of the LR. Microbial DOC consumption and CO2 production were elevated when DOC concentration was high. Despite dramatic hydrologic changes, DOC composition appeared stable, with optical analyses suggesting that LR DOC is primarily composed of three terrestrial humic‐like fluorescence groups. If the current climatic trend of intensifying droughts, elevated temperatures and decreased discharge continue, our results suggest the net effect may be for a more localized riverine carbon cycle with reduced downstream transport of DOC, but higher local mineralization rates due to elevated DOC concentrations.

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.

A Source of Terrestrial Organic Carbon to Investigate the Browning of Aquatic Ecosystems

There is growing evidence that terrestrial ecosystems are exporting more dissolved organic carbon (DOC) to aquatic ecosystems than they did just a few decades ago. This “browning” phenomenon will alter the chemistry, physics, and biology of inland water bodies in complex and difficult-to-predict ways. Experiments provide an opportunity to elucidate how browning will affect the stability and functioning of aquatic ecosystems. However, it is challenging to obtain sources of DOC that can be used for manipulations at ecologically relevant scales. In this study, we evaluated a commercially available source of humic substances (“Super Hume”) as an analog for natural sources of terrestrial DOC. Based on chemical characterizations, comparative surveys, and whole-ecosystem manipulations, we found that the physical and chemical properties of Super Hume are similar to those of natural DOC in aquatic and terrestrial ecosystems. For example, Super Hume attenuated solar radiation in ways that will not only influence the physiology of aquatic taxa but also the metabolism of entire ecosystems. Based on its chemical properties (high lignin content, high quinone content, and low C:N and C:P ratios), Super Hume is a fairly recalcitrant, low-quality resource for aquatic consumers. Nevertheless, we demonstrate that Super Hume can subsidize aquatic food webs through 1) the uptake of dissolved organic constituents by microorganisms, and 2) the consumption of particulate fractions by larger organisms (i.e., Daphnia). After discussing some of the caveats of Super Hume, we conclude that commercial sources of humic substances can be used to help address pressing ecological questions concerning the increased export of terrestrial DOC to aquatic ecosystems.

Factors controlling nitrogen and dissolved organic carbon exports across timescales in two watersheds with different land uses

AbstractInvestigating factors controlling the temporal patterns of nitrogen (N) and dissolved organic carbon (DOC) exports on the basis of a comparative study of different land uses is beneficial for managing water resources, especially in agricultural watersheds. We focused our research on an agricultural watershed (AW) and a forested watershed (FW) located in the Shibetsu watershed of eastern Hokkaido, Japan, to investigate the temporal patterns of N and DOC exports and factors controlling those patterns at different timescales (inter‐annual, seasonal, and hydrological event scales). Results showed that the annual patterns of N and DOC exports significantly varied over time and were probably controlled by climate. Higher discharge volumes in 2003, a wet year, showed higher N and DOC loadings in both watersheds. However, this process was also regulated by land use associated with N inputs. Higher concentrations and loadings were shown in the agricultural watershed. At the seasonal scale, N and DOC exports in the AW and the FW were more likely controlled by sources associated with land use. The Total N (TN) and Nitrate‐N (NO3−‐N) had higher concentrations during snowmelt season in the AW, which may be attributed to manure application in late autumn or early winter in the agricultural watershed. Concentrations of TN, NO3−‐N, dissolved organic nitrogen (DON), and DOC showed higher values during the summer rainy season in the FW, related to higher litter decomposition during summer and autumn and the fertilizer application in the agricultural area during summer. Higher DOC concentrations and loadings were observed during the rainy season in the AW, which is probably attributed to higher DOC production related to temperature and microbial activity during summer and autumn in grasslands. Correlations between discharge and concentrations differed during different periods or in different watersheds, suggesting that weather discharge can adequately represent the fact that N export depends on N concentrations, discharge level, and other factors. The differing correlations between N/DOC concentrations and the Si concentration indicated that the N/DOC exports might occur along different flow paths during different periods. During baseflow, the high NO3−‐N exports were probably derived from deep groundwater and might have percolated from uplands during hydrological events. During hydrological events, NO3−‐N exports may occur along near‐surface flow paths and in deep groundwater, whereas DOC exports could be related to near‐surface flow paths. At the event scale, the relationships between discharge and concentrations of N and DOC were regulated by antecedent soil moisture (shallow groundwater condition) in each watershed. These results indicated that factors controlling N and DOC exports varied at different timescales in the Shibetsu area and that better management of manure application during winter in agricultural lands is urgently needed to control water pollution in streams. Copyright © 2013 John Wiley &amp; Sons, Ltd.

