Dissolved Organic Carbon Transport Research Articles

Severe flood modulates the sources and age of dissolved organic carbon in the Yangtze River Estuary

Floods in global large rivers modulate the transport of dissolved organic carbon (DOC) and estuarine hydrological characteristics significantly. This study investigated the impact of a severe flood on the sources and age of DOC in the Yangtze River Estuary (YRE) in 2020. Comparing the flood period in 2020 to the non-flood period in 2017, we found that the flood enhanced the transport of young DOC to the East China Sea (ECS), resulting in significantly enriched Δ14C-DOC values. During the flood period, the proportion of modern terrestrial organic carbon (OC) was significantly higher compared to the non-flood period. Conversely, the proportion of pre-aged sediment OC was significantly lower during the flood period. The high turbidity associated with the flood facilitated rapid transformation and mineralization of sedimentary and fresh terrestrial OC, modifying the sources of DOC. The flux of modern terrestrial OC transported to the ECS during the flood period was 1.58 times higher than that of the non-flood period. These findings suggest that floods can modulate the sources and decrease the age of DOC, potentially leading to increased greenhouse gas emissions. Further research is needed to understand the long-term impacts of floods on DOC dynamics in global estuaries.

Seasonal and interannual dissolved organic carbon transport process dynamics in a subarctic headwater catchment revealed by high-resolution measurements

Abstract. Dissolved organic carbon (DOC) dynamics are evolving in the rapidly changing Arctic and a comprehensive understanding of the controlling processes is urgently required. For example, the transport processes governing DOC dynamics are prone to climate-driven alteration given their strong seasonal nature. Hence, high-resolution and long-term studies are required to assess potential seasonal and interannual changes in DOC transport processes. In this study, we monitored DOC at a 30 min resolution from September 2018 to December 2022 in a headwater peatland-influenced stream in northern Finland (Pallas catchment, 68° N). Temporal variability in transport processes was assessed using multiple methods: concentration–discharge (C–Q) slope for seasonal analysis, a modified hysteresis index for event analysis, yield analysis, and random forest regression models to determine the hydroclimatic controls on transport. The findings revealed the following distinct patterns: (a) the slope of the C–Q relationship displayed a strong seasonal trend, indicating increasing transport limitation each month after snowmelt began; (b) the hysteresis index decreased post-snowmelt, signifying the influence of distal sources and DOC mobilization through slower pathways; and (c) interannual variations in these metrics were generally low, often smaller than month-to-month fluctuations. These results highlight the importance of long-term and detailed monitoring to enable separation of inter- and intra-annual variability to better understand the complexities of DOC transport. This study contributes to a broader comprehension of DOC transport dynamics in the Arctic, specifically quantifying seasonal variability and associated mechanistic drivers, which is vital for predicting how the carbon cycle is likely to change in Arctic ecosystems.

Spatially distributed tracer‐aided modelling to explore DOC dynamics, hot spots and hot moments in a tropical mountain catchment

AbstractTracer‐aided rainfall‐runoff modelling is a promising tool for understanding catchment hydrology, particularly when tracers provide information about coupled hydrological‐biogeochemical processes. Such models allow for predicting the quality and quantity of water under changing climatic and anthropogenic conditions. Here, we present the Spatially‐distributed Tracer‐Aided Rainfall‐Runoff model with a coupled biogeochemical reactive tracer module (STARR‐DOC) to simulate dissolved organic carbon (DOC) dynamics and sources. The STARR‐DOC model was developed and tested for a humid high Andean ecosystem (páramo) using high‐resolution hourly DOC and hydrometeorological data to simulate hourly discharge and DOC at a fine spatial (10 × 10 m) resolution. Overall, the model was able to acceptably reproduce discharge (KGE ~ 0.45) and stream DOC (KGE ~ 0.69) dynamics. Spatially distributed DOC simulations were independently compared using point DOC measurements for different soil types across the catchment, which allowed for identifying DOC production hot spots and hot moments. Results showed higher hydrological connectivity between slopes and valleys with increasing precipitation. Wetter conditions also favoured DOC production (wet month = 82 mg L−1, dry month = 5 mg L−1) and transport to the stream network (DOC concentrations: during events ~15 mg L−1, during baseflows ~4 mg L−1). Our results also suggest that minor changes in meteorological conditions directly affect páramo soil water dynamics and biogeochemistry. Knowledge of when and where DOC production in mountain catchments is greatest is important for water managers to understand when they make decisions about water security, especially considering climate change predictions for the Andean region.

