Dissolved Organic Carbon Loads Research Articles

DOC and CO2-C Releases from Pristine and Drained Peat Soils in Response to Water Table Fluctuations: A Mesocosm Experiment

Hydrological conditions are considered to be among the main drivers influencing the export of dissolved organic carbon (DOC) from terrestrial to aquatic ecosystems, and hydrology is likely to alter due to climate change. We built a mesocosm experiment by using peat profiles from a pristine and from a drained (drained in 1978) peatland. A several-week-long low water table period followed by a high water table period, that is, a setting mimicking drought followed by flood, released relatively more DOC from pristine peat than from drained peat. From pristine peat profiles DOC was released into soil water in such quantities that the concentration of DOC remained stable despite dilution caused by added spring water to the mesocosms. In drained peat the DOC concentrations decreased during the high water table period indicating stronger dilution effect in comparison to pristine peat. At the landscape level DOC load from a drained peatland to the recipient water body may, however, increase during flooding because of high water runoff out of the peatland containing high DOC concentrations relative to the forest and agricultural areas. During the high water table period neither peat type nor water table had any clear impact on carbon dioxide (CO2-C) fluxes.

An investigation of size-fractionated organic matter from Lake Superior and a tributary stream using radiocarbon, stable isotopes and NMR

This study investigated the concentration and isotopic composition of different size fractions of organic matter (OM) in Lake Superior and in one of its many tributary streams and rivers (Amity Creek, Duluth, Minnesota, USA). Structural compositional drivers of the Δ14C of high molecular weight (HMW, >1kDa) dissolved organic carbon (DOC) in the lake were also evaluated. Low molecular weight (LMW, <1kDa) DOC was the fraction containing the largest proportion (68–88%) of organic carbon (OC) in the lake. Particulate organic carbon (POC, >0.7μm) was generally 13C-depleted (−29±1.2‰) relative to “bulk” (<0.7μm) DOC (−26.4±0.7‰), “init” (<0.2μm) DOC (−26.6±0.8‰), HMW DOC (−26.9±0.3‰) and LMW DOC (−26.5±0.9‰), and had more variable 14C content (Δ14C of −94‰ to 53‰; 735years BP to modern) than the other size fractions. Init DOC (Δ14C of 17–59‰), HMW DOC (Δ14C of 23–64‰) and LMW DOC (Δ14C of 16–62‰) all reflected contemporary (modern) radiocarbon signatures. Bulk DOC (Δ14C of −19‰ to 57‰; 90years BP to modern) had modern radiocarbon values in the offshore sites (Δ14C of 2–57‰) with pre-aged samples (Δ14C of −8‰ to −19‰) seen at the nearshore site. HMW DOM was relatively N-poor (C:N of 12–19) compared to particulate organic matter (POM, C:N of 8–10) revealing either a more diagenetically altered state or contrasting sources. 13C NMR data showed that biochemical composition of HMW DOC in Lake Superior was dominated by carbohydrates (53–65%) with only trace aromatic components (2–4%). Structurally complex components such as heteropolysaccharides (HPS), amide/peptides and amino sugars (AMS) constitute 75–84% of HMW DOC whereas carboxylic-rich alicyclic molecules (CRAM) made up 16–25% of HMW DOC in the lake. Combined HPS and AMS, O-alkyl carbohydrate carbon, and total carbohydrate carbon contents were significantly positively correlated to the Δ14C of HMW DOC suggesting they contribute a contemporary 14C-enriched component to the HMW DOC in the lake. In contrast, CRAM and aliphatic carbons were significantly inversely correlated with Δ14C of HMW DOC implying that these represent 14C-depleted (older) components of HMW DOC in the lake. At Amity Creek, storm events led to larger loads of bulk DOC and POC, which were both 14C-enriched (modern). However, in baseflow conditions 14C-depleted bulk DOC and POC were exported. LMW DOC from the creek was considerably 14C-depleted compared to concurrent HMW DOC.