The age of terrestrial carbon export and rainfall intensity in a temperate river headwater system

Riverine dissolved organic carbon (DOC) supports the production of estuaries and coastal ecosystems, constituting one of the most actively recycled pools of the global carbon cycle. A substantial proportion of DOC entering oceans is highly aged, but its origins remain unclear. Significant fluxes of old DOC have never been observed in temperate headwaters where terrestrial imports take place. Here, we studied the radiocarbon age of DOC in three streams draining forested headwater catchments of the river Mulde (Ore Mountains, Germany). In a 4 week summer precipitation event DOC aged at between 160 and 270 years was delivered into the watershed. In one stream, the DOC was modern but depleted in radiocarbon compared to other hydrological conditions. The yield was substantial and corresponded to 20–52 % of the annual DOC yields in wet and dry years, respectively. The analysis of long-term data suggested that the DOC export in extreme precipitation events added to the annual yield and was not compensated for by lower exports in remaining periods. We conclude that climate change, along with additional processes associated with human activities, channels old soil carbon into more rapidly cycled carbon pools of the hydrosphere.

Influence of stratification on marine dissolved organic carbon (DOC) dynamics: The Mediterranean Sea case

Vertical distributions of dissolved organic carbon (DOC) were determined in different seasons in the southern Adriatic and Tyrrhenian Seas to study the role of stratification in DOC patterns. These two seas are located at similar latitude but differ in extents of vertical stratification. Stratification affects DOC dynamics in both basins, but with interesting differences. In the Tyrrhenian Sea, where the upper water column was stratified during all cruises, DOC showed high surface layer values without substantial seasonality. By contrast, in the southern Adriatic Sea, the seasonal cycle of stratification forced opposite trends in the 0–50m and 50–800m stocks, with DOC removal from the upper 50m associated with DOC increase below 50m. Regarding DOC export via deep water formation in the southern Adriatic Sea, we estimate that 0.19TgCyr−1 was exported to the 50–800m layer by convective overturn, while 0.85–1.19TgCyr−1 was sequestered below 1000m due to a continental shelf pump mechanism. We hypothesize that enhanced stratification associated with a warmer ocean could further increase DOC concentrations in the mixed layer, changing the role of DOC in the oceanic carbon cycle.

Searching for similarity in topographic controls on carbon, nitrogen and phosphorus export from forested headwater catchments

Topography influences hydrological processes that in turn affect biogeochemical export to surface water on forested landscapes. The differences in long-term average annual dissolved organic carbon (DOC), organic and inorganic nitrogen [NO3−-N, dissolved organic nitrogen (DON)], and phosphorus (total dissolved phosphorus, TDP) export from catchments in the Algoma Highlands of Ontario, Canada, with similar climate, geology, forest and soil were established. Topographic indicators were designed to represent topographically regulated hydrological processes that influence nutrient export, including (1) hydrological storage potential (i.e. effects of topographic flats/depressions on water storage) and (2) hydrological flushing potential (i.e. effects of topographic slopes on potential for variable source area to expand and tap into previously untapped areas). Variations in NO3−-N export among catchments could be explained by indicators representing both hydrological flushing potential (91%, p < 0.001) and hydrological storage potential (65%, p < 0.001), suggesting the importance of hydrological flushing in regulating NO3−-N export as well as surface saturated areas in intercepting NO3−-N-loaded runoff. In contrast, hydrological storage potential explained the majority of variations among catchments in DON (69%, p < 0.001), DOC (94%, p < 0.001) and TDP (82%, p < 0.001) export. The lower explanatory power of DON (about 15% less) compared with that of DOC and TDP suggests another mechanism influencing N export, such as controls related to alternative fates of nitrogen (e.g. as gas). This study shows that simple topographic indicators can be used to track nutrient sources, sinks and their transport and export to surface water from catchments on forest landscapes. Copyright © 2013 John Wiley & Sons, Ltd.