Top-down control over dissolved organic carbon in the bottom water of the Weddell Sea and its implication for the continental shelf pump

Dense water out of the Antarctic shelves is expected to drive the transport of carbon into the deep Southern Ocean via the formation of Antarctic Bottom Water. However, bottom water formation’s capacity to sequester carbon into the deep ocean is poorly constrained. Here, dissolved organic carbon (DOC), dissolved black carbon (DBC), and particulate organic carbon (POC) were examined to reveal the influence of the Weddell Sea Deep Water (WSDW) on DOC transport during its flowing out of the Weddell Sea. High DOC concentrations (>60 μM-C) and low DBC/DOC ratios (<1.5%) were observed in surface water near the South Orkney Islands, ascribing to sea ice melt-induced phytoplankton blooms. Seawater at the mid-deep depths exhibited a higher DOC concentration (averaging 48.1 ± 3.7 μM-C) than the incoming water source, resulting from the release of DOC from sinking particles. Bottom water had higher DOC concentration compared to the mid-deep layer water (t-test, p < 0.005), while the DBC concentrations were comparable. In addition, the excess DOC (relative to WSDW) in bottom water showed a close relation with POC in surface water. These results reveal a top-down control over the DOC concentration in bottom water through a quick sinking of diatom detritus and subsequently solubilization in bottom water and/or sediment. With an estimate, the WSDW carries 5.1 ± 2.8 Tg-C/yr of excess DOC out of the Powell Basin, representing an important carbon source to the deep Southern Ocean. This study highlights the key role of the Antarctic continental shelf pump in carbon sequestration.

Impacts of anthropogenic water regulation on global riverine dissolved organic carbon transport

Abstract. Anthropogenic water regulation activities, including reservoir interception, surface water withdrawal, and groundwater extraction, alter riverine hydrologic processes and affect dissolved organic carbon (DOC) export from land to rivers and oceans. In this study, schemes describing soil DOC leaching, riverine DOC transport, and anthropogenic water regulation were developed and incorporated into the Community Land Model 5.0 (CLM5.0) and the River Transport Model (RTM). Three simulations by the developed model were conducted on a global scale from 1981–2013 to investigate the impacts of anthropogenic water regulation on riverine DOC transport. The validation results showed that DOC exports simulated by the developed model were in good agreement with global river observations. The simulations showed that DOC transport in most rivers was mainly influenced by reservoir interception and surface water withdrawal, especially in central North America and eastern China. Four major rivers, including the Danube, Yangtze, Mississippi, and Ganges rivers, have experienced reduced riverine DOC flows due to intense water management, with the largest effect occurring in winter and early spring. In the Danube and Yangtze river basins, the impact in 2013 was 4 to 5 times greater than in 1981, with a retention efficiency of over 50 %. The Ob river basin was almost unaffected. The total impact of anthropogenic water regulation reduced global annual riverine DOC exports to the ocean by approximately 13.36 ± 2.45 Tg C yr−1, and this effect increased from 4.83 % to 6.20 % during 1981–2013, particularly in the Pacific and Atlantic oceans.

