Sources and flowpaths of dissolved organic carbon during storms in two forested watersheds of the Precambrian Shield

Dissolved organic carbon (DOC) concentrations and export were studied in two small catchments in central Ontario to examine DOC sources and to assess the hypothesis that organic matter adjacent to the stream is a significant contributor of DOC during storms. Different DOC dynamics and exports were observed according to the depth of the riparian water table. In Harp 4-21, riparian flowpaths were predominantly through A and upper B soil horizons and riparian soils contributed between 73 and 84% of the stream DOC export during an autumn storm. In Harp 3A, riparian flowpaths were predominantly through lower B horizons. Consequently, riparian soils were less important and hillslopes contributed more than 50% of the stream DOC export in subcatchments without wetlands during storms. Wetlands and adjacent soils contributed significantly to DOC export in Harp 3A; 8% of the total catchment area exported 32 to 46% of the storm runoff DOC. DOC export dynamics in wetlands and riparian soils were distinctly different. In wetlands, transport was affected by leaching and flushing of DOC at the wetland surface leading to lower DOC concentrations with successive storms. In riparian soils, groundwater flowpaths were more important and stronger positive relationships between discharge and DOC concentration were observed. Precipitation, throughfall and stemflow were minor sources of stream DOC during storms and contributed less than 20% of the total export.     

Contrasting nutrient exports from a forested and an agricultural catchment in south-eastern Australia

Dissolved organic carbon (DOC) and total and inorganic nitrogen and phosphorus concentrations were determined over 3 years in headwater streams draining two adjacent catchments. The catchments are currently under different land use; pasture/grazing vs plantation forestry. The objectives of the work were to quantify C and nutrient export from these landuses and elucidate the factors regulating export. In both catchments, stream water dissolved inorganic nutrient concentrations exhibited strong seasonal variations. Concentrations were highest during runoff events in late summer and autumn and rapidly declined as discharge increased during winter and spring. The annual variation of stream water N and P concentrations indicated that these nutrients accumulated in the catchments during dry summer periods and were flushed to the streams during autumn storm events. By contrast, stream water DOC concentrations did not exhibit seasonal variation. Higher DOC and NO₃ ⁻ concentrations were observed in the stream of the forest catchment, reflecting greater input and subsequent breakdown of leaf-litter in the forest catchment. Annual export of DOC was lower from the forested catchment due to the reduced discharge from this catchment. In contrast however, annual export of nitrate was higher from the forest catchment suggesting that there was an additional NO₃ ⁻ source or reduction of a NO₃ ⁻ sink. We hypothesize that the denitrification capacity of the forested catchment has been significantly reduced as a consequence of increased evapotranspiration and subsequent decrease in streamflow and associated reduction in the near stream saturated area.     

Seasonal Variations of Dissolved Nitrogen and DOC:DON Ratios in an Intermittent Mediterranean Stream

Seasonal variations of dissolved inorganic nitrogen (DIN) (NO₃–N and NH₄–N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km²) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000–2001 was 70 kg km⁻¹ year⁻¹, of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO₃–N, DON and NH₄–N, respectively.     

Hydrologic Drivers and Seasonality of Dissolved Organic Carbon Concentration, Nitrogen Content, Bioavailability, and Export in a Forested New England Stream

We present the results of a full year of high-resolution monitoring of hydrologic event-driven export of stream dissolved organic matter (DOM) from the forested Bigelow Brook watershed in Harvard Forest, Massachusetts, USA. A combination of in situ fluorescent dissolved organic matter (FDOM) measurement, grab samples, and bioassays was utilized. FDOM was identified as a strong indicator of concentration for dissolved organic carbon (DOC, r ² = 0.96), dissolved organic nitrogen (DON, r ² = 0.81), and bioavailable DOC (BDOC, r ² = 0.81). Relationships between FDOM and concentration were utilized to improve characterization of patterns of hydrological event-driven export and the quantification of annual export. This characterization was possible because DOM composition remained relatively consistent seasonally; however, a subtle shift to increased fluorescence per unit absorbance was observed for summer and fall seasons and percent BDOC did increase slightly with increasing concentrations. The majority of export occurred during pulsed hydrological events, so the greatest impact of bioavailable exports may be on downstream aquatic ecosystems. Export from individual events was highly seasonal in nature with the highest flow weighted mean concentrations (DOC_FW) being observed in late summer and fall months, but the highest total export being observed for larger winter storms. Seasonal trends in DOC export coincide with weather driven changes in surface and subsurface flow paths, potential for depletion and rebuilding of a flushable soil organic matter pool, and the availability of terrestrial carbon sources such as leaf litter. Our approach and findings demonstrate the utility of high frequency FDOM measurement to improve estimates of intra-annual temporal trends of DOM export.     

