Dissolved Organic Carbon Export Research Articles

Warming drives dissolved organic carbon export from pristine alpine soils

Despite decades of research, the influence of climate on the export of dissolved organic carbon (DOC) from soil remains poorly constrained, adding uncertainty to global carbon models. The limited temporal range of contemporary monitoring data, ongoing climate reorganisation and confounding anthropogenic activities muddy the waters further. Here, we reconstruct DOC leaching over the last ~14,000 years using alpine environmental archives (two speleothems and one lake sediment core) across 4° of latitude from Te Waipounamu/South Island of Aotearoa New Zealand. We selected broadly comparable palaeoenvironmental archives in mountainous catchments, free of anthropogenically-induced landscape changes prior to ~1200 C.E. We show that warmer temperatures resulted in increased allochthonous DOC export through the Holocene, most notably during the Holocene Climatic Optimum (HCO), which was some 1.5–2.5 °C warmer than the late pre-industrial period—then decreased during the cooler mid-Holocene. We propose that temperature exerted the key control on the observed doubling to tripling of soil DOC export during the HCO, presumably via temperature-mediated changes in vegetative soil C inputs and microbial degradation rates. Future warming may accelerate DOC export from mountainous catchments, with implications for the global carbon cycle and water quality.

Response times as explanatory variable for export of dissolved organic carbon (DOC) from small forested catchments

Rising trends in the concentrations of dissolved organic carbon (DOC) are observed in many inland waters, including the headwater catchment of the Große Ohe river in the Bavarian Forest National Park (Germany). During flood events, DOC is mobilized via different hydrological pathways, affecting the hydrochemistry of aquatic ecosystems and the viability of drinking water supply. In our field experiments we observed different phases of DOC mobilization during six intensively studied rainfall-runoff events with contrasting antecedent wetness conditions. We propose response time diagrams to link the different phases of DOC mobilization to different response times along a hillslope-riparian-zone-transect. Depending on the antecedent wetness conditions, the hillslope and riparian zone participated differently to phases of the runoff event and shaped the flow hydrograph and DOC export at the catchment outlet. The hillslope always responded with little time delay to precipitation events regardless of the antecedent wetness condition. In contrast, response times in the riparian zone varied. For wet antecedent conditions we observed little delay between the hillslope- and riparian zone peak response, which caused high peak discharge, fast DOC mobilization and high DOC export from the catchment. In contrast, for dry antecedent conditions, the riparian zone response was significantly lower and much delayed. This led not only to attenuated peak discharge but also to larger time lags between the flow hydrograph- and the DOC concentration peak. The combination of low runoff rates and delayed DOC concentration peaks resulted into lower DOC export from the catchment. Thus, the antecedent wetness condition and response times of the hillslope and riparian zone are important indicators for DOC export in the catchment.

Exploring hydrological controls on dissolved organic carbon export dynamics in a typical flash flood catchment using a process-based model

The dynamics of dissolved organic carbon (DOC) export from headwater catchments are of critical importance for the global carbon balance and are driven by complex runoff processes. Most previous studies have used statistical relationships between runoff and DOC concentration to estimate DOC export dynamics. Thus, the coupling mechanisms between runoff generation and DOC export dynamics at the process level were obscured in the fitting parameters and have rarely been addressed. In this study, high-frequency (hourly) discharge and DOC export from a typical flash flood experimental headwater catchment with an area of 1.8 km2 were simulated using a process-based model (INCA-C). The results showed that the INCA-C model successfully captured the hourly dynamics of both discharge and DOC concentrations with a Nash–Sutcliffe efficiency (NSE) of 0.47–0.81 and 0.28–0.70 among moderate events and 0.81–0.85 and 0.19–0.90 among extreme events, respectively. The DOC was exported with distinct concentration dynamics, fluxes, and contributions from the four flow pathways under different storm intensities. At higher intensities, the DOC fluxes were exported by subsurface flows, particularly from shallow organic soil, with greater peaks and shorter time-to-peaks. Exported DOC is primarily sourced from subsurface runoff from the mineral layer (73 %–77 %) during moderate events, whereas it is primarily sourced from subsurface runoff from the organic layer (61 %–79 %) during extreme events. The two contrasting contributions suggest that hydrological pathway controls and DOC dynamic patterns can shift owing to runoff generation influenced by storm intensity. The distinct and variable controls of different flow pathways on DOC export highlight the need to explain the role of hydrology in regulating DOC storm exports through process-based modelling.