Annual carbon balance of a peatland 10 yr following restoration

Abstract. Undisturbed peatlands represent long-term net sinks of carbon; however, peat extraction converts these systems into large and persistent sources of greenhouse gases. Although rewetting and restoration following peat extraction have taken place over the last several decades, very few studies have investigated the longer term impact of this restoration on peatland carbon balance. We determined the annual carbon balance of a former horticulturally-extracted peatland restored 10 yr prior to the study and compared these values to the carbon balance measured at neighboring unrestored and natural sites. Carbon dioxide (CO2) and methane (CH4) fluxes were measured using the chamber technique biweekly during the growing season from May to October 2010 and three times over the winter period. Dissolved organic carbon (DOC) export was measured from remnant ditches in the unrestored and restored sites. During the growing season the restored site had greater uptake of CO2 than the natural site when photon flux density was greater than 1000 μmol m−2 s−1, while the unrestored site remained a source of CO2. Ecosystem respiration was similar between natural and restored sites, which were both significantly lower than the unrestored site. Methane flux remained low at the restored site except from open water pools, created as part of restoration, and remnant ditches. Export of DOC during the growing season was 5.0 and 28.8 g m−2 from the restored and unrestored sites, respectively. Due to dry conditions during the study year all sites acted as net carbon sources with annual balance of the natural, restored and unrestored sites of 250.7, 148.0 and 546.6 g C m−2, respectively. Although hydrological conditions and vegetation community at the restored site remained intermediate between natural and unrestored conditions, peatland restoration resulted in a large reduction in annual carbon loss from the system resulting in a carbon balance more similar to a natural peatland.

Extreme storms and changes in particulate and dissolved organic carbon in runoff: Entering uncharted waters?

AbstractWe determined the runoff exports of particulate (POC) and dissolved organic carbon (DOC) from a 12 ha forested catchment which received more than 14 storm events in a 16‐month period and three extreme events associated with hurricanes. While POC and DOC exports for the small events were comparable, POC exports for the hurricane‐associated events were six to eight times the DOC values. Hurricane Irene alone contributed to 56% (21.2 kg C ha–1) and 19% (3.3 kg C ha–1) of the 2011 exports of POC and DOC, respectively. A precipitation threshold beyond which POC fluxes rapidly outpaced the DOC values was also identified. Our study suggests that large, high‐intensity storm events that are predicted to increase under future climate‐change scenarios will dramatically alter the runoff C regime by enhancing the POC inputs to aquatic ecosystems. Such shift in C forms could have important consequences for aquatic biota, atmospheric C cycling, and ecosystem and human health.

Effects of climate and land-surface processes on terrestrial dissolved organic carbon export to major U.S. coastal rivers

This study aims to understand the influences of climate change and land surface processes on the variation of in-stream DOC concentrations in coastal rivers crossing different climate zones. Monthly observations spanning multiple years in seven major rivers in four different climate zones within the U.S. were analyzed for correlations between dissolved organic carbon (DOC) concentration and surface air temperature, precipitation, land cover and discharge. The major watersheds were the Altamaha River (GA), the Apilachicola River (FL), the Columbia River (OR), the Delaware River (NJ), the Sacramento and San Joaquin Rivers (CA) and the Susquehanna River (MD). One minor watershed, the Neponset River (MA) was also analyzed. Results indicate that temperature is the most important variable for DOC export when the variation of annual mean temperatures is large (e.g., >5°C) with sufficient precipitation levels. Land-surface characteristics and discharges are better correlated to DOC concentrations when the variations of annual mean temperatures are small (e.g. <2°C). However, results from the small watershed (Neponset) showed that land surface processes can vary annual DOC concentrations about ±1.65mg/L from mean value. This study is the first to examine DOC relationships in watersheds in multiple climate zones, and it was determined that weak correlations between temperature and DOC found in previous studies may be attributable to the fact that those studies examined small watersheds contained within a single climate zone. DOC flux per square meter was calculated based on incremental temperature increases. The results indicate that an increase of 1°C would result in a 0.476mg/L increase of in-stream DOC in large watersheds. Climate warming would have a greater impact on riverine DOC yields in cooler climate zones (up to 26% per °C) than on those in warmer climate zones (up to 6% per °C).