Insights and issues with simulating terrestrial DOC loading of Arctic river networks

Terrestrial carbon dynamics influence the contribution of dissolved organic carbon (DOC) to river networks in addition to hydrology. In this study, we use a biogeochemical process model to simulate the lateral transfer of DOC from land to the Arctic Ocean via riverine transport. We estimate that, over the 20th century, the pan-Arctic watershed has contributed, on average, 32 Tg C/yr of DOC to river networks emptying into the Arctic Ocean with most of the DOC coming from the extensive area of boreal deciduous needle-leaved forests and forested wetlands in Eurasian watersheds. We also estimate that the rate of terrestrial DOC loading has been increasing by 0.037 Tg C/yr2 over the 20th century primarily as a result of climate-induced increases in water yield. These increases have been offset by decreases in terrestrial DOC loading caused by wildfires. Other environmental factors (CO2 fertilization, ozone pollution, atmospheric nitrogen deposition, timber harvest, agriculture) are estimated to have relatively small effects on terrestrial DOC loading to Arctic rivers. The effects of the various environmental factors on terrestrial carbon dynamics have both offset and enhanced concurrent effects on hydrology to influence terrestrial DOC loading and may be changing the relative importance of terrestrial carbon dynamics on this carbon flux. Improvements in simulating terrestrial DOC loading to pan-Arctic rivers in the future will require better information on the production and consumption of DOC within the soil profile, the transfer of DOC from land to headwater streams, the spatial distribution of precipitation and its temporal trends, carbon dynamics of larch-dominated ecosystems in eastern Siberia, and the role of industrial organic effluents on carbon budgets of rivers in western Russia.

Strong seasonal effect of moderate experimental warming on plankton respiration in a temperate estuarine plankton community

Climate change projections forecast a 1.1–6.4 °C global increase in surface water temperature and a 3 °C increase for the Baltic Sea. This study examined the short-term interactive effects of a realistic future temperature increase (3 °C) on pelagic respiration and bacterioplankton growth and phytoplankton photosynthesis in situ. This study was undertaken throughout a full seasonal cycle in the northern Baltic Sea. We found marked positive short-term effects of temperature on plankton respiration but no significant effect on bacterioplankton growth or phytoplankton photosynthesis. Absolute respiration rates remained similar to other comparable environments at the in situ temperature. With the 3 °C temperature increase, respiration rates in situ increased up to 5-fold during the winter and 2-fold during the summer. A maximum seasonal Q10 value of 332 was observed for respiration during the cold winter months (twater ≈ 0 °C), and summer Q10 values were comparatively high (9.1). Q10 values exhibited a significant inverse relationship to water temperature during winter. Our results thereby suggest that plankton respiration in this coastal zone is more temperature sensitive than previously reported. In addition, field data indicated that plankton respiration switched from being temperature limited to being limited by dissolved organic carbon (DOC) after the simulated temperature increase. Assuming that our observations are relevant over longer time scales, climate change may worsen hypoxia, increase CO2 emissions and create a more heterotrophic food web in coastal zones with a high load of riverine DOC.

The role of irrigation runoff and winter rainfall on dissolved organic carbon loads in an agricultural watershed

We investigated the role of land use/land cover and agriculture practices on stream dissolved organic carbon (DOC) dynamics in the Willow Slough watershed (WSW) from 2006 to 2008. The 415km2 watershed in the northern Central Valley, California is covered by 31% of native vegetation and the remaining 69% of agricultural fields (primarily alfalfa, tomatoes, and rice). Stream discharge and weekly DOC concentrations were measured at eight nested subwatersheds to estimate the DOC loads and yields (loads/area) using the USGS developed stream load estimation model, LOADEST. Stream DOC concentrations peaked at 18.9mgL−1 during summer irrigation in the subwatershed with the highest percentage of agricultural land use, demonstrating the strong influence of agricultural activities on summer DOC dynamics. These high concentrations contributed to DOC yields increasing up to 1.29gm−2 during the 6 month period of intensive agricultural activity. The high DOC yields from the most agricultural subwatershed during the summer irrigation period was similar throughout the study, suggesting that summer DOC loads from irrigation runoff would not change significantly in the absence of major changes in crops or irrigation practices. In contrast, annual DOC yields varied from 0.89 to 1.68gm−2yr−1 for the most agricultural watershed due to differences in winter precipitation. This suggests that variability in the annual DOC yields will be largely determined by the winter precipitation, which can vary significantly from year to year. Changes in precipitation patterns and intensities as well as agricultural practices have potential to considerably alter the DOC dynamics.

