Dissolved Organic Carbon Export Research Articles

Distribution, seasonality, and fluxes of dissolved organic matter in the Pearl River (Zhujiang) estuary, China

Abstract. Dissolved organic carbon (DOC) concentration in the Pearl River estuary (PRE) of China was measured in May, August, and October 2015 and January 2016. Chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) in the latter three seasons were characterized by absorption and fluorescence spectroscopy. CDOM and FDOM exhibited negligible seasonal variations, while DOC displayed a significant seasonality, with the average concentration being highest in May (156 µmol L−1), lowest in November (87 µmol L−1), and comparable between January (118 µmol L−1) and August (112 µmol L−1). Although DOC, CDOM, and FDOM in surface water were generally higher than in bottom water, the difference between the two layers was statistically insignificant. DOC showed little cross-estuary variations in all seasons, while CDOM and FDOM in January were higher on the west side of the estuary than on the east side. All three variables showed rapid drawdowns in the head region of the estuary (salinity <5); their dynamics in the main estuary were primarily controlled by conservative mixing, leading to linearly declining or relatively constant (for DOC in May and November only) contents with increasing salinity. The decrease in FDOM with salinity was 5 %–35 % faster than that of CDOM, which in turn was 2–3 times quicker than that of DOC. Salinity and CDOM absorption coefficients could serve as indicators of DOC in August and January. Freshwater endmembers in all seasons mainly contained fresh, protein-rich DOM of microbial origin, a large part of it likely being pollution-derived. Protein-like materials were preferentially consumed in the head region but the dominance of the protein signature was maintained throughout the estuary. Exports of DOC and CDOM (in terms of the absorption coefficient at 330 nm) into the South China Sea were estimated as 195×109 g and 266×109 m2 for the PRE and 362×109 g and 493×109 m2 for the entire Pearl River Delta. The PRE presents the lowest concentrations and export fluxes of DOC and CDOM among the world's major estuaries. DOM delivered from the PRE is, however, protein-rich and thus may enhance heterotrophs in the adjacent coastal waters. Overall, the PRE manifests lower abundance and smaller spatiotemporal variability of DOM than expected for a sizable estuary with a marked seasonality of river runoff due supposedly to the poorly forested watershed of the Pearl River, the rapid degradation of the pollution-derived DOM in the upper reach, and the short residence time of freshwater.

Environmental Controls on the Riverine Export of Dissolved Black Carbon

AbstractEach year, tropical rivers export a dissolved organic carbon (DOC) flux to the global oceans that is equivalent to ~4% of the global land sink for atmospheric CO2. Among the most refractory fractions of terrigenous DOC is dissolved black carbon (DBC), which constitutes ~10% of the total DOC flux and derives from the charcoal and soot (aerosol) produced during biomass burning and fossil fuel combustion. Black carbon (BC) has disproportionate storage potential in oceanic pools and so its export has implications for the fate and residence time of terrigenous organic carbon (OC). In contrast to bulk DOC, there is limited knowledge of the environmental factors that control riverine fluxes of DBC. We thus completed a comprehensive assessment of the factors controlling DBC export in tropical rivers with catchments distributed across environmental gradients of hydrology, topography, climate, and soil properties. Generalized linear models explained 70 and 64% of the observed variance in DOC and DBC concentrations, respectively. DOC and DBC concentrations displayed coupled responses to the dominant factors controlling their riverine export (soil moisture, catchment slope, and catchment stocks of OC or BC, respectively) but varied divergently across gradients of temperature and soil properties. DBC concentrations also varied strongly with aerosol BC deposition rate, indicating further potential for deviation of DBC fluxes from those of DOC due to secondary inputs of DBC from this unmatched source. Overall, this study identifies the specific drivers of BC dynamics in river catchments and fundamentally enhances our understanding of refractory DOC export to the global oceans.