Groundwater‐derived dissolved inorganic and organic carbon exports from a mangrove tidal creek: The missing mangrove carbon sink?

A majority of the global net primary production of mangroves is unaccounted for by current carbon budgets. It has been hypothesized that this “missing carbon” is exported as dissolved inorganic carbon (DIC) from subsurface respiration and groundwater (or pore‐water) exchange driven by tidal pumping. We tested this hypothesis by measuring concentrations and δ13C values of DIC, dissolved organic carbon (DOC), and particulate organic carbon (POC), along with radon (222Rn, a natural submarine groundwater discharge tracer), in a tidal creek in Moreton Bay, Australia. Concentrations and δ13C values displayed consistent tidal variations, and mirrored the trend in 222Rn in summer and winter. DIC and DOC were exported from, and POC was imported to, the mangroves during all tidal cycles. The exported DOC had a similar δ13C value in summer and winter (∼ −30‰). The exported δ13C‐DIC showed no difference between summer and winter and had a δ13C value slightly more enriched (∼ −22.5‰) than the exported DOC. The imported POC had differing values in summer (∼ −16‰) and winter (∼ −22‰), reflecting a combination of seagrass and estuarine particulate organic matter (POM) in summer and most likely a dominance of estuarine POM in winter. A coupled 222Rn and carbon model showed that 93–99% of the DIC and 89–92% of the DOC exports were driven by groundwater advection. DIC export averaged 3 g C m−2 d−1 and was an order of magnitude higher than DOC export, and similar to global estimates of the mangrove missing carbon (i.e., ∼ 1.9–2.7 g C m−2 d−1).

Concentrations and fluxes of dissolved organic carbon in runoff from a forested catchment: insights from high frequency measurements

Abstract. Concentrations of dissolved organic carbon (DOC) in runoff from catchments are often subject to substantial short-term variations. The aim of this study was to identify the compartmental sources of DOC in a forested catchment and the causes for short-term variations in runoff. Furthermore, we investigated the implication of short-term variations for the calculation of annual runoff fluxes. High frequency measurements (30 min intervals) of DOC in runoff, of discharge and groundwater table were conducted for one year in the 4.2 km2 forested Lehstenbach catchment, Germany. Riparian wetland soils represent about 30% of the catchment area. The quality of DOC was investigated by three dimensional fluorescence excitation-emission matrices in samples taken from runoff, deep groundwater and shallow groundwater from the riparian wetland soils. The concentrations of DOC in runoff were highly variable at an hourly to daily time scale, ranging from 2.6 mg L−1 to 34 mg L−1 with an annual average of 9.2 mg L−1. The concentrations were positively related to discharge, with a counter clockwise hysteresis. Relations of DOC to discharge were steeper and the degree of hysteresis larger in the summer/fall than in the winter/spring period. Dynamics of groundwater table, discharge, DOC concentrations and DOC quality parameters indicated that DOC in runoff originated mainly from the riparian wetland soils, both under low and high flow conditions. The annual export of DOC from the catchment was 84 kg C ha−1 yr−1 when calculated from the high frequency measurements. If the annual export was calculated by simulated samplings of &gt;2 days intervals substantial deviations resulted. Predicted changes in precipitation and discharge patterns as well as generally increasing temperatures likely will cause raising DOC exports from this catchment.

DOC-dynamics in a small headwater catchment as driven by redox fluctuations and hydrological flow paths – are DOC exports mediated by iron reduction/oxidation cycles?