Dissolved organic matter in relation to nutrients (N and P) and heavy metals in surface runoff water as affected by temporal variation and land uses – A case study from Indian River Area, south Florida, USA

Transport of dissolved organic matter in runoff water plays a critical role in the increased inputs of nutrients (N and P) and metals (Fe, Cu and Zn) in water, and the deposition of muck sediments in the Saint Lucie Estuary and Indian River Lagoon, Florida. The objectives of this study were to investigate the variation of dissolved organic carbon (DOC) and dissolved nitrogen (DN) in surface runoff water with varying land uses, and to explore their possible relations with the leaching losses of nutrients N and P, and metals Fe, Cu, and Zn from different land uses. For this, surface runoff water from two major agricultural production systems (citrus grove and vegetable farm) in south Florida was collected weekly using pre-installed autosamplers during Jan. 2008–Mar. 2010. Meanwhile, runoff water from twelve field sites with varying land uses (such as ranch, golf course, and forest) was collected monthly using a grab sampler from Aug. 2008 to April, 2010, and soils were collected at the depth of 0–15cm and 15–30cm twice a year from autosampler-installed locations and on Nov. 25th, 2008 from grab sampling locations as well. In the field, water samples were measured for electrical conductivity, dissolved oxygen, and temperature; in the laboratory, water samples were analyzed for pH, total solids, turbidity, DOC, DN, total P (TP), and metals Cu, Fe and Zn with or without passing a 0.45μm filter membrane. After passing a 2mm sieve, fresh soil samples were analyzed for moisture, pH, electrical conductivity, 0.5M K2SO4 solution extractable DOC/DN, and Mehlich 3 extractable P, Fe, Cu, and Zn. Spatial and temporal variations in the concentrations of DOC and DN in runoff water were observed, with the greatest average concentrations of DOC and DN from ranch (∼28 and ∼2.3mgL−1) and the lowest from golf course (∼16mgL−1) for DOC and from forest and nursery for DN (∼1.5–1.6mgL−1), respectively. Temporal fluctuations of DOC and DN in runoff water from varying land uses were in good accord with rainfall intensity and frequency of rainfall. Temporal variations of TP and metals Fe, Cu, and Zn were influenced by climatic condition as well. Citrus grove exported greater amounts of DOC and DN in runoff water (27.6 and 3.51mgL−1 of median value) than vegetable farms (13.8 and 2.26mgL−1 of median value). Annual cumulative loads of DOC and total N in 2008, 2009 or 2010 were >20kgha−1, greater than those of TP (<13.5kgha−1) and metals Fe, Cu, and Zn (<1.5kgha−1). Ranch soil had the highest extractable DOC concentration (>100mgkg−1), accounting for 1% of its total organic carbon. Soil extractable DN had significant correlations with extractable P and Cu/Zn in soils. In conclusion, soil total organic carbon pool and its availability had an important effect on DOC and DN in runoff water. Land use type and rainfall event were the two influential factors affecting concentrations of DOC and DN, and the export of nutrients N and P, and metals in runoff water.

Crop Residue Biomass Effects on Agricultural Runoff

High residue loads associated with conservation tillage and cover cropping may impede water flow in furrow irrigation and thus decrease the efficiency of water delivery and runoff water quality. In this study, the biomass residue effects on infiltration, runoff, and export of total suspended solids (TSS), dissolved organic carbon (DOC), sediment-associated carbon (TSS-C), and other undesirable constituents such as phosphate (soluble P), nitrate (), and ammonium () in runoff water from a furrow-irrigated field were studied. Furrow irrigation experiments were conducted in 91 and 274 m long fields, in which the amount of residue in the furrows varied among four treatments. The biomass residue in the furrows increased infiltration, and this affected total load of DOC, TSS, and TSS-C. Net storage of DOC took place in the long but not in the short field because most of the applied water ran off in the short field. Increasing field length decreased TSS and TSS-C losses. Total load of , , and soluble P decreased with increasing distance from the inflow due to infiltration. The concentration and load of P increased with increasing residue biomass in furrows, but no particular trend was observed for and . Overall, the constituents in the runoff decreased with increasing surface cover and field length.