Hurricane pulses: Small watershed exports of dissolved nutrients and organic matter during large storms in the Southeastern USA

Extreme weather events, such as hurricanes, can cause ecological disturbances that alter energy and nutrients across terrestrial-aquatic boundaries. Yet, relatively few studies have considered the impacts of extreme weather events on biogeochemical dynamics in watersheds at larger spatial scales. Here, we assessed the effects of Hurricanes Harvey and Irma on the export of dissolved organic matter (DOM) and nutrients in ten watersheds from five southeastern states of the United States. We quantified the magnitude of dissolved organic carbon (DOC) and nutrients exported during the storms and assessed the changes in DOM sources and bioreactivity after storms. Our results show that the storm-mobilized DOC and nutrients fluxes were primarily driven by water discharge. The proportions of terrestrial, humic-like DOM compounds increased, and percent autochthonous, protein-like DOM decreased during high flows. Percent bioreactive DOC decreased with increasing discharge. Bioreactivity increased with increasing nitrate concentration, but decreased as percent terrestrial humic-like DOM, aromaticity, and molecular weight increased. These observations suggest that storms may have shifted flow paths to shallower depths that promoted the addition of biorefractory organic matter from topsoils into the water column. Notably, the total flux of bioreactive DOC was at least nearly twice as high at peak discharge, indicating materials transported by large storm flows could strongly enhance microbial activity in streams, although the position of storm-mediated microbial hotspots would depend on the flow rate and other instream parameters. Additionally, compared to forest-dominated watersheds, urban watersheds exported high loads of nutrients and bioreactive DOC, and a wetland-dominated watershed had a prolonged, but relatively subdued export of DOC and nutrients. Together, our findings highlight the ecological significance of extreme weather and climate events in leading to rapid, large-magnitude changes in energy and nutrient availability within drainage networks, and the potential interactions between land use and climate change on watershed biogeochemistry.

Riverine dissolved organic carbon and its optical properties in a permafrost region of the Upper Heihe River basin in the Northern Tibetan Plateau

Riverine dissolved organic carbon (DOC) and its optical properties were investigated in two sub-river basins (Yeniugou and Hulugou river) of the Upper Heihe river basin in the northern Tibetan Plateau. The results showed that DOC concentrations ranged from 0.25 to 12.2 and 0.18–1.04 mg L−1 for Yeniugou and Hulugou river basin with an average of 0.82 and 0.33 mg L−1, respectively. Export of DOC from the studied river (YNG: ~0.86 Gg C yr−1) was lesser compared with other large river in the Tibetan Plateau and Arctic regions because of the small drainage area and lower DOC concentrations. There exhibited significant seasonality for DOC in Yeniugou river basin with higher values observed during late spring and summer; however, no such distinct trend was observed for DOC in the studied rivers of Hulugou river basin. In contrast, total dissolved nitrogen showed a slightly lower value during the summer season. A strong relationship was determined between DOC concentrations and spectral UV absorbance at 254 nm (SUVA254), absorption coefficients and spectral slope for both sub-river basins, attributing that the riverine DOM in the northern Tibetan Plateau has a remarkably high content of aromatic compounds in late spring and summer. Considering the less snow cover percentage, this study highlights the potential impacts of permafrost thaw on the riverine DOC and its characteristics in the permafrost region under climate change.

Estuarine Dissolved Organic Carbon Flux From Space: With Application to Chesapeake and Delaware Bays

AbstractThis study uses a neural network model trained with in situ data, combined with satellite data and hydrodynamic model products, to compute the daily estuarine export of dissolved organic carbon (DOC) at the mouths of Chesapeake Bay (CB) and Delaware Bay (DB) from 2007 to 2011. Both bays show large flux variability with highest fluxes in spring and lowest in fall as well as interannual flux variability (0.18 and 0.27 Tg C/year in 2008 and 2010 for CB; 0.04 and 0.09 Tg C/year in 2008 and 2011 for DB). Based on previous estimates of total organic carbon (TOCexp) exported by all Mid‐Atlantic Bight estuaries (1.2 Tg C/year), the DOC export (CB + DB) of 0.3 Tg C/year estimated here corresponds to 25% of the TOCexp. Spatial and temporal covariations of velocity and DOC concentration provide contributions to the flux, with larger spatial influence. Differences in the discharge of fresh water into the bays (74 billion m3/year for CB and 21 billion m3/year for DB) and their geomorphologies are major drivers of the differences in DOC fluxes for these two systems. Terrestrial DOC inputs are similar to the export of DOC at the bay mouths at annual and longer time scales but diverge significantly at shorter time scales (days to months). Future efforts will expand to the Mid‐Atlantic Bight and Gulf of Maine, and its major rivers and estuaries, in combination with coupled terrestrial‐estuarine‐ocean biogeochemical models that include effects of climate change, such as warming and CO2 increase.