Effects of the Dry–Wet Hydrological Shift on Dissolved Organic Carbon Dynamics and Fate Across Stream–Riparian Interface in a Mediterranean Catchment

Abstract The stream–riparian interface, characterized by a dynamic and complex hydrology, is an important control point for nutrient fluxes and processing between terrestrial and aquatic systems. Predicted alterations in the discharge regime in Mediterranean climate regions make it necessary to understand the effects of abrupt hydrological transition between dry and wet conditions on the transport and fate of dissolved organic carbon (DOC) across the stream–riparian interface. In this study, the concentrations and fate of total DOC (TDOC) and a subset of four molecular weight fractions (<1 kDa, 1–10 kDa, 10–100 kDa, >100 kDa) were investigated in stream water and riparian groundwater during autumn of 2003 and 2004. The two study periods were characterized by contrasting antecedent hydrological conditions: the streamflow was interrupted in summer 2003 but was permanent in summer 2004. Comparison of the two study periods indicates that an abrupt dry–wet hydrological transition amplifies the water exchange across the stream–riparian interface and favors retention of up to 57% of the TDOC that flows across the interface. Furthermore, the efficiency of DOC retention across the stream-riparian interface also varies greatly depending on DOC molecular size. More than 70% of DOC fractions higher than 10 kDa were retained, whereas the smaller fraction (less than 1 kDa) was nearly conserved. Consequently, our study helps to clarify the effects of extreme hydrological events on DOC transport in running waters in Mediterranean regions.     

Effect of Reduced Winter Precipitation and Increased Temperature on Watershed Solute Flux, 1988–2002, Northern Michigan

Since 1987 we have studied weekly change in winter (December–April) precipitation, snowpack, snowmelt, soil water, and stream water solute flux in a small (176-ha) Northern Michigan watershed vegetated by 65–85 year-old northern hardwoods. Our primary study objective was to quantify the effect of change in winter temperature and precipitation on watershed hydrology and solute flux. During the study winter runoff was correlated with precipitation, and forest soils beneath the snowpack remained unfrozen. Winter air temperature and soil temperature beneath the snowpack increased while precipitation and snowmelt declined. Atmospheric inputs declined for H⁺, NO₃⁻, NH₄⁺, dissolved inorganic nitrogen (DIN), and SO₄²⁻. Replicated plot-level results, which could not be directly extrapolated to the watershed scale, showed 90% of atmospheric DIN input was retained in surface shallow (<15 cm deep) soils while SO₄²⁻ flux increased 70% and dissolved organic carbon (DOC) 30-fold. Most stream water base cation (C_B), HCO₃⁻, and Cl⁻ concentrations declined with increased stream water discharge, K⁺, NO₃⁻, and SO₄²⁻ remained unchanged, and DOC and dissolved organic nitrogen (DON) increased. Winter stream water solute outputs declined or were unchanged with time except for NO₃⁻ and DOC which increased. DOC and DIN outputs were correlated with the percentage of winter runoff and stream discharge that occurred when subsurface flow at the plot-level was shallow (<25 cm beneath Oi). Study results suggest that the percentage of annual runoff occurring as shallow lateral subsurface flow may be a major factor regulating solute outputs and concentrations in snowmelt-dominated ecosystems.     