Contribution of different effluent organic matter fractions to membrane fouling in ultrafiltration of treated domestic wastewater

In the present work, effluent organic matter (EfOM) in treated domestic wastewater was separated into hydrophobic neutrals, colloids, hydrophobic acids, transphilic acids and neutrals and hydrophilic compounds. Their contribution to dissolved organic carbon (DOC) was identified. Further characterization was conducted with respect to molecular size and hydrophobicity. Each isolated fraction was dosed into salt solution to identify its fouling potential in ultrafiltration (UF) using a hydrophilized polyethersulfone membrane. The results show that each kind of EfOM leads to irreversible fouling. At similar delivered DOC load to the membrane, colloids present the highest fouling effect in terms of both reversible and irreversible fouling. The hydrophobic organics show much lower reversibility than the biopolymers present. However, as they are of much smaller size than the membrane pore opening, they cannot lead to such severe fouling as biopolymers do. In all of the isolated fractions, hydrophilics show the lowest fouling potential. For either colloids or hydrophobic substances, increasing their content in feedwater leads to worse fouling. The co-effect between biopolymers and other EfOM fractions has also been identified as one of the mechanisms contributing to UF fouling in filtering EfOM-containing waters.

Agriculture has changed the amount and composition of dissolved organic matter in Central European headwater streams

Dissolved organic matter (DOM) is an important part of the global carbon cycle and significantly influences aquatic ecosystem functions. Recent studies suggest that its amount and composition in freshwaters may be altered by agricultural land use, whereby the influence of preceding in-stream production and processing is not clear. To assess the land use effect on DOM amount and composition for the export from terrestrial to freshwater systems at the land–water interface, we sampled headwater streams draining agricultural and near-pristine catchments (forested and wetland) in the North German plains. To account for spatial and seasonal variation, we conducted a screening of DOM amount (53 sites) and composition (42 sites), and conducted bi-weekly samplings to investigate seasonal variation at eight sites over one year.Concentrations of dissolved organic carbon (DOC) were significantly higher for agricultural and wetland catchments than for forested catchments. Moreover, DOC loads exhibited higher seasonal variation for agricultural and wetland catchments than for forested catchments, which was due to higher variation in discharge. Parallel Factor Analysis revealed that the composition of DOM in agricultural catchments was significantly different from the other studied catchment types, and was characterized by low redox state and high structural complexity. Moreover, a gradient from protein- to humic-like fluorescence significantly separated forested from agricultural and wetland catchments. The contribution of humic-like DOM was strongly and positively related to DOC concentration, suggesting a mechanistic coupling of both. The effects of land use on patterns of DOC concentration and DOM composition were consistent across seasons, implying that land use strongly regulates DOM export. Overall, this study clearly shows the seasonally independent importance of agricultural land use for the amount and composition of DOM fluxes from the terrestrial zone to surface waters. These altered fluxes may affect ecosystem metabolism and health of agricultural headwaters and downstream situated aquatic ecosystems.

The influence of reservoirs, climate, land use and hydrologic conditions on loads and chemical quality of dissolved organic carbon in the Colorado River