Carbon and mercury export from the Arctic rivers and response to permafrost degradation

Arctic rivers export a large amount of organic carbon (OC) and mercury (Hg) to Arctic oceans. Because there are only a few direct calculations of OC and Hg exports from these large rivers, very little is known about their response to changes in the active layer in northern permafrost-dominated areas. In this study, multiyear data sets from the Arctic Great Rivers Observatory (ArcticGRO) are used to estimate the export of dissolved organic carbon (DOC), particulate organic carbon (POC), total mercury (THg) and methylmercury (MeHg) from the six largest rivers (Yenisey, Lena, Ob, Mackenzie, Yukon and Kolyma) draining to the Arctic Ocean. From 2003 to 2017, annual DOC and POC export to the Arctic Ocean was approximately 21612 Gg and 2728 Gg, and the exports of Hg and MeHg to the Arctic Ocean were approximately 20090 kg and 110 kg (0.002% of the total Hg stored in the northern hemisphere active layer). There were great variations in seasonal OC and Hg concentrations and chemical characteristics, with higher fluxes in spring and lower fluxes in winter (baseline). DOC and Hg concentrations are significantly positively correlated to discharge, as discharge continues to increase in response to a deepening active layer thickness during recent past decades. This study shows that previous results likely underestimated DOC exports from rivers in the circum-Arctic regions, and both OC and Hg exports will increase under predicted climate warming scenarios.

Supply, Demand, and In-Stream Retention of Dissolved Organic Carbon and Nitrate During Storms in Mediterranean Forested Headwater Streams

The capacity of headwater streams to transform and retain organic matter and nutrients during base flow conditions has been largely demonstrated in the literature. Yet, most solute export occurs during storms, and thus, it becomes essential to understand the role of in-stream processes on regulating solute concentrations and exports during storm flow conditions. In this study, we explored patterns of solute supply, solute demand, and resulting in-stream solute retention for a number of individual storms from two Mediterranean streams (intermittent and perennial) that together encompassed a wide range of hydrological conditions. Our results indicate that >70% of the individual storms were chemodynamic (i.e. solute concentrations either increased or decreased with increasing discharge) at the two sites and for both dissolved organic carbon (DOC) and nitrate (NO3-). At the perennial stream, DOC and NO3- concentrations did not show any clear pattern of storm response during both dry and wet periods, though deviations from chemostasis were generally larger for those events showing higher concentrations during storm flow. At the intermittent stream, DOC and NO3- showed positive divergences from chemostasis during the wet period. In this site, DOC showed no clear pattern of storm response during the dry period, while many storms showed low NO3- concentrations compared to chemostasis, suggesting either limited NO3- sources or in-stream retention. At the two streams, in-stream biogeochemical demand during individual storms was either similar or higher than during base flow conditions for both DOC and NO3-. In-stream NO3- demand resulted in substantial whole-reach retention during storms (up to 40%), indicating that in-stream biogeochemical processes substantially reduced downstream flux of terrestrial NO3- inputs during storm events. Conversely, whole-reach DOC retention was relatively low during storms (< 10%), suggesting little ability to regulate DOC export and an energy subsidy to downstream ecosystems during storms. This study indicates that in-stream biogeochemical demand during storms can counterbalance solute supply to some extent, and stresses the importance of considering the potential role of in-stream processes on shaping stream solute export during storms.

Carbon Exports from Terrestrial Ecosystems: A Critical-Zone Framework

In terrestrial ecosystems, vascular plants photosynthesize and respire to produce organic matter and CO2, respectively. Fractions of these products dissolve and are processed belowground to become dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) for export in landscape drainage to receiving inland waters. This paper gathers findings from across the Earth and ecological sciences to assemble a simple framework explaining the interplay among these processes in the critical zone (CZ), the skin of Earth from the top of the vegetation canopy through soil and subsoil to the base of active groundwater circulation. In this framework, chemical weathering, that is the dissolution of soil and subsoil mineral material, is a keystone process fueling carbon and energy flow through the system. The ecologic function of weathering dynamically generates nutrients positively feeding back into biosynthesis (and DOC generation) supporting heterotrophy. Root and heterotrophic respiration in turn drive the geologic function of weathering, entraining and stabilizing CO2 in solution (DIC generation) for storage in the hydrosphere and lithosphere. The CZ framework supports straightforward explanations of spatiotemporal patterns of DOC and DIC exports from catchments, including how they differentially respond to hydrologic and ecosystem development dynamics. Mechanisms that generate and export dissolved C also fuel and affect dynamics of stream emission of CO2 to the atmosphere. At larger time scales, terrestrial C-export rates and dynamics co-evolve with CZ development and disturbance. Ultimately terrestrial C exports are plate-tectonically and thermostatically capped and floored by volcanic CO2 production and carbonate chemistry in Earth’s crust.