Site‐to‐site variability and temporal trends of DOC concentrations and fluxes in temperate forest soils

Here, we report site‐to‐site variability and 12–14 year trends of dissolved organic carbon (DOC) from organic layers and mineral soils of 22 forests in Bavaria, Germany. DOC concentrations in the organic layer were negatively correlated with mean annual precipitation and elevation whereas air temperature had a positive effect on DOC concentrations. DOC fluxes in subsoils increased by 3 kg ha^?1 yr^?1 per 100 mm precipitation or per 100 m elevation. The highest DOC concentrations were found under pine stands with mor humus. Average DOC concentrations in organic layer leachates followed the order: pine>oak>spruce>beech. However, the order was different for mean DOC fluxes (spruce>pine>oak>beech) because of varying precipitation regimes among the forest types. In 12 of 22 sites, DOC concentrations of organic layer leachates significantly increased by 0.5 to 3.1 mg C L^?1 yr^?1 during the sampling period. The increase in DOC concentration coincided with decreasing sulfate concentration, indicating that sulfate concentration is an important driver of DOC solubility in the organic layer of these forest sites. In contrast to the organic layer, DOC concentrations below 60 cm mineral soil depth decreased by <0.1–0.4 mg C L^?1 yr^?1 at eight sites. The negative DOC trends were attributed to (i) increasing adsorption of DOC by mineral surfaces resulting from desorption of sulfate and (ii) increasing decay of DOC resulting from decreasing stabilization of DOC by organo‐Al complexes. Trends of DOC fluxes from organic layers were consistent with those of DOC concentrations although trends were only significant at seven sites. DOC fluxes in the subsoil were with few exceptions small and trends were generally not significant. Our results suggest that enhanced mobilization of DOC in forest floors contributed to the increase of DOC in surface waters while mineral horizons did not contribute to increasing DOC export of forest soils.     

Permafrost conditions in peatlands regulate magnitude,timing, and chemical composition of catchment dissolved organic carbon export

Permafrost thaw in peatlands has the potential to alter catchment export of dissolved organic carbon (DOC) and thus influence downstream aquatic C cycling. Subarctic peatlands are often mosaics of different peatland types, where permafrost conditions regulate the hydrological setting of each type. We show that hydrological setting is key to observed differences in magnitude, timing, and chemical composition of DOC export between permafrost and nonpermafrost peatland types, and that these differences influence the export of DOC of larger catchments even when peatlands are minor catchment components. In many aspects, DOC export from a studied peatland permafrost plateau was similar to that of a forested upland catchment. Similarities included low annual export (2–3 g C m^?2) dominated by the snow melt period (~70%), and how substantial DOC export following storms required wet antecedent conditions. Conversely, nonpermafrost fens had higher DOC export (7 g C m^?2), resulting from sustained hydrological connectivity during summer. Chemical composition of catchment DOC export arose from the mixing of highly aromatic DOC from organic soils from permafrost plateau soil water and upland forest surface horizons with nonaromatic DOC from mineral soil groundwater, but was further modulated by fens. Increasing aromaticity from fen inflow to outlet was substantial and depended on both water residence time and water temperature. The role of fens as catchment biogeochemical hotspots was further emphasized by their capacity for sulfate retention. As a result of fen characteristics, a 4% fen cover in a mixed catchment was responsible for 34% higher DOC export, 50% higher DOC concentrations and ~10% higher DOC aromaticity at the catchment outlet during summer compared to a nonpeatland upland catchment. Expansion of fens due to thaw thus has potential to influence landscape C cycling by increasing fen capacity to act as biogeochemical hotspots, amplifying aquatic C cycling, and increasing catchment DOC export.     

A worldwide view of organic carbon export from catchments

Growing interest in the effects of global change on the metabolism, stoichiometry and cycling of carbon in aquatic ecosystems has motivated research on the export of organic carbon (OCE) from catchments. In this article, quantitative and functional features of the annual export rates of total, particulate and dissolved organic carbon (TOC, POC and DOC) were reviewed, and the stoichiometry of export (OC:N, OC:P and N:P) from 550 catchments worldwide was reported. TOC export ranged 2.1–92,474?kg?C?km^?2?year^?1, POC export ranged 0.4–73,979?kg?C?km^?2?year^?1 and DOC export ranged 1.2–56,946?kg?C?km^?2?year^?1. Exports of TOC and DOC were strongly linked, but POC export was unrelated to DOC. The DOC fraction comprised on average 73?±?21% of TOC export. The export rates of organic carbon were poorly related to those of total nitrogen and total phosphorus. Discrete and continuous environmental variables failed to predict TOC export, but DOC export was influenced by discharge and catchment area worldwide. Models of OCE in different catchment types were controlled by different environmental variables; hydrological variables were generally better predictors of OCE than anthropogenic and soil variables. Elemental ratios of carbon export in most catchments were above the Redfield ratio, suggesting that phosphorus may become the limiting nutrient for downstream plant growth. These ratios were marginally related to environmental data. More detailed hydrological data, consideration of in-stream processes and the use of quasi-empirical dynamical models are advocated to improve our knowledge of OCE rates and those of other nutrients.     