Investigating hydrological transport pathways of dissolved organic carbon in cold region watershed based on a watershed biogeochemical model

Dissolved organic carbon (DOC) is a significant component of regional and global carbon cycles and an important surface water quality indicator. DOC affects the processes of solubility, bioavailability and transport for a number of contaminants, such as heavy metals. Therefore, it is crucial to understand DOC fate and transport in the watershed and the transport pathways of DOC load. We modified a previously developed watershed-scale organic carbon model by incorporating the DOC load from glacier melt runoff and used the modified model to simulate periodic daily DOC load in the upper Athabasca River Basin (ARB) in the cold region of western Canada. The calibrated model achieved an overall acceptable performance for simulating daily DOC load with model uncertainties mainly from the underestimation of peak loads. Parameter sensitivity analysis indicates that the fate and transport of DOC load in upper ARB are mainly controlled by DOC production in the soil layers, DOC transport at the soil surface, and reactions in the stream system. The modeling results indicated that the DOC load is mainly from the terrestrial sources and the stream system was a negligible sink in the upper ARB. It also indicated that rainfall-induced surface runoff was the major transport pathway of DOC load in the upper ARB. However, the DOC loads transported by glacier melt runoff were negligible and only accounted for 0.02% of the total DOC loads. In addition, snowmelt-induced surface runoff and lateral flow contributed 18.7% of total DOC load, which is comparable to the contribution from the groundwater flow. Our study investigated the DOC dynamics and sources in the cold region watershed in western Canada and quantified the contribution of different hydrological pathways to DOC load, which could provide a useful reference and insight for understanding watershed-scale carbon cycle processes.

Modeling dissolved organic carbon exchange across major straits and shelf breaks in the South China Sea

The South China Sea (SCS) is the largest semienclosed marginal sea in the western Pacific. The advective supply of dissolved organic carbon (DOC) from the open ocean to the SCS influences its carbon cycle and biogeochemical processes. However, quantitative studies of DOC exchange across different straits and the shelf to the SCS are particularly limited. In this study, we investigated seasonal DOC transport across four major straits and cross-shelf breaks using a coupled physical–biogeochemical model. The modeling results show that the seasonal variations in DOC transport through the major straits are largely controlled by the monsoon. In winter, associated with the northeast monsoon, the magnitudes of DOC transport from the four straits are generally larger than those in summer with the southwest monsoon. DOC transport is eastward at the Luzon Strait and northward at the Taiwan and Karimata straits during the southwest monsoon and changes direction during the northeast monsoon. The annual mean of depth-integrated DOC transport is 86.7 Tg C yr−1 westward at the Luzon Strait, 19.1 Tg C yr−1 northward at the Taiwan Strait, 25.9 Tg C yr−1 southward at the Karimata Strait, and 46.0 Tg C yr−1 southward at the Mindoro Strait. DOC transport across the shelf break is a result of geostrophic or quasigeostrophic balance. In the north and west, the annual net cross-shelf break transports are onshore with magnitudes of 8.8 and 31.1 Tg C yr−1, respectively. In the southern SCS, the annual net transport is offshore with a magnitude of 18.3 Tg C yr−1. The net export of total labile and semilabile DOC from the SCS has a flux of 10.7 Tg C yr−1. This study provides seasonal budgets of DOC exchange between the SCS and its adjacent oceans, which contribute to a better evaluation of the carbon cycle in the SCS.

Estimating the lateral transfer of organic carbon through the European river network using a land surface model

Abstract. Lateral carbon transport from soils to the ocean through rivers has been acknowledged as a key component of the global carbon cycle, but it is still neglected in most global land surface models (LSMs). Fluvial transport of dissolved organic carbon (DOC) and CO2 has been implemented in the ORCHIDEE LSM, while erosion-induced delivery of sediment and particulate organic carbon (POC) from land to river was implemented in another version of the model. Based on these two developments, we take the final step towards the full representation of biospheric carbon transport through the land–river continuum. The newly developed model, called ORCHIDEE-Clateral, simulates the complete lateral transport of water, sediment, POC, DOC, and CO2 from land to sea through the river network, the deposition of sediment and POC in the river channel and floodplains, and the decomposition of POC and DOC in transit. We parameterized and evaluated ORCHIDEE-Clateral using observation data in Europe. The model explains 94 %, 75 %, and 83 % of the spatial variations of observed riverine water discharges, bankfull water flows, and riverine sediment discharges in Europe, respectively. The simulated long-term average total organic carbon concentrations and DOC concentrations in river flows are comparable to the observations in major European rivers, although our model generally overestimates the seasonal variation of riverine organic carbon concentrations. Application of ORCHIDEE-Clateral for Europe reveals that the lateral carbon transfer affects land carbon dynamics in multiple ways, and omission of this process in LSMs may lead to an overestimation of 4.5 % in the simulated annual net terrestrial carbon uptake over Europe. Overall, this study presents a useful tool for simulating large-scale lateral carbon transfer and for predicting the feedbacks between lateral carbon transfer and future climate and land use changes.