Longitudinal patterns in dissolved organic carbon (DOC) loads and chemical quality were identified in the Colorado River from the headwaters in the Rocky Mountains to the United States–Mexico border from 1994 to 2011. Watershed‐ and reach‐scale climate, land use, river discharge and hydrologic modification conditions that contribute to patterns in DOC were also identified. Principal components analysis (PCA) identified site‐specific precipitation and reach‐scale discharge as being correlated with sites in the upper basin, where there were increases in DOC load from the upstream to downstream direction. In the lower basin, where DOC load decreased from upstream to downstream, sites were correlated with site‐specific temperature and reach‐scale population, urban land use and hydrologic modification. In the reaches containing Lakes Powell and Mead, the two largest reservoirs in the United States, DOC quantity decreased, terrestrially derived aromatic DOC was degraded and/or autochthonous less aromatic DOC was produced. Taken together, these results suggest that longitudinal patterns in the relatively unregulated upper basin are influenced by watershed inputs of water and DOC, whereas DOC patterns in the lower basin are reflective of a balance between watershed contribution of water and DOC to the river and loss of water and DOC due to hydrologic modification and/or biogeochemical processes. These findings suggest that alteration of constituent fluxes in rivers that are highly regulated may overshadow watershed processes that would control fluxes in comparable unregulated rivers. Further, these results provide a foundation for detailed assessments of factors controlling the transport and chemical quality of DOC in the Colorado River.

DIEL CHANGES OF DISSOLVED ORGANIC MATTER IN STREAMS OF VARYING WATERSHED LAND USE

ABSTRACTWe examined diel changes in dissolved organic matter (DOM) under summer low‐flow conditions for six streams selected across a gradient of agricultural land use intensity (21%–73% cropland). Trends in concentration [dissolved organic carbon (DOC) concentration] and optical properties of DOM showed a high degree of synchrony. During a 24‐h period, DOC changed by between 2.9 and 4.3 mg L−1. For all sites, the highest DOC was observed during dark periods and the lowest concentration during light periods. DOM sampled during daylight hours was more complex (humic) as determined using specific absorbance. DOM source, measured using fluorescence spectroscopy, usually varied during the 24‐h period, but few similarities in temporal fluorescence trends were identified between sites. Human landscape alterations appear to have little direct influence on the temporal nature of diel DOM dynamics in our study region. However, our results indicate that landscape characteristics are likely more broadly important in defining the magnitude temporal change in DOC. Woodland areas were negatively correlated with early afternoon DOC (r2 = 0.89, p = 0.005), positively correlated with diel percent change in DOC (r2 = 0.87, p = 0.007) and positively with diel change in discharge (r2 = 0.64, p = 0.054). Together, these results indicate the likely influence of differing riparian transpiration rates and changing subsurface inputs on DOC loading. Reduced DOC and discharge rates during daylight hours resulted in a mean underestimation of DOC export rate by 61% for early afternoon versus early morning sampling. Variation in the dominant source of DOM entering theses streams was strongly related to agricultural land use, and DOM in high cropland streams was more microbially derived. In addition, high cropland streams also exhibited the lowest minimum dissolved oxygen concentrations, a pattern related to high rates of productivity at these sites. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Designing a Dynamic Data-Driven Application System for Estimating Real-Time Load of Dissolved Organic Carbon in a River

Understanding the dynamics of naturally occurring dissolved organic carbon (DOC) in a river is central to estimating surface water quality, aquatic carbon cycling, and global climate change. Currently, determination of DOC in surface water is primarily accomplished by manually collecting samples for laboratory analysis, which requires at least 24 h. In other words, no effort has been devoted to monitoring real-time variations of DOC in a river due to the lack of suitable and/or cost-effective wireless sensors. However, when considering human health, carbon footprints, effects of urbanization, industry, and agriculture on water supply, timely DOC information may be critical. We have developed here a new paradigm of a dynamic data-driven application system (DDDAS) for estimating the real-time load of DOC into a river. This DDDAS was validated with field measurements prior to its applications. Results show that the real-time load of DOC in the river varied over a range from −13,143 to 29,248 kg/h at the selected site. The negative loads occurred because of the back flow in the estuarine reach of the river. The cumulative load of DOC in the river for the selected site at the end of the simulation (178 h) was about 1.2 tons. Our results support the utility of the DDDAS developed in this study for estimating the real-time variation of DOC in a river ecosystem.