Seasonal relationships between planktonic microorganisms and dissolved organic material in an alpine stream

The relationships between the abundance and activity of planktonic, heterotrophic microorganisms and the quantity and characteristics of dissolved organic carbon (DOC) in a Rocky Mountain stream were evaluated. Peak values of glucose uptake, 2.1 nmol L⁻¹ hr⁻¹, and glucose concentration, 333 nM, occurred during spring snowmelt when the water temperature was 4.0°C and the DOC concentration was greatest. The turnover time of thein situ glucose pool ranged seasonally from 40–1110 hours, with a mean of 272 hr. Seasonal uptake of³H-glucose, particulate ATP concentrations, and direct counts of microbial biomass were independent of temperature, but were positively correlated with DOC concentrations and negatively correlated with stream discharge. Heterotrophic activity in melted snow was generally low, but patchy. In the summer, planktonic heterotrophic activity and microbial biomass exhibited small-scale diel cycles which did not appear to be related to fluctuations in discharge or DOC, but could be related to the activity of benthic invertebrates. Leaf-packs placed under the snow progressively lost weight and leachable organic material during the winter, indicating that the annual litterfall in the watershed may be one source of the spring flush of DOC. These results indicate that the availability of labile DOC to the stream ecosystem is the primary control on seasonal variation in heterotrophic activity of planktonic microbial populations.     

Flux of organic carbon in a riverine mangrove wetland in the Florida Coastal Everglades

Short-term (daily) and seasonal variations in concentration and flux of dissolved organic carbon (DOC) were examined over 15 tidal cycles in a riverine mangrove wetland along Shark River, Florida in 2003. Due to the influence of seasonal rainfall and wind patterns on Shark River’s hydrology, samplings were made to include wet, dry and transitional (Norte) seasons. We used a flume extending from a tidal creek to a basin forest to measure vertical (vegetated soil/water column) and horizontal (mangrove forest/tidal creek) flux of DOC. We found significant (p < 0.05) variations in surface water temperature, salinity, conductivity, pH and mean concentration of DOC with season. Water temperature and salinity followed seasonal patterns of air temperature and rainfall, while mean DOC concentration was highest during the dry season (May), followed by the wet (October) and ‘Norte’ (December) seasons. This pattern of DOC concentration may be due to a combination of litter production and inundation pattern of the wetland. In contrast to daily (between tides) variation in DOC flux between the mangrove forest and tidal creek, daily variations of mean water quality were not significant. However, within-tide variation of DOC flux, dissolved oxygen content and salinity was observed. This indicated that the length of inundation and water source (freshwater vs. saltwater) variation across tidal cycles influenced water quality and DOC flux in the water column. Net DOC export was measured in October and December, suggesting the mangrove forest was a source of DOC to the adjacent tidal creek during these periods. Net annual export of DOC from the fringe mangrove to both the tidal creek and basin mangrove forest was 56 g C m⁻² year⁻¹. The seasonal pattern in our flux results indicates that DOC flux from this mangrove forest may be governed by both freshwater discharge and tidal range.     

Dissolved organic carbon concentrations in runoff from shallow heathland catchments: effects of frequent excessive leaching in summer and autumn

Transport and turnover of dissolved organic carbon (DOC) is important in the C cycle of organic soils. The concentration of DOC in soil water is buffered by adsorption to the soil matrix, and has been hypothesized to depend on the pool size of adsorbed DOC. We have studied the effect of frequent artificial excessive leaching events on concentration and flux of DOC in shallow, organic rich mountain soils. Assuming a constant Kd value for DOC adsorption to the soil matrix, we used these data to assess the change in the pool of adsorbed (or potential) DOC in the soil. The study involved manipulation of precipitation amount and frequency in summer and autumn in small, heathland catchments at Storgama, southern Norway. The shallow soils (16–34 cm deep on average) limit the possibility for changes in water flow paths during events. The mini-catchments range in size from 75 to 98 m². Our data show that after leaching of about 1.2 g DOC m⁻² the DOC concentration in runoff declines by approximately 50%. From this we conclude that the pool size of adsorbed potential DOC in the shallow soils at any time is of the order 2–3 g m⁻². Frequent episodes suggest that the replenishment rate, which depends on the decomposition rate of soil organic matter, is fast and the potential DOC pool could be fully restored probably within days during summer, but with some more time required in autumn, due to lower temperatures. Both pool size of potential DOC and replenishment rate are seasonally dependent. The pool of potential DOC, and thus the DOC concentration in discharge, is at their maximum in the growing season. However, under non-leaching conditions, the concentration of DOC in soil water and thus the pool size of potential DOC seems to level off, possibly due to conversion of DOC to less reversibly bound forms, or to further decomposition to CO₂.     