Groundwater travel times predict DOC in streams and riparian soils across a heterogeneous boreal landscape

Dissolved organic carbon (DOC) in surface waters is an important component of the boreal landscape carbon budget and a critical variable in water quality. A dominant terrestrial DOC source in the boreal landscape is the riparian zone. These near stream areas play a key role in regulating DOC transport between land and aquatic ecosystems. The groundwater dynamics at this interface have been considered a major controlling variable for DOC export to streams. This study focuses on the regulating role of groundwater levels and mean travel times (MTT) on riparian DOC concentrations and, subsequently, stream DOC. This is done by comparing them as explanatory variables to capture the spatial and intra-annual variability of the stream and riparian groundwater DOC. We used a physically based 3D hydrological model, Mike SHE, to simulate DOC concentrations of the riparian zones for 14 sub-catchments within the Krycklan catchment (Sweden). The model concept assumes that DOC concentrations will be higher in groundwater moving through shallow flow paths. In the model, this can be linked to the position of the groundwater table at a point of observation or the travel time, which will generally be shorter for water that has travelled through shallow and more conductive soil layers. We compared the results with both observed stream and groundwater concentrations. The analysis revealed that the correlation between modelled and observed annual averages of stream DOC increased from r = 0.08 to r = 0.87 by using MTT instead of groundwater level. MTT also better captured the observed spatial variability in riparian DOC concentrations and more successfully represented seasonal variability of stream DOC. We, therefore, suggest that MTT is a better predictor than groundwater level for riparian DOC concentration because it can capture a greater variety of catchment heterogeneities, such as variation in soil properties, catchment size, and input from deep groundwater sources.

Distribution of particulate and dissolved organic carbon in surface waters of northern scottish fjords

Coastal waters can act concurrently as a source and sink of carbon (C), with coastal sediments trapping and storing significant quantities of C, while C is simultaneously released to the atmosphere through biogeochemical processes in the water column. The mid-to high-latitude fjords are recognised as hotspots for the burial and storage of OC in their sediments and for their potential to provide a long-term climate regulation service. Yet the distribution of particulate (POC) and dissolved organic carbon (DOC) in fjord water columns remains a significant knowledge gap, that is particularly acute in temperate vegetated systems such as those in the North Atlantic. Here we present POC and DOC data from surface waters across four Scottish fjords with differing meteorological, catchment and submarine geomorphological characteristics to both quantify and understand the factors governing the spatial distribution of POC and DOC in fjords. The measured POC and DOC concentrations in the surface waters of the four fjords are broadly analogous to other temperate vegetated fjord systems around the world. Within these systems, local factors such as submarine geomorphology is the primary factor driving POC and DOC dispersal in fjord surface waters. The data highlights that fjords are important pathways for the transport and storage of POC and DOC in the coastal ocean and that a greater focus is required on the water column OC to allow the role fjords play in near- (water column) and long-term (sediment) climate regulation to be quantified.