Assessment of sediment and organic carbon exports into the Arctic ocean: The case of the Yenisei River basin

The export of organic carbon export by the rivers to the oceans either as particulate organic carbon (POC) or dissolved organic carbon (DOC) is very sensitive to climate change especially in permafrost affected catchments where soils are very rich in organic carbon. With global warming, organic carbon export in both forms is expected to increase in Arctic regions. It should affect contemporary biogeochemical cycles in rivers and oceans and therefore modify the whole food web. This study tries to understand complex processes involved in sediment, POC and DOC riverine transport in the Yenisei River basin and to quantify their respective fluxes at the river outlet. The SWAT (Soil and Water Assessment Tool) hydrological model is used in this study to simulate water and suspended sediment transfers in the largest Arctic river. POC and DOC export have been quantified with empirical models, adapted from literature for the study case. First, the hydrological model has been calibrated and validated at a daily time step for the 2003–2008 and the 2009–2016 periods respectively, and its output has been compared with field data for water and sediment fluxes. Based on conceptualization of transfer processes, calibration on climate and soil properties has been performed in order to correctly represent hydrology and sediment transfer in permafrost basins. Second, calibration of empirical models for DOC/POC transport have been performed by comparing their output with field data, available from 2003 to 2016. Our study reveals that SWAT is capable of correctly representing hydrology, sediment transfer, POC and DOC fluxes and their spatial distribution at a daily timescale, and outlines the links between these fluxes and permafrost features. Our simulation effort results in specific sediment, POC and DOC fluxes of 2.97 t km−2 yr−1, 0.13 t km−2 yr−1 and 1.14 t km−2 yr−1 for the period 2003–2016 which are in the range of previous estimates. About 60% of the total fluxes of sediment, DOC and POC to the Arctic Ocean are exported during the two months of the freshet. Spatial analysis show that permafrost-free areas have returned higher daily organic carbon export than permafrost affected zones, highlighting the thawing permafrost effect on carbon cycle in climate change feedback.

Sources of dissolved organic carbon (DOC) in a mixed land use catchment (Exe, UK)

Many catchment management schemes in the UK have focussed on peatland restoration to improve ecosystem services such as carbon sequestration, water quality and biodiversity. The effect of these schemes on dissolved organic carbon (DOC) flux is critical in understanding peatland carbon budgets as well as the implications for drinking water treatment. In many catchments, however, peatland areas are not the only source of DOC, meaning that their significance at the full catchment scale is unclear. In this paper we have evaluated the importance of different land uses as sources of DOC by combining three datasets obtained from the Exe catchment, UK. The first dataset comprises a weekly monitoring record at three sites for six years, the second, a monthly monitoring record of 25 sites in the same catchment for one year, and the third, an assessment of DOC export from litter and soil carbon stocks. Our results suggest that DOC concentration significantly increased from the peaty headwaters to the mixed land-use areas (ANOVA F = 12.52, p < 0.001, df = 2), leading to higher flux estimates at the downstream sites. We present evidence for three possible explanations: firstly, that poor sampling of high flows may lead to underestimation of DOC flux, second, that there are significant sources of DOC besides the peatland headwaters, and finally, that biological- and photo-degradation decreases the influence of upstream DOC sources. Our results provide evidence both for the targeting of catchment management in peatland areas as well as the need to consider DOC from agricultural and forested areas of the catchment.

Unusual Roles of Discharge, Slope and SOC in DOC Transport in Small Mountainous Rivers, Taiwan

Riverine dissolved organic carbon (DOC), responsible for riverine productivity, is rarely documented in subtropical small mountainous rivers (SMRs) where high rainfall and steep slopes are the main features. This study investigated the DOC export at eight sites in three Taiwan SMRs to characterize the dynamics and controlling factors of DOC transport. Results showed that the mean DOC concentration of ~0.78 mg L−1 is much lower than the global average of ~5.29 mg L−1. However, the mean DOC yield, ~22.51 kg-C ha−1 yr−1, is higher than the global average of 14.4–19.3 kg-C ha−1 yr−1. Comparing with worldwide rivers from literature, the annual discharge, slope, and SOC (soil organic carbon) are controlling factors as expected, though they influence in different ways. SOC stock likely regulated by elevation-dependent biomes dominate the DOC supply, while slope restrains the DOC generation due to shallow soil depth and fast runoff velocity. However, the abundant discharge flushing this persistent low supply leads to a large DOC export in the SMRs. Furthermore, the DOC dynamics during typhoon periods showed a clockwise hysteresis, suggesting that the DOC is mainly from the riparian zone or downslope area during the rising limb of the hydrograph. This study elucidates the DOC transport in SMRs and provides an atypical yet significant piece of understanding on DOC transport in a global context.