Controls on the composition and lability of dissolved organic matter in Siberia's Kolyma River basin

High‐latitude northern rivers export globally significant quantities of dissolved organic carbon (DOC) to the Arctic Ocean. Climate change, and its associated impacts on hydrology and potential mobilization of ancient organic matter from permafrost, is likely to modify the flux, composition, and thus biogeochemical cycling and fate of exported DOC in the Arctic. This study examined DOC concentration and the composition of dissolved organic matter (DOM) across the hydrograph in Siberia's Kolyma River, with a particular focus on the spring freshet period when the majority of the annual DOC load is exported. The composition of DOM within the Kolyma basin was characterized using absorbance‐derived measurements (absorbance coefficienta330, specific UV absorbance (SUVA254), and spectral slope ratio SR) and fluorescence spectroscopy (fluorescence index and excitation‐emission matrices (EEMs)), including parallel factor analyses of EEMs. Increased surface runoff during the spring freshet led to DOM optical properties indicative of terrestrial soil inputs with high humic‐like fluorescence, SUVA254, and low SRand fluorescence index (FI). Under‐ice waters, in contrast, displayed opposing trends in optical properties representing less aromatic, lower molecular weight DOM. We demonstrate that substantial losses of DOC can occur via biological (∼30% over 28 days) and photochemical pathways (&gt;29% over 14 days), particularly in samples collected during the spring freshet. The emerging view is therefore that of a more dynamic and labile carbon pool than previously thought, where DOM composition plays a fundamental role in controlling the fate and removal of DOC at a pan‐Arctic scale.

Dissolved organic carbon mobilisation in relation to variable discharges and environmental flows in a highly regulated lowland river

The relationships between discharge and dissolved organic carbon (DOC) have been extensively studied in rainfall runoff-driven stream systems. Less is known about discharge and DOC relationships in river systems dependent on floodplain inundation. We examined DOC dynamics and mobilisation over low discharge periods and several larger discharge events in the highly regulated lowland Namoi River, Australia. Stable isotope signatures (δ13C) of various water-column fractions (e.g. 200 µm, fine particulate organic matter) were used to determine the sources of DOC. DOC concentrations over low discharge periods were fairly similar among sites and ranged between 5 and 10 mg L–1. Concentrations during a high-discharge event increased substantially with a mean of 20.4 mg L–1 and a maximum of 44 mg L–1. Significant positive linear relationships were found between DOC concentrations and discharge (P &lt; 0.001, r2 = 0.45). The 13C composition of DOC sampled across the three sites (e.g. –26.2‰) suggests a mixture of terrestrial and aquatic sources, with little downstream variation; however, we would envisage that during periods of high discharge allochthonous sources would dominate. Environmental flows (that are ecologically beneficial, potentially reversing changes brought about by flow regulation) have been allocated to the river, with the intention to increase the amount of DOC delivered to the river. The relationship between DOC and discharge was used to estimate DOC loads to the river under different modelled flow-management scenarios, including without environmental flow, with environmental flow, and simulated natural (low development) flow. On the basis of the modelling results, environmental flows should increase the amount of allochthonous DOC transported within the river in years with moderate and large flow events. Years with low flows did not deliver large loads of allochthonous DOC. The present results showed the potential variability in DOC delivery in relation to floodplain inundation in a lowland river that may not otherwise be detected in rainfall/runoff-driven headwater streams.

Diagnosis of dissolved organic matter removal by GAC treatment in biologically treated papermill effluents using advanced organic characterisation techniques