Abstract. Dissolved organic carbon (DOC) exports from many catchments in Europe and North-America are steadily increasing. Several studies have sought to explain this observation. As possible causes, a decrease in acid rain or sulfate deposition, concomitant reductions in ionic strength and increasing temperatures were identified. DOC often originates from riparian wetlands; but here, despite higher DOC concentrations, ionic strength in pore waters usually exceeds that in surface waters. In the catchment under study, DOC concentrations were synchronous with dissolved iron concentrations in pore and stream water. This study aims at testing the hypothesis that DOC exports are mediated by iron reduction/oxidation cycles. Following the observed hydrographs, δ18O of water and DOC fluorescence, the wetlands were identified as the main source of DOC. Antecedent biogeochemical conditions, i.e., water table levels in the wetlands, influenced the discharge patterns of nitrate, iron and DOC during an event. The correlation of DOC with pH was positive in pore waters, but negative in surface waters; it was negative for DOC with sulfate in pore waters, but only weak in surface waters. Though, the positive correlation of DOC with iron was universal for pore and surface water. The decline of DOC and iron concentrations in transition from anoxic wetland pore water to oxic stream water suggests a flocculation of DOC with oxidising iron, leading to a drop in pH in the stream during high DOC fluxes. The pore water did not per se differ in pH. There is, thus, a need to consider processes more thoroughly of DOC mobilisation in wetlands when interpreting DOC exports from catchments. The coupling of DOC with iron fluxes suggested that increased DOC exports could at least, in part, be caused by increasing activities in iron reduction, possibly due to increases in temperature, increasing wetness of riparian wetlands, or by a shift from sulfate dominated to iron reduction dominated biogeochemical regimes.

Change in winter climate will affect dissolved organic carbon and water fluxes in mid‐to‐high latitude catchments

AbstractThere is a growing awareness that mid‐to‐high latitude regions will be strongly affected by climate change. These changes are predicted to be especially pronounced during winter, particularly at higher latitudes. To test how water quality in northern catchments could be affected by warmer winter climates, we assembled long‐term data from eight well‐studied catchments in Sweden, Scotland, Canada and the USA across a climatic gradient spanning from −2 to +9 °C in mean annual temperature and between −11.6 and + 6.1 °C in average winter temperature. We used the climatic gradient combined with inter‐annual variability among catchments to examine how warmer winters could affect the seasonality (seasonal timing) and synchroneity (coupling) of water and dissolved organic carbon (DOC) fluxes. In general, sites with colder winters (less than −5 °C) experienced an export concentrated in spring, whereas sites with warmer winters (&gt;0 °C) displayed a more evenly distributed export across all seasons. Catchments with warmer winters also displayed less synchroneity between water and DOC flux during winter compared with colder sites, whereas the opposite was found for the spring. Patterns from the climatic gradient were supported by inter‐annual variability at individual sites where both seasonality and synchroneity in the spring were related to the temperature during the preceding winter. Our findings suggest that one likely consequence of warmer winters in northern regions is that the proportion of the annual DOC and water export will increase during winter and decrease during spring and summer. This is of importance as it is the latter seasons during which downstream utilization of both water and DOC often is largest. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Simulation of monthly dissolved organic carbon concentrations in small forested watersheds

Dissolved organic carbon (DOC) plays an important role in both terrestrial and aquatic ecosystems. In-stream DOC concentration is an important indicator of water quality, because high in-stream DOC concentrations naturally lead to reductions in dissolved oxygen. Recent studies indicate that DOC is responsible in transferring toxic metals (e.g., mercury, Hg) from terrestrial sources to aquatic ecosystems. However, assessing DOC or Hg dynamics is fairly challenging as a result of extreme spatiotemporal variation. Typically, process-based models with short-to-medium term temporal resolutions are required to model DOC and Hg dynamics. The objective of this research is to develop a watershed-based monthly DOC production and export model that integrates the treatment of (i) forest-litter decomposition (DOC production), (ii) wetland-to-watershed area ratio (DOC storage), and (iii) relevant hydrological processes (DOC export) in the simulation of in-stream concentrations. Model results are compared against DOC concentrations collected at the stream outlets of two separate forested watersheds in Kejimkujik National Park, Atlantic Canada. Comparisons for the two watersheds show that predicted monthly DOC concentrations are generally in good agreement with field-based concentrations, giving R2-values of 0.61 and 0.63 for Pine Marten Brook watershed and 0.72 and 0.75 for Moose Pit Brook watershed for model calibration and validation, respectively.