The immediate effects of downslope cornstalk mulch (DCM) on sediment yield, runoff and runoff-associated dissolved carbon loss in a representative hillslope, Southwestern China

Dissolved carbon plays a pivotal role in carbon cycles and potentially affects ecological processes of water body. While stalk mulch can regulate soil and water losses, its effects on the export of dissolved carbon from soils are poorly understood. The main objective of this study was to determine the effects of rainfall intensity and downslope cornstalk mulch (the lengths of cornstalks along the slope, DCM) on runoff, sediment yield and dissolved carbon losses, including dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and dissolved total carbon (DTC). A series of field rainfall simulations were conducted on the steep fallow land (20 °) locating in a representative hillslope of southwestern China that experiences severe soil erosion, and data for runoff, sediment and dissolved carbon loss rates were obtained from triplet parallel plots (1.5 m long and 1 m wide for each one) using 60 min rainfall simulations at 30 and 90 mm h−1 intensities on bare soils and 90 mm h−1 intensity on soils with DCM (air-dried whole plants, 60% coverage and weight 1.65 kg), respectively. Rainfall intensity showed positive effects on runoff generation, sediment yield and loss rates of DIC, DOC and DTC. The plots with immediate DCM had no difference in runoff generation, but were 71.1% lower in sediment yield and 53.9% higher in DOC loss rate relative to the bare plots. The contribution of DIC to DTC increased from 35.9% to 59.6% with the increasing rainfall intensity. Cornstalk mulching appeared efficient in reducing sediment yield, but its adoption is likely to dramatically enhance DOC exports from soils with concerns for the quality of inland waters. Further research studies are required to discriminate between soil and cornstalk mulch contribution to DOC exports by overland flow.

Longitudinal discontinuities in riverine greenhouse gas dynamics generated by dams and urban wastewater

Abstract. Surface water concentrations of CO2, CH4, and N2O have rarely been measured simultaneously in river systems modified by human activities, contributing to large uncertainties in estimating global riverine emissions of greenhouse gases (GHGs). Basin-wide surveys of the three GHGs were combined with a small number of measurements of C isotope ratios in dissolved organic matter (DOM), CO2, and CH4 in the Han River basin, South Korea, to examine how longitudinal patterns of the three gases and DOM are affected by four cascade dams along a middle section of the North Han River (hereafter termed “middle reach”) and treated wastewater discharged to the lower Han River (“lower reach”) traversing the Seoul metropolitan area. Monthly monitoring and two-season comparison were conducted at 6 and 15 sites, respectively, to measure surface water gas concentrations and ancillary water quality parameters including concentrations of dissolved organic carbon (DOC) and optical properties of DOM. The basin-wide surveys were complemented with a sampling cruise along the lower reach and synoptic samplings along an urban tributary delivering effluents from a large wastewater treatment plant (WWTP) to the lower reach. The levels of pCO2 were relatively low in the middle reach (51–2465 µatm), particularly at the four dam sites (51–761 µatm), compared with those found in the largely forested upper basin with scattered patches of croplands (163–2539 µatm), the lower reach (78–11298 µatm), and three urban tributaries (2120–11970 µatm). The upper and middle reaches displayed generally low concentration ranges of CH4 and N2O, with some local peaks influenced by agricultural runoff and impoundments. By comparison, the lower reach exhibited exceptionally high concentrations of CH4 (1.2–15766 nmol L−1) and N2O (7.5–1396 nmol L−1), which were significantly correlated with different sets of variables such as DO and PO43- for CH4 and NH4+ and NO3- for N2O. Downriver increases in the levels of DOC and optical properties such as fluorescence index (FI) and protein-like fluorescence indicated an increasing DOM fraction of anthropogenic and microbial origin. The concentrations of the three GHGs and DOC were similar in magnitude and temporal variation at a WWTP discharge and the receiving tributary, indicating a disproportionate contribution of the WWTP effluents to the tributary gas and DOC exports to the lower reach. The values of δ13C in surface water CO2 and CH4 measured during the sampling cruise along the lower reach, combined with δ13C and Δ14C in DOM sampled across the basin, implied a strong influence of the wastewater-derived gases and aged DOM delivered by the urban tributaries. The downstream enrichment of 13C in CO2 and CH4 suggested that the spatial distribution of these gases across the eutrophic lower reach may also be constrained by multiple concomitant processes including outgassing, photosynthesis, and CH4 oxidation. The overall results suggest that dams and urban wastewater may create longitudinal discontinuities in riverine metabolic processes leading to large spatial variations in the three GHGs correlating with different combinations of DOM properties and nutrients. Further research is required to evaluate the relative contributions of anthropogenic and in-stream sources of the three gases and DOM in eutrophic urbanized river systems and constrain key factors for the contrasting impoundment effects such as autotrophy-driven decreases in pCO2 and in-lake production of CH4 and N2O.