Seasonal Changes in Hydrology and Permafrost Degradation Control Mineral Element‐Bound DOC Transport From Permafrost Soils to Streams

AbstractMineral elements bind to dissolved organic carbon (DOC) in permafrost soils, and this may contribute to the stabilization or the degradation of organic carbon along the soil to river continuum. Permafrost thaw enlarges the pool of soil constituents available for soil to river transfer. The unknown is how changes in hydrology upon permafrost degradation affect the connection between soil‐derived mineral element‐bound DOC and headwater streams. Here, we study Al, Fe, Ca, and DOC concentrations in water from a headwater stream at Eight Mile Lake, Alaska, USA (colloidal [0.22 μm–1 kDa] and truly dissolved [&lt;1 kDa] fractions) and in soil pore waters sampled across a gradient of permafrost degradation at the same location. We target the peak flow to base flow transition to show that there is a narrow window of mineral element‐bound DOC colloid transport from soils to streams. We show that during spring thaw and maximum thaw there is an enhanced lateral transfer of mineral element‐bound DOC colloids in extensively degraded sites compared to minimally degraded sites. This is explained by a more rapid response of hydrology at peak flow to base flow transition at degraded sites. Our results suggest that ongoing permafrost degradation and the associated response of soils to changing hydrology can be detected by targeting the composition and size of mineral element‐DOC associations in soil waters and headwater streams during peak flow‐baseflow transitions.

Landscape, Soil, Lithology, Climate and Permafrost Control on Dissolved Carbon, Major and Trace Elements in the Ob River, Western Siberia

In order to foresee possible changes in the elementary composition of Arctic river waters, complex studies with extensive spatial coverage, including gradients in climate and landscape parameters, are needed. Here, we used the unique position of the Ob River, draining through the vast partially frozen peatlands of the western Siberia Lowland and encompassing a sizable gradient of climate, permafrost, vegetation, soils and Quaternary deposits, to assess a snap-shot (8–23 July 2016) concentration of all major and trace elements in the main stem (~3000 km transect from the Tom River confluence in the south to Salekhard in the north) and its 11 tributaries. During the studied period, corresponding to the end of the spring flood-summer baseflow, there was a systematic decrease, from the south to the north, of Dissolved Inorganic Carbon (DIC), Specific Conductivity, Ca and some labile trace elements (Mo, W and U). In contrast, Dissolved Organic Carbon (DOC), Fe, P, divalent metals (Mn, Ni, Cu, Co and Pb) and low mobile trace elements (Y, Nb, REEs, Ti, Zr, Hf and Th) sizably increased their concentration northward. The observed latitudinal pattern in element concentrations can be explained by progressive disconnection of groundwaters from the main river and its tributaries due to a northward increase in the permafrost coverage. A northward increase in bog versus forest coverage and an increase in DOC and Fe export enhanced the mobilization of insoluble, low mobile elements which were present in organo-ferric colloids (1 kDa—0.45 µm), as confirmed by an in-situ dialysis size fractionation procedure. The chemical composition of the sampled mainstream and tributaries demonstrated significant (p &lt; 0.01) control of latitude of the sampling point; permafrost coverage; proportion of bogs, lakes and floodplain coverage and lacustrine and fluvio-glacial Quaternary deposits of the watershed. This impact was mostly pronounced on DOC, Fe, P, divalent metals (Mn, Co, Ni, Cu and Pb), Rb and low mobile lithogenic trace elements (Al, Ti, Cr, Y, Zr, Nb, REEs, Hf and Th). The pH and concentrations of soluble, highly mobile elements (DIC, SO4, Ca, Sr, Ba, Mo, Sb, W and U) positively correlated with the proportion of forest, loesses, eluvial, eolian, and fluvial Quaternary deposits on the watershed. Consistent with these correlations, a Principal Component Analysis demonstrated two main factors explaining the variability of major and trace element concentration in the Ob River main stem and tributaries. The DOC, Fe, divalent metals and trivalent and tetravalent trace elements were presumably controlled by a northward increase in permafrost, floodplain, bogs, lakes and lacustrine deposits on the watersheds. The DIC and labile alkaline-earth metals, oxyanions (Mo, Sb and W) and U were impacted by southward-dominating forest coverage, loesses and eluvial and fertile soils. Assuming that climate warming in the WSL will lead to a northward shift of the forest and permafrost boundaries, a “substituting space for time” approach predicts a future increase in the concentration of DIC and labile major and trace elements and a decrease of the transport of DOC and low soluble trace metals in the form of colloids in the main stem of the Ob River. Overall, seasonally-resolved transect studies of large riverine systems of western Siberia are needed to assess the hydrochemical response of this environmentally-important territory to on-going climate change.