Granular activated carbon (GAC) exhaustion rates on pulp and paper effluent from South East Australia were found to be a factor of three higher (3.62cf. 1.47kgm−3) on Kraft mills compared to mills using Thermomechanical pulping supplemented by Recycled Fibre (TMP/RCF). Biological waste treatment at both mills resulted in a final effluent COD of 240mgL−1. The dissolved organic carbon (DOC) was only 1.2 times higher in the Kraft effluent (70 vs. 58mgL−1), however, GAC treatment of Kraft and TMP/RCF effluent was largely different on the DOC persisted after biological treatment. The molecular mass (636 vs. 534gmol−1) and aromaticity (5.35 vs. 4.67Lmg−1m−1) of humic substances (HS) were slightly higher in the Kraft effluent. The HS aromaticity was decreased by a factor of 1.0Lmg−1m−1 in both Kraft and TMP/RCF effluent. The molecular mass of the Kraft effluent increased by 50gmol−1 while the molecular mass of the TMP/RCF effluent was essentially unchanged after GAC treatment; the DOC removal efficiency of the GAC on Kraft effluent was biased towards the low molecular weight humic compounds. The rapid adsorption of this fraction, coupled with the slightly higher aromaticity of the humic components resulted in early breakthrough on the Kraft effluent. Fluorescence excitation–emission matrix analysis of the each GAC treated effluent indicated that the refractory components were higher molecular weight humics on the Kraft effluent and protein-like compounds on the TMP/RCF effluent. Although the GAC exhaustion rates are too high for an effective DOC removal option for biologically treated pulp and paper mill effluents, the study indicates that advanced organic characterisation techniques can be used to diagnose GAC performance on complex effluents with comparable bulk DOC and COD loads.

Fate of Allochthonous Dissolved Organic Carbon in Lakes: A Quantitative Approach

Inputs of dissolved organic carbon (DOC) to lakes derived from the surrounding landscape can be stored, mineralized or passed to downstream ecosystems. The balance among these OC fates depends on a suite of physical, chemical, and biological processes within the lake, as well as the degree of recalcintrance of the allochthonous DOC load. The relative importance of these processes has not been well quantified due to the complex nature of lakes, as well as challenges in scaling DOC degradation experiments under controlled conditions to the whole lake scale. We used a coupled hydrodynamic-water quality model to simulate broad ranges in lake area and DOC, two characteristics important to processing allochthonous carbon through their influences on lake temperature, mixing depth and hydrology. We calibrated the model to four lakes from the North Temperate Lakes Long Term Ecological Research site, and simulated an additional 12 ‘hypothetical’ lakes to fill the gradients in lake size and DOC concentration. For each lake, we tested several mineralization rates (range: 0.001 d−1 to 0.010 d−1) representative of the range found in the literature. We found that mineralization rates at the ecosystem scale were roughly half the values from laboratory experiments, due to relatively cool water temperatures and other lake-specific factors that influence water temperature and hydrologic residence time. Results from simulations indicated that the fate of allochthonous DOC was controlled primarily by the mineralization rate and the hydrologic residence time. Lakes with residence times <1 year exported approximately 60% of the DOC, whereas lakes with residence times >6 years mineralized approximately 60% of the DOC. DOC fate in lakes can be determined with a few relatively easily measured factors, such as lake morphometry, residence time, and temperature, assuming we know the recalcitrance of the DOC.

The supply and characteristics of colored dissolved organic matter (CDOM) in the Arctic Ocean: Pan Arctic trends and differences

A comprehensive data set of dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) absorption measurements is analysed in light of tracing the supply and distribution of dissolved organic matter in the Arctic Ocean. Two years of river data from six major Arctic rivers (Kolyma, Lena, Ob, Mackenzie, Yenisei, and Yukon) and measurements from a transect across the Arctic Ocean are presented. The results show that although the Lena River currently dominates the supply of DOC and CDOM, climate change induced increases in base flow discharge will likely increase the contribution of the Yenisei River. Seasonal variations in the spectral characteristics of CDOM in the rivers reflected the shift in the dominant source of organic matter from modern plant litter in the spring freshet to older more degraded material during winter low flow periods. Strong correlations were found between the river loading of CDOM and DOC across the systems studied indicating that in situ CDOM sensors could be used in the future to improve estimates of riverine DOC loading. CDOM in the surface waters of the Eurasian Basin was largely characterised as riverine material although extrapolations to riverine end member concentrations suggested that approximately half the riverine CDOM is removed during its transport across the shelf. In contrast the Canadian Basin surface waters were characterised by a much greater proportion of autochthonous CDOM. These differences in DOM quality in the surface waters of the two basins are hypothesised to also influence the extent to which material is remineralised during its passage through the Arctic Ocean.