Dissolved organic carbon in glaciers of the southeastern Tibetan Plateau: Insights into concentrations and possible sources

Dissolved organic carbon (DOC) released from glaciers has an important role in the biogeochemistry of glacial ecosystems. This study focuses on DOC from glaciers of the southeastern Tibetan Plateau, where glaciers are experiencing rapid shrinkage. We found that concentrations of DOC in snowpits (0.16±0.054 μg g−1), aged snow (0.16±0.048 μg g−1), and bare ice (0.18±0.082 μg g−1) were similar across the southeastern Tibetan Plateau, but were slightly lower than those in other glaciers on the Tibetan Plateau. Vertical variations of DOC, particulate organic carbon, black carbon, and total nitrate in snowpit showed no systematic variations in the studied glaciers, with high values of DOC occurring in the ice or dusty layers. We estimated the export of DOC and particulate organic carbon from glaciers to be 1.96±0.66 Gg yr–1 and 5.88±2.15 Gg yr–1 in this region, respectively, indicating that organic carbon released from glacier meltwater may be affecting downstream ecosystems. Potential sources of the air masses arriving at the southeastern Tibetan glaciers include South Asia, Central Asia, Middle East, and northwest China. Emissions from biomass burning of South Asia played an important role in the deposition of DOC to the glacier, which can be evidenced by backward trajectories and fire spot distributions from MODIS and CALIPSO images. Our findings suggest that anthropogenic aerosols contribute abundant DOC to glaciers on the southeastern Tibetan Plateau. The pronounced rate of glacial melting in the region may be delivering increased quantities of relic DOC to downstream rivers.

Hydrological control of dissolved organic carbon dynamics in a rehabilitated &amp;lt;i&amp;gt;Sphagnum&amp;lt;/i&amp;gt;-dominated peatland: a water-table based modelling approach

Abstract. Hydrological disturbances could increase dissolved organic carbon (DOC) exports through changes in runoff and leaching, which reduces the potential carbon sink function of peatlands. The objective of this study was to assess the impact of hydrological restoration on hydrological processes and DOC dynamics in a rehabilitated Sphagnum-dominated peatland. A conceptual hydrological model calibrated on the water table and coupled with a biogeochemical module was applied to La Guette peatland (France), which experienced a rewetting initiative on February 2014. The model (eight calibrated parameters) reproduced water-table (0.1&lt;NS&lt;0.61) and pore-water DOC concentrations (2&lt;RMSE&lt;11 mg L−1) in a time series (1 April 2014 to 15 December 2017) in two contrasting locations (rewetted and control) in the peatland. Hydrological restoration was found to impact the water balance through a decrease in slow deep drainage and an increase in fast superficial runoff. Observed DOC concentrations were higher in summer in the rewetted location compared to the control area and were linked to a difference in dissolved organic matter composition analyzed by fluorescence. Hydrological conditions, especially the severity of the water-table drawdown in summer, were identified as the major factor controlling DOC-concentration dynamics. The results of the simulation suggest that the hydrological restoration did not affect DOC loads, at least in a short-term period (3 years). However, it impacted the temporal dynamics of DOC exports, which were the most episodic and were mainly transported through fast surface runoff in the area affected by the restoration, while slow deep drainage dominated DOC exports in the control area. In relation to dominant hydrological processes, exported DOC is expected to be derived from more recent organic matter in the top peat layer in the rewetted area, compared to the control area. Since it is calibrated on water-table and DOC concentration, the model presented in this study proved to be a relevant tool in identifying the main hydrological processes and factors controlling DOC dynamics in different areas of the same peatland. It is also a suitable alternative to a discharge-calibrated catchment model when the outlet is not easy to identify or to monitor.