Aerobic respiration in riparian exchange zones of regulated river corridors

AbstractRiver stage fluctuations drive surface water‐groundwater exchanges within river corridors. This study evaluates how repeated daily stage fluctuations, representative of hydropeaking conditions, influence aerobic respiration of river‐sourced dissolved organic carbon (DOC) in the riparian exchange zone using reactive flow and transport simulations. Over 50 hypothetical scenarios were modelled to evaluate how the duration of the daily flood signal, river DOC concentration, aquifer hydraulic conductivity and ambient groundwater flow condition affect the fate and transport of DOC and DO in the riparian aquifer. Time series subsurface snapshots highlight how the various factors influence the subsurface distribution of DOC and DO. The total mass of DOC respired per meter of river had a wide range depending on the parameters, spanning from 1.4 to 71 g over 24‐h, with high hydraulic conductivity and losing ambient groundwater flow conditions favouring the largest amount of DOC respired. The ratio of DOC mass entering the riparian zone with the mass returning to the river showed that as little as 5% to as much as 76% of the DOC that enters the bank during stage fluctuations returns to the river. This return ratio is dependent on river DOC concentration, hydraulic conductivity and ambient groundwater flow. The results illustrate that stage variations due to river regulation can be a significant control on aerobic respiration in riparian exchange zones.

Distribution and Sources of Carbon in Serpentinized Mantle Peridotites at the Atlantis Massif (IODP Expedition 357)

AbstractThe carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered during IODP Expedition 357 on the Atlantis Massif (AM) was examined to characterize carbon sources and the fate of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in seawater during long‐lived hydrothermal circulation and serpentinization. Carbon isotopes reveal three stages of carbonate formation, starting at least 38,000 yr ago: (a) Early dispersed carbonate precipitation, with low water/rock ratios and high temperatures (50°C–190°C); (b) carbonate vein formation related to high and focused fluid fluxes still at high temperatures (30°C–200°C); and (c) seawater circulation leading to cold carbonate precipitation controlled by late, brittle fractures during uplift, and unroofing of the oceanic core complex. Our study reveals three main DIC sources in the system: (a) DIC from abiotic hydrothermal degradation of dissolved organic matter (OM); (b) DIC from seawater; and (c) DIC from mantle‐derived volatiles. Basement rocks containing dispersed carbonates are characterized by high concentrations (∼800 ppm) of total non‐carbonate carbon (NCC) and 13C‐depleted carbonates. We propose that high seawater fluxes in the southern part of the AM likely favor the transport and incorporation of marine DOC in serpentinites and that carbonates record isotopic signals of OM decay. Our study indicates that organic carbon accounts for a significant proportion of the total carbon stored in the AM and suggests that serpentinites may be an important sink of DOC from seawater.

Hydrological and catchment controls on event‐scale dissolved organic carbon dynamics in boreal headwater streams