The significance of storms for the concentration and export of dissolved organic carbon from two Precambrian Shield catchments

Dissolved organic carbon (DOC) concentrations and DOC export arestudied during storms to examine the relationship between DOCconcentration and stream discharge and to assess the importance of stormson DOC export. Storms were monitored in seven subcatchments within twosmall watersheds (Harp 4--21 and Harp 3A) on the Precambrian Shield inCentral Ontario, Canada. Stream DOC concentrations increase during stormsby as much as 100% and 410% in Harp3A and Harp 4--21 respectively. The seasonal regression between DOC andstream discharge is significant in subcatchments without wetlands(r² > 0.7) but is not significant in thetwo subcatchments with small wetland areas (r² <0.06). On average, regressions based on weekly data yield accurate estimatesof DOC export but the variation in regressions among individual storms andthe small number of high DOC samples result in uncertainties of more than30% in DOC export. The period-weighted calculation ofDOC export from weekly data underestimates export by 14%and 22% in Harp 3A and Harp 4--21 respectively. Stormswere responsible for 57% to 68% of theDOC export in the autumn and 29% to 40%of the DOC export in the spring. A single large storm accounted for31% of the autumn DOC export in Harp 3A. The importanceof individual storms for DOC export and the variation in the relationshipbetween DOC and stream discharge among storms make it difficult to predictthe effects of climate change on DOC export and DOC concentrations.     

The influence of catchment characteristics on the seasonality of carbon and nitrogen species concentrations in upland rivers of Northern Scotland

Data from 13 catchments with no arable land in Northern Scotland were used to develop empirical linear regression models of average monthly NO₃ ^? concentrations and average summer and winter concentrations for NH₄ ⁺, dissolved organic N (DON) and dissolved organic carbon (DOC) as a function of catchment characteristics. All catchments displayed a pronounced seasonal NO₃ ^? cycle. Variation in monthly mean NO₃ ^? concentration within and between catchments could be predicted from mean monthly air temperature using separate regression equations for temperatures < and ≥ 5 °C. Soil type, climate and land use influenced NH₄ ⁺ concentrations. In summer, concentrations of NH₄ ⁺ were largest in catchments with extensive areas of brown forest soils, which are less acidic and more base-rich than other upland soils. However, concentrations declined with increasing conifer cover and summer rainfall. In winter, however, % conifer cover had a positive effect, while higher temperature and higher humus iron podzol cover had negative influences. DON concentration decreased with increasing catchment elevation in both summer and winter. Surprisingly, concentrations of DON only displayed a positive relationship with percentage peat cover in the summer. The most important factor controlling DOC concentration was soil type, with a positive relationship being observed between DOC and peat and humus iron podzol coverage. Elevation was also important, but only in the winter when concentrations were negatively correlated with maximum catchment elevation. Overall, multivariate regression equations explained the spatial and seasonal variability in N species concentrations over a range of catchments within Northern Scotland.     

Hydrological controls on dissolved organic carbon during snowmelt in the Snake River near Montezuma,Colorado