Influence of water color and catchment lake cover on stream macroinvertebrate communities: Ecological insights into browning effects

Browning of streams due to increased export of dissolved organic carbon (DOC) and iron has been observed in vast areas of the northern hemisphere with likely adverse ecological effects. Lake basins in stream catchments can moderate DOC export and influence stream communities, which complicates understanding of the effects of DOC. In this study, we explored the independent and interactive effects of water color (proxy for DOC and iron) and catchment lake cover on benthic macroinvertebrate communities in 94 medium-sized boreal forest streams. We first investigated the role of lake basins and other catchment characteristics in controlling water color. We then studied the effects of water color and catchment lake cover on macroinvertebrate community composition, biodiversity, and functional feeding traits. Water color correlated negatively with catchment lake cover, whereas the correlation with peatland cover and drainage intensity was positive. PERMANOVA and GLS analyses indicated that both color and catchment lake cover had a distinct independent effect on invertebrate community composition and community attributes, without significant interactions. Color had an independent negative effect on EPT (Ephemeroptera, Plecoptera, Trichoptera) taxa richness irrespective of lake cover. Increasing water color had negative effect on relative abundance of grazer, but no significant effect on shredder trait, while lake cover had a negative effect on both of the traits. Lake cover exhibited a negative influence on collector-gatherers, and a positive effect on filter feeders, while the predators were positively affected by both factors. The results highlight that water color influences the community structure of boreal stream ecosystems, and the effects are similar regardless of catchment lake cover. Mitigation measures should be emphasized, aimed at reducing DOC and iron runoff, in land use planning and river basin management.

Geomorphologic controls and anthropogenic impacts on dissolved organic carbon from mountainous rivers: insights from optical properties and carbon isotopes

Abstract. Mountainous rivers are critical in transporting dissolved organic carbon (DOC) from terrestrial environments to downstream ecosystems. However, how geomorphologic factors and anthropogenic impacts control the composition and export of DOC in mountainous rivers remains largely unclear. Here, we explore DOC dynamics in three subtropical mountainous catchments (i.e., the Yinjiang, Shiqian, and Yuqing catchments) in southwest China, which are heavily influenced by anthropogenic activities. Water chemistry, stable and radioactive carbon isotopes of DOC (δ13CDOC and Δ14CDOC), and optical properties (UV absorbance and fluorescence spectra) were employed to assess the biogeochemical processes and controlling factors on riverine DOC. The radiocarbon ages of DOC in the Yinjiang River varied widely from 928 years BP to the present. Stepwise multiple regression analyses and partial least square path models revealed that geomorphology and anthropogenic activities were the major drivers controlling DOC concentrations and DOM characteristics. Catchments with higher catchment slope gradients were characterized by lower DOC concentrations, enriched δ13CDOC and Δ14CDOC, and more aromatic dissolved organic matter (DOM), which were opposite to catchments with gentle catchment slopes. Variabilities in DOC concentrations were also regulated by land use, with higher DOC concentrations in urban and agricultural areas. Furthermore, DOM in catchments with a higher proportion of urban and agricultural land uses was less aromatic, less recently produced, and exhibited a higher degree of humification and more autochthonous humic-like DOM. This research highlights the significance of incorporating geomorphologic controls on DOC sources and anthropogenic impacts on DOM composition into the understanding of DOC dynamics and the quality of DOM in mountainous rivers, which are globally abundant.

A Tidal Flat Adjacent to a Fringe Mangrove Forest Mitigates pCO2 Increases and Enhances Lateral Export of Dissolved Carbon

Carbon export from mangrove forests to the oceans partly acts as a sink for atmospheric CO2, exceeding the rate of carbon burial in mangrove soils. Primary production in ecosystems adjacent to mangroves may prevent degassing and enhance further carbon export from mangroves to the oceans. In this study, we continuously monitored carbonate chemistry parameters (pCO2, dissolved inorganic carbon (DIC), total alkalinity (TA)) and dissolved organic carbon (DOC) in a tidal flat adjacent to a fringe mangrove forest over a spring-neap tidal cycle. Mean pCO2 during the entire period was 923 ± 318 μatm, and the export of TA, DIC, and DOC from the mangroves to the ocean was 36 ± 26 mmol m−2 d−1, 42 ± 39 mmol m−2 d−1, and 10 ± 9 mmol m−2 d−1, respectively. Semi-monthly pCO2 variations in the mangrove front were controlled by the tidal level during spring tide and by photosynthesis and respiration on the tidal flat during neap tide. This means that during neap tide, photosynthesis on the tidal flat offset the increase in pCO2 caused by the porewater export from the mangrove soil. The DIC/TA export ratio in this study was 1.17 ± 0.08, which was lower than the global average of 1.41 ± 1.39, indicating that the tidal flat adjacent to the mangrove forest may act as a buffer zone to mitigate the increase in pCO2, resulting in much of the exported DIC being stored in the ocean.