AbstractHydrological events transport large proportions of annual or seasonal dissolved organic carbon (DOC) loads from catchments to streams. The timing, magnitude and intensity of these events are very sensitive to changes in temperature and precipitation patterns, particularly across the boreal region where snowpacks are declining and summer droughts are increasing. It is important to understand how landscape characteristics modulate event‐scale DOC dynamics in order to scale up predictions from sites across regions, and to understand how climatic changes will influence DOC dynamics across the boreal forest. The goal of this study was to assess variability in DOC concentrations in boreal headwater streams across catchments with varying physiographic characteristics (e.g., size, proportion of wetland) during a range of hydrological events (e.g., spring snowmelt, summer/fall storm events). From 2016 to 2017, continuous discharge and sub‐daily chemistry grab samples were collected from three adjacent study catchments located at the International Institute for Sustainable Development‐Experimental Lakes Area in northwestern Ontario, Canada. Catchment differences were more apparent in summer and fall events and less apparent during early spring melt events. Hysteresis analysis suggested that DOC sources were proximal to the stream for all events at a catchment dominated by a large wetland near the outlet, but distal from the stream at the catchments that lacked significant wetland coverage during the summer and fall. Wetland coverage also influenced responses of DOC export to antecedent moisture; at the wetland‐dominated catchment, there were consistent negative relationships between DOC concentrations and antecedent moisture, while at the catchments without large wetlands, the relationships were positive or not significant. These results emphasize the utility of sub‐daily sampling for inferring catchment DOC transport processes, and the importance of considering catchment‐specific factors when predicting event‐scale DOC behaviour.

Natural and anthropogenic impacts on the DOC characteristics in the Yellow River continuum

The Yellow River is the second largest river in China. Carbon transport by the Yellow River has significant influence on riverine carbon cycles in Asia. During the wet season, the riverine carbon was mainly found in dissolved form, i.e., dissolved organic carbon (DOC), along the entire course of the river. The distinct spatial variations of DOC concentration were observed at different reaches of the mainstream (p < 0.01), while the highest mean DOC concentration was generally observed at midstream (4.13 ± 0.91 mg/L). Carbon stable isotope analysis δ13C and C: N ratio of DOC, evidenced the sources of DOC in headwater and upstream were primarily the terrestrial plants (94% and 61%), but it was changed to soil organic matter (SOM) in mid- and downstream (36% and 37%), and the contribution of sewage to DOC were also increased to 17% and 18%. In the whole mainstream of the Yellow River, water temperature (WT) had a significant impact on DOC concentration, and it could explain 67% of the DOC variance. However, in a large catchment, the driving mechanisms on the DOC variations in headwaters will not necessarily be those controlling DOC trends in downstream. The study firstly quantified, in headwater and upstream, the natural factors explained as much as 65% and 73% of the DOC variations, respectively. In mid- and downstream areas, DOC was significantly influenced by the amount of wastewater discharged by the industry and the use of chemical fertilizers (p < 0.05). These findings may facilitate a better assessment of global riverine carbon cycling and may help to reveal the importance of the balance between development and environmental sustainability with the changing DOC transport features in the Yellow River due to human disturbances.

Impact of freeze-thaw cycles on organic carbon and metals in waters of permafrost peatlands

Despite the importance of soil and surface waters freezing in permafrost landscapes, the behaviour of dissolved organic carbon (DOC), nutrients and metals during periodic freeze-thaw cycles (FTC) remains poorly known. The on-going climate warming is likely to increase the frequency of FTC in continental aquatic settings, which could modify the chemical composition of waters. In this study, we conducted 9 repetitive cycles of overnight freezing (−20 °C) and 5 h thawing (4 °C) in the laboratory using representative 0.22 μm-filtered waters from NE European permafrost peatland: leachates of vegetation and soil, and natural surface waters (depression, thermokarst lake and river). Only minor (<5%–15%) changes of DOC concentrations, SUVA254 and molecular weight were observed in all leachates and the depression water. In contrast, several trace elements (Fe, Al, P, Mn, As, and REE) exhibited sizable variations during FTC (>10%). The leachates and the depression water were enriched in trace elements, whereas the thermokarst lake and the river demonstrated a decrease in concentration of Fe (−39 and −94%, respectively), Al (−9 and −85%), and Mn (−10 and −79%) during FTC. Overall, the observations demonstrated an increase in aliphatic low molecular weight organic matter (OM), and the precipitation of Fe, Al hydroxides and organo-mineral particles. Therefore, enhanced of frequency of FTC can favour the release of metals and toxicants from acidic OM-rich surface waters and maintain stable OM-metals-colloids in large lakes and rivers, thus regulating aquatic transport of DOC and metals from soils to the Arctic Ocean.