Seasonal ice and hydrologic controls on dissolved organic carbon and nitrogen concentrations in a boreal‐rich fen

Boreal wetland carbon cycling is vulnerable to climate change in part because hydrology and the extent of frozen ground have strong influences on plant and microbial functions. We examined the response of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) across an experimental manipulation of water table position (both raised and lowered water table treatments) in a boreal‐rich fen in interior Alaska. DOC and TDN responses to water table manipulation exhibited an interaction with seasonal ice dynamics. We observed consistently higher DOC and TDN concentrations in the lowered water table treatment (71.7 ± 6.5 and 3.0 ± 0.3 mg−L) than in both the control (55.6 ± 5.1 and 2.3 ± 0.2 mg−L) and raised (49.1 ± 4.3 and 1.9 ± 0.1 mg L−1, respectively) water table treatments. Across all plots, pore water DOC concentrations at 20 cm increased as the depth to water table increased (R2 = 0.43, p &lt; 0.001). DOC concentrations also increased as the seasonal thaw depth increased, with solutes increasing most rapidly in the drained plot (R2 = 0.62, p &lt; 0.001). About half of the TDN pool was composed of dissolved organic N (DON). Inorganic N and DON were both highly correlated with changes in DOC, and their respective constraints to mineralization are discussed. These results demonstrate that a declining water table position and dryer conditions affect thaw depth and peat temperatures, and interactions among these ecosystem properties will likely increase DOC and TDN loading and potential for export in these systems.

Identifying sources of dissolved organic carbon in agriculturally dominated rivers using radiocarbon age dating: Sacramento–San Joaquin River Basin, California

We used radiocarbon measurements of dissolved organic carbon (DOC) to resolve sources of riverine carbon within agriculturally dominated landscapes in California. During 2003 and 2004, average Δ14C for DOC was −254‰ in agricultural drains in the Sacramento–San Joaquin Delta, −218‰ in the San Joaquin River, −175‰ in the California State Water Project and −152‰ in the Sacramento River. The age of bulk DOC transiting the rivers of California’s Central Valley is the oldest reported for large rivers and suggests wide-spread loss of soil organic matter caused by agriculture and urbanization. Using DAX 8 adsorbent, we isolated and measured 14C concentrations in hydrophobic acid fractions (HPOA); river samples showed evidence of bomb-pulse carbon with average Δ14C of 91 and 76‰ for the San Joaquin and Sacramento Rivers, respectively, with older HPOA, −204‰, observed in agricultural drains. An operationally defined non-HPOA fraction of DOC was observed in the San Joaquin River with seasonally computed Δ14C values of between −275 and −687‰; the source of this aged material was hypothesized to be physically protected organic-matter in high clay-content soils and agrochemicals (i.e., radiocarbon-dead material) applied to farmlands. Mixing models suggest that the Sacramento River contributes about 50% of the DOC load in the California State Water Project, and agricultural drains contribute approximately one-third of the load. In contrast to studies showing stabilization of soil carbon pools within one or two decades following land conversion, sustained loss of soil organic matter, occurring many decades after the initial agricultural-land conversion, was observed in California’s Central Valley.

Utilizing chromophoric dissolved organic matter measurements to derive export and reactivity of dissolved organic carbon exported to the Arctic Ocean: A case study of the Yukon River, Alaska

The quality and quantity of dissolved organic matter (DOM) exported by Arctic rivers is known to vary with hydrology and this exported material plays a fundamental role in the biogeochemical cycling of carbon at high latitudes. We highlight the potential of optical measurements to examine DOM quality across the hydrograph in Arctic rivers. Furthermore, we establish chromophoric DOM (CDOM) relationships to dissolved organic carbon (DOC) and lignin phenols in the Yukon River and model DOC and lignin loads from CDOM measurements, the former in excellent agreement with long‐term DOC monitoring data. Intensive sampling across the historically under‐sampled spring flush period highlights the importance of this time for total export of DOC and particularly lignin. Calculated riverine DOC loads to the Arctic Ocean show an increase from previous estimates, especially when new higher discharge data are incorporated. Increased DOC loads indicate decreased residence times for terrigenous DOM in the Arctic Ocean with important implications for the reactivity and export of this material to the Atlantic Ocean.