A quantitative understanding of the factors controlling the variation of dissolved organic carbon (DOC) in headwater streams is of scientific concern for at least two reasons. First, quantifying the overall carbon budgets of lotic systems is needed for a fundamental understanding of these systems. Second, DOC interacts strongly with other dissolved substances (heavy metals in particular) and plays an important role in the transport of contaminants.In the Snake River near Montezuma, Colorado, measurements of DOC from 1980 to 1986 show rapid decreases in concentration from a peak very early in the snowmelt period. Peak DOC concentrations occur approximately one month prior to peak discharge in the stream. The decline in DOC with time is approximately exponential, suggesting that a simple flushing mechanism can explain the response. We examined hydrological mechanisms to explain the observed variability of DOC in the Snake River by simulating the hydrological response of the catchment using TOPMODEL and routing the predicted flows through a simple model that accounted for temporal changes in DOC. Conceptually the DOC model represents a terrestrial (soil) reservoir in which DOC builds up during low flow periods and is flushed out by infiltrating meltwaters. The model reproduces the main features of the observed variation in DOC in the Snake River and thus lays the foundation for quantitatively linking hydrological processes with carbon cycling through upland catchments. Model results imply that a significant fraction of the soils in the Snake River catchment contribute DOC to the stream during peak discharge. Our work represents one of the first attempts to quantitatively describe the hydrological controls on DOC dynamics in a headwater stream. These controls are studied through the model by imposing mass balance constraints on both the flux of water through the various DOC source areas and the amount of DOC that can accumulate in these areas.     

Long term records of riverine dissolved organic matter

This presents the longest, consistent records of dissolved organic carbon in rivers ever published. This study presents long-term records of organic matter as indicated by water colour that were constructed for three catchments in Northern England for as far back as 1962. Observations show that there have been large increases in DOC concentrations over the period of study with in one case a doubling of the concentration over a period of 29 years. However, in one of the catchments no significant change was observed over a 31-year period. All catchments show common inter-annual control on carbon release in response to droughts, but no step increases in DOC concentrations were observed in response to such perturbations with pre-drought levels being restored within a period 3–4 years. Observed increasing trends do not correlate with changes in river discharge, pH, alkalinity or rainfall, but do coincide with increasing average summer temperatures in the region. The times series of DOC concentration over the period of the record appears stationary, but the distribution of daily values suggests a change in sources of colour over the increasing trend. The evidence supports a view that increases in carbon release are in equilibrium with temperature increases accentuated by land-use factors.     

Biogeochemical and hydrological controls on carbon export from a forested catchment in central Japan

Here we review research on the links between hydrological processes and the biogeochemical environment controlling the dynamics of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in temperate forested catchments. In addition, we present the results of original experiments. The spatial and temporal changes in DIC and DOC concentrations were investigated in tandem with observations of elementary belowground hydrological processes for a forested headwater catchment in central Japan. The soil CO₂ gas concentration, which is the source of DIC, increased with depth. The hydrological characteristics of groundwater also affected the spatial variation of partial pressure of dissolved CO₂ (pCO₂) in groundwater. The temporal variations in the soil CO₂ gas concentration and the pCO₂ values of groundwater suggested that the dynamics of DIC were strongly affected by biological activity. However, the geographical differences in DIC leaching were affected not only by the link between climatological conditions and biological activity, but also by other factors such as geomorphologic conditions. The DOC concentrations decreased with selective removal of hydrophobic acid during vertical infiltration. The major DOC-removal mechanisms were retention of metal-organic complexes to soil solids in the upper mineral soil layer and decomposition of DOC in the lower mineral soil layer. The responses of the DIC and DOC concentrations to changes in discharge during storm events were explained by the spatial variation in the DIC and DOC concentrations. Seasonal variation, which represents a long-term change, in stream water DOC concentrations was affected not only by the temporal variation in DOC concentrations in the topsoil, which may be affected by biological activity, but also by water movement, which transports DOC from the topsoil to stream water. These results indicate that both a biogeochemical approach and a method for evaluating the hydrological effects on carbon dynamics are critical for clarifying the carbon accumulation-and-release processes in forested ecosystems.     

Seasonal and hydrologic drivers of dissolved organic matter and nutrients in the upper Kuparuk River, Alaskan Arctic