Different storm responses of organic carbon transported to Lake Taihu by the eutrophic Tiaoxi River, China

Low-frequency high-magnitude storms can flush disproportionate amounts of terrigenous dissolved organic carbon (DOC) and particulate organic carbon (POC) into rivers during a short period. However, previous studies focused on the impacts of storms on organic carbon transport in headwater streams that are minimally influenced by human activities and are far from lakes. To better estimate the lake carbon budget and manage lake water environments, we need to understand the transport of storm-induced organic carbon into lakes by eutrophic rivers. Based on daily and hourly time-series monitoring data, this paper systematically studied the influences of storm precipitation on DOC and POC transport in the eutrophic Tiaoxi River entering Lake Taihu, the 3rd largest freshwater lake in China. The results showed that seven storms transported 59% of the annual total organic carbon into Lake Taihu in 2019, and all storms resulted in transport peaks. During the storm period on August 9–16, 2019, DOC was negatively related to the water level (r = −0.44, p < 0.05), but POC responded positively (r = 0.52, p < 0.05); allochthonous organic carbon contents were elevated, but the autochthonous components were diluted. Moreover, the storm-induced input of riverine organic carbon influenced the lake water environment across a large region, and the impacts lasted more than 10 days. These findings have important implications for accurately estimating riverine organic carbon fluxes into lakes and making better-informed decisions about when to pump drinking water from lakes.

Surface transport of DOC acts as a trophic link among Mediterranean sub-basins

Lateral advection affects the spatial distribution of dissolved substances in the ocean but very few studies, so far, have been devoted to describe and quantify its impact on the distribution of dissolved organic carbon (DOC) which, in oligotrophic environments, accounts for the largest fraction of chemical energy. In this contribution, using an integrated approach, we explore the importance of surface advection on carbon dynamics in the Western Mediterranean Sea, where strong inter-basin differences in primary production do exist. Detailed information on the surface circulation, derived from high-resolution model simulations, is combined with the analysis of spatially resolved, accurate DOC vertical profiles, repeated over time. Our data show that surface circulation plays a crucial role in regulating DOC concentrations and distributions in the Tyrrhenian Sea and that horizontal transport of DOC into the Tyrrhenian Sea is of the same order of magnitude as in-situ DOC production. Our study addresses this process for the first time in the Mediterranean Sea, whose small size allows for fast, inter-basin transfer times which, in turn, favors the preservation of the DOC stock produced elsewhere. We highlight that this mechanism may be important also in other regions of the oceans, where surface advection may set up a sort of compensation among regions with different trophic regimes, thus smoothing trophic gradients. We posit that understanding these transport processes is a crucial and preliminary step to understand and quantify all the other processes (biological, chemical, geological) that influence DOC distribution on a variety of timescales.

Dissolved organic carbon transport in the Qilian mountainous areas of China

AbstractThe Dissolved Organic Carbon (DOC) content of rivers is the most significant part of the carbon cycle migration in the basin under consideration, and it is the basis for a comprehensive understanding of the regional carbon cycle. In this study, we periodically collected samples from four monitoring stations in the Xiying River Basin of the Qilian Mountains in the northern Qinghai‐Tibet Plateau. We calculated the fluxes of organic carbon in the rivers within the study area and have discussed the influencing factors of DOC concentration in these rivers. The results showed that: (a) The DOC concentration and transport flux of the Xiying river showed significant seasonal changes. The DOC concentration during summer and autumn was higher than that in winter and spring, and the output flux in summer and autumn accounted for approximately 88.3% of the total annual output. (b) Precipitation runoff has a higher DOC concentration than meltwater runoff. Climate factors, river‐water chemical characteristics, and seasonal frozen‐soil changes in the river basin have significant effects on the river DOC concentration and transport flux. (c) Larger runoff causes higher DOC concentrations in rivers. Runoff is the primary means of carbon migration in the inland river basin. Carbon migration is significant from the upstream to the middle and downstream sections of the inland river basin.