Hydrological connectivity controls dissolved organic carbon exports in a peatland-dominated boreal catchment stream

Abstract. The magnitudes of dissolved organic carbon (DOC) exports from boreal peatlands to streams through lateral subsurface flow vary during the ice-free season. Peatland water table depth and the alternation of low and high flow in peat-draining streams are thought to drive this DOC export variability. However, calculation of the specific DOC exports from a peatland can be challenging considering the multiple potential DOC sources within the catchment. A calculation approach based on the hydrological connectivity between the peat and the stream could help to solve this issue, which is the approach used in the present research. This study took place from June 2018 to October 2019 in a boreal catchment in northeastern Canada, with 76.7 % of the catchment being covered by ombrotrophic peatland. The objectives were to (1) establish relationships between DOC exports from a headwater stream and the peatland hydrology; (2) quantify, at the catchment scale, the amount of DOC laterally exported to the draining stream; and (3) define the patterns of DOC mobilization during high-river-flow events. At the peatland headwater stream outlet, the DOC concentrations were monitored at a high frequency (hourly) using a fluorescent dissolved organic matter (fDOM) sensor, a proxy for DOC concentration. Hydrological variables, such as stream outlet discharge and peatland water table depth (WTD), were continuously monitored at hourly intervals for 2 years. Our results highlight the direct and delayed control of subsurface flow from peat to the stream and associated DOC exports. Rain events raised the peatland WTD, which increased hydrological connectivity between the peatland and the stream. This led to increased stream discharge (Q) and a delayed DOC concentration increase, typical of lateral subsurface flow. The magnitude of the WTD increase played a crucial role in influencing the quantity of DOC exported. Based on the observations that the peatland is the most important contributor to DOC exports at the catchment scale and that other DOC sources were negligible during high-flow periods, we propose a new approach to estimate the specific DOC exports attributable to the peatland by distinguishing between the surfaces used for calculation during high-flow and low-flow periods. In 2018–2019, 92.6 % of DOC was exported during flood events despite the fact that these flood events accounted for 59.1 % of the period. In 2019–2020, 93.8 % of DOC was exported during flood events, which represented 44.1 % of the period. Our analysis of individual flood events revealed three types of events and DOC mobilization patterns. The first type is characterized by high rainfall, leading to an important WTD increase that favours the connection between the peatland and the stream and leading to high DOC exports. The second is characterized by a large WTD increase succeeding a previous event that had depleted DOC available to be transferred to the stream, leading to low DOC exports. The third type corresponds to low rainfall events with an insufficient WTD increase to reconnect the peatland and the stream, leading to low DOC exports. Our results suggest that DOC exports are sensitive to hydroclimatic conditions; moreover, flood events, changes in rainfall regime, ice-free season duration, and porewater temperature may affect the exported DOC and, consequently, partially offset the net carbon sequestration potential of peatlands.

Seasonal and Storm Event‐Based Dynamics of Dissolved Organic Carbon (DOC) Concentration in a Mediterranean Headwater Catchment

AbstractThis study investigates the spatial and temporal dynamics of Dissolved Organic carbon (DOC) concentration in a Mediterranean headwater catchment (Turbolo River catchment, southern Italy) equipped with two multi‐parameter sondes providing more than two‐year (May 2019–November 2021) continuous high‐frequency measurements of several DOC‐related parameters. The sondes were installed in two nested sections, a quasi‐pristine upstream sub‐catchment and a downstream outlet with anthropogenic water quality disturbances. DOC estimates were achieved by correcting the fluorescent dissolved organic matter—fDOM—values through an original procedure not requiring extensive laboratory measurements. Then, DOC dynamics at the seasonal and storm event scales were analyzed. At the seasonal scale, results confirmed the climate control on DOC production, with increasing background concentrations in hot and dry summer months. The hydrological regulation proved crucial for DOC mobilization and export, with the top 10th percentile of discharge associated with up to 79% of the total DOC yield. The analysis at the storm scale using flushing and hysteresis indices highlighted substantial differences between the two catchments. In the steeper upstream catchment, the limited capability of preserving hydraulic connection over time with DOC sources determined the prevalence of transport as the limiting factor to DOC export. In the downstream catchment, transport‐ and source‐limited processes were observed almost equally. The correlation between the hysteretic behavior and antecedent precipitation was not linear since the process reverted to transport‐limited for high accumulated rainfall values. Exploiting high‐resolution measurements, the study provided insights into DOC export dynamics in nested headwater catchments at multiple time scales.

Aquatic Carbon Export and Dynamics in Mountain Headwater Streams of the Western U.S.

AbstractMountain headwater streams actively cycle carbon, receiving it from terrestrial landscapes