As the planet warms, widespread changes in Arctic hydrology and biogeochemistry have been documented and these changes are expected to accelerate in the future. Improved understanding of the behavior of water-borne constituents in Arctic rivers with varying hydrologic conditions, including seasonal variations in discharge?Cconcentration relationships, will improve our ability to anticipate future changes in biogeochemical budgets due to changing hydrology. We studied the relationship between seasonal water discharge and dissolved organic carbon and nitrogen (DOC and DON) and nutrient concentrations in the upper Kuparuk River, Arctic Alaska. Fluxes of most constituents were highest during initial snowmelt runoff in spring, indicating that this historically under-studied period contributes significantly to total annual export. In particular, the initial snowmelt period (the stream is completely frozen during the winter) accounted for upwards of 35% of total export of DOC and DON estimated for the entire study period. DOC and DON concentrations were positively correlated with discharge whereas nitrate (NO₃ ^?) and silicate were negatively correlated with discharge throughout the study. However, discharge-specific DOC and DON concentrations (i.e. concentrations compared at the same discharge level) decreased over the summer whereas discharge-specific concentrations of NO₃ ^? and silicate increased. Soluble reactive phosphorus (SRP) and ammonium (NH₄ ⁺) were negatively correlated with discharge during the spring thaw, but were less predictable with respect to discharge thereafter. These data provide valuable information on how Arctic watershed biogeochemistry will be affected by future changes in temperature, snowfall, and rainfall in the Arctic. In particular, our results add to a growing body of research showing that nutrient export per unit of stream discharge, particularly NO₃ ^?, is increasing in the Arctic.     

Composition and variations in the occurrence of dissolved free simple organic compounds of an unproductive lake ecosystem in northern Sweden

Low molecular weight organic carbon compounds are potentially important carbon and energy substrates to heterotrophic production in the aquatic environment. We studied the occurrence of dissolved free amino acids (AA), monosaccharides (CHO), and carboxylic acids (CA) in the subarctic Lake Diktar-Erik. The lake is unproductive with slightly humic water, and receives water via one major inlet stream draining a birch forest environment. The concentration of dissolved organic carbon (DOC) in the inlet stream was strongly correlated with the discharge. This relationship changed from season to season, indicating changes in the sources of the DOC entering the stream. AA and CHO each accounted for an average of less than 0.5% of the DOC. After high discharge events during the ice-free period, AA and CHO occurred in especially high concentrations. CA occurred in higher concentrations during the ice-free period, when it generally accounted for 20–30% of the DOC pool. The CA content relative to the total DOC pool was strongly inversely correlated with overall DOC concentration, and at low DOC levels the relative content of CA was high and vice versa. This followed a seasonal trend, with CA accounting for a smaller proportion of the DOC in winter and a larger part in spring/early summer. A conservative estimate suggested that the studied simple organic carbon compounds potentially could cover 30% of the bacterial gross production in the lake and therefore potentially also was an important source of CO₂ that occur in supersaturated concentrations in the lake.     

Export of dissolved organic carbon and nitrogen from Gleysol dominated catchments – the significance of water flow paths

In this study, we estimated whether changes in hydrological pathwaysduring storms could explain the large temporal variations of dissolvedorganic carbon (DOC) and nitrogen (DON) in the runoff of threecatchments: a forest and a grassland sub-catchment of 1600m² delineated by trenches, and a headwater catchment of 0.7km².The average annual DOC export from the sub-catchments was 185 kg DOCha^–1 y^–1 for the forest, 108 kg DOCha^–1 y^–1 for the grassland and 84 kgDOC ha^–1 y^–1 for the headwatercatchment. DON was the major form of the dissolved N in soil and streamwater. DON export from all catchments was approximately 6 kg Nha^–1 y^–1, which corresponded to 60% ofthe total N export and to 50% of the ambient wet N deposition. DOC andDON concentrations in weekly samples of stream water were positivelycorrelated with discharge. During individual storms, concentrations andproperties of DOC and DON changed drastically. In all catchments, DOCconcentrations increased by 6 to 7 mg DOC l^–1 comparedto base flow, with the largest relative increment in the headwatercatchment (+350%). Concentrations of DON, hydrolysable amino acids, andphenolics showed comparable increases, whereas the proportion ofcarbohydrates in DOC decreased at peak flow. Prediction of DOC and DONconcentrations by an end-member mixing analysis (EMMA) on the base ofinorganic water chemistry showed that changes in water flow pathslargely explained these temporal variability. According to the EMMA, thecontribution of throughfall to the runoff peaked in the initial phase ofthe storm, while water from the subsoil dominated during base flow only.EMMA indicated that the contribution of the DOC and DON-rich topsoil washighest in the later stages of the storm, which explained the highestDOC and DON concentrations as the hydrograph receded. Discrepanciesbetween observed and predicted concentrations were largest for thereactive DOC compounds such as carbohydrates and phenolics. Theyoccurred at base flow and in the initial phase of storms. This suggeststhat other mechanisms such as in-stream processes or a time-variantrelease of DOC also played an important role.