Forest harvest legacies control dissolved organic carbon export in small watersheds, western Oregon

This study examined physical and biological controls on dissolved organic carbon (DOC) fluxes from conifer-forest watersheds in the H.J. Andrews Experimental Forest of Oregon. We tested how DOC export was related to streamflow and legacies of wood on the forest floor three to five decades after harvest of old-growth forest in seven watersheds spanning the rain to snow elevation gradient. Three watersheds had old-growth forest and four had 30 to 50-year-old forest established after clearcutting of old-growth forest. Mean annual DOC flux in the watersheds was related to the biomass of forest floor wood, which was two or three times higher in watersheds with old-growth forest compared to young forest, and was inversely related to elevation, a measure of snowpack depth and duration. In contrast, fluxes of inorganic elements such as Si and Ca did not vary with harvest history or forest floor characteristics. Annual fluxes of DOC, Si, and Ca were linearly related to annual runoff, and annual volume-weighted concentrations of these ions declined by < 0.6% with several-fold increases in annual runoff. Across all years, DOC concentrations peaked before the peak of the hydrograph in all watersheds, which we interpret as representing movement, likely via preferential and surficial flow, of organic materials mineralized and solubilized during the long dry summers in this ecosystem. DOC concentrations relative to stream flow exhibited clockwise hysteresis loops in each water year, also suggesting that soluble DOC produced in the dry summer is exported in the fall. DOC concentration differences between reference and harvested watersheds also peaked in late summer or early fall, suggesting that the source of the additional DOC from reference watersheds came from coarse woody debris that remains moist during the dry summers and that was significantly greater in watersheds with elevated DOC. Taken together, our results suggest that forest floor wood is a previously unappreciated control on the supply of DOC that can be exported, and runoff is a secondary control on total DOC flux to streams. The legacy of forest harvest on DOC flux can be observed for decades, as total ecosystem carbon stocks, especially coarse woody debris, may require centuries to develop after old-growth forest harvest.

Seasonal shifts in export of DOC and nutrients from burned and unburned peatland-rich catchments, Northwest Territories, Canada

Abstract. Boreal peatlands are major catchment sources of dissolved organic carbon (DOC) and nutrients and thus strongly regulate the landscape carbon balance, aquatic food webs, and downstream water quality. Climate change is likely to influence catchment solute yield directly through climatic controls on run-off generation, but also indirectly through altered disturbance regimes. In this study we monitored water chemistry from early spring until fall at the outlets of a 321 km2 catchment that burned 3 years prior to the study and a 134 km2 undisturbed catchment. Both catchments were located in the discontinuous permafrost zone of boreal western Canada and had ∼ 60 % peatland cover. The two catchments had strong similarities in the timing of DOC and nutrient yields, but a few differences were consistent with anticipated effects of wildfire based on peatland porewater analysis. The 4-week spring period, particularly the rising limb of the spring freshet, was crucial for accurate characterization of the seasonal solute yield from both catchments. The spring period was responsible for ∼ 65 % of the seasonal DOC and nitrogen and for ∼ 85 % of the phosphorous yield. The rising limb of the spring freshet was associated with high phosphorous concentrations and DOC of distinctly high aromaticity and molecular weight. Shifts in stream DOC concentrations and aromaticity outside the early spring period were consistent with shifts in relative streamflow contribution from precipitation-like water in the spring to mineral soil groundwater in the summer, with consistent relative contributions from organic soil porewater. Radiocarbon content (14C) of DOC at the outlets was modern throughout May to September (fraction modern carbon, fM: 0.99–1.05) but likely reflected a mix of aged DOC, e.g. porewater DOC from permafrost (fM: 0.65–0.85) and non-permafrost peatlands (fM: 0.95–1.00), with modern bomb-influenced DOC, e.g. DOC leached from forest litter (fM: 1.05–1.10). The burned catchment had significantly increased total phosphorous (TP) yield and also had greater DOC yield during summer which was characterized by a greater contribution from aged DOC. Overall, however, our results suggest that DOC composition and yield from peatland-rich catchments in the discontinuous permafrost region likely is more sensitive to climate change through impacts on run-off generation rather than through altered fire regimes.

Estimation of Lateral DOC Transport in Marginal Sea Based on Remote Sensing and Numerical Simulation

AbstractDue to high temporal and spatial dynamics and complex biological processes, it is difficult to estimate lateral transportations of the dissolved organic carbon (DOC) in marginal seas by conventional field observations and model simulations. Taking the East China Sea (ECS), for example, this study proposed a novel model for estimating the lateral DOC flux in marginal sea. Three‐dimensional distributions of DOC concentrations were retrieved by satellite remote sensing. Furthermore, three‐dimensional currents were simulated using the numerical model. Finally, the lateral DOC flux in the whole ECS was estimated based on the satellite‐derived DOC concentrations and the simulated currents. The results showed that despite seasonal variations, the unit area lateral DOC flux remained high throughout the year in the Changjiang River estuary, Zhejiang‐Fujian Coast, Taiwan Strait, and Kuroshio regions. There were three eastward transportation channels which were strong in summer half year and weak in winter half year. Moreover, a northward transport zone extending from north of Taiwan to the south of the Changjiang River estuary was detected. In terms of annually net flux, highest net DOC import (30.65 TgC yr−1) was from the Taiwan Strait, mainly through the eastern side of the strait. The second highest net DOC import (18.75 TgC yr−1) was from the shelf slope (200 m isobath), mainly through the south part of the slope between 26°N and 26.5°N. Most of the net DOC export (−52.75 TgC yr−1) from the ECS was observed through the northern boundary (32°N), particularly in the outer shelf with water depth of 100–200 m.

Variations in concentrations and sources of bioavailable organic compounds in the Indian estuaries and their fluxes to coastal waters

Estuarine organic matter (EOM) supports heterotrophic carbon demand in the estuaries. However, the fraction of bioavailable organic compounds, which are easily assimilated by biological organisms, in the EOM is unclear. We hypothesize that a significant fraction of dissolved EOM is bioavailable and supports heterotrophic activity in the Indian estuaries. In order to test this hypothesis, 26 major, medium and minor Indian estuaries were sampled during wet and dry periods (2011–2012) for dissolved bioavailable organic compounds and dissolved organic carbon (DOC). The mean concentration of total dissolved carbohydrates (TDCHO) and total dissolved proteins (TDPRO) were higher during wet than dry period in contrary to the observations of dissolved free Amino Acids (DFAA). TDPRO:TDCHO and statistical analysis revealed that terrestrial oganic matter contributed significantly to the EOM in the major and medium estuaries during wet period whereas fresh organic matter derived from in situ production contributed in all estuaries during dry period and minor estuaries during wet period. The lower concentrations of TDCHO and DFAA and their contribution to DOC were observed in the Indian than global estuaries suggesting that either efficient utilization of bioavailable organic compounds or less inputs (autochthonous/allochthonous) in the Indian estuaries. Indian estuaries transported an order of magnitude higher concentration of bioavailable organic compounds to the coastal Bay of Bengal (490 × 109 gC y−1) than to the Arabian Sea (50 × 109 gC y−1). The contribution of bioavailable organic compounds to the DOC export was 25% and 7% in the Bay of Bengal and Arabian Sea respectively. This study suggests that EOM contains a significant fraction of bioavailable organic compounds, as hypothesized, which might support heterotrophic carbon demand in the Indian estuaries as well as adjacent coastal waters.

Riverine dissolved organic carbon in Rukarara River Watershed, Rwanda

Dissolved organic carbon (DOC) loading is rarely estimated in tropical watersheds. This study quantifies DOC loading in the Rukarara River Watershed (RRW), a Rwandan tropical forest and agricultural watershed, and evaluates its relationship with hydrological factors, land use and land cover (LULC), and topography to better understand the impact of stream DOC export on watershed carbon budgets. The annual average load for the study period was 977.80 kg C, which represents approximately 8.44% of the net primary productivity of the watershed. The mean daily exports were 0.37, 0.14, 0.075 and 0.32 kg C/m2 in streams located in natural forest, tea plantation, small farming areas, and at the outlet of the river, respectively. LULC is a factor that influences DOC loading. The quick flow was the main source of stream DOC at all study sites. Stream DOC increases with increasing water flow, indicating a positive relationship. Thus, the expectation is that a change in land cover and/or rainfall will result in a change of stream DOC dynamics within the watershed. Topography was also found to influence the dynamics of stream DOC through its effect on overland flow in terms of drainage area and total length of flow paths. Tea plantations were located in areas of high drainage density and projected increase of rainfall in the region, as a consequence of climate change, could increase stream DOC content and affect stream water quality, biodiversity, balance between autotrophy and heterotrophy, and bioavailability of toxic compounds within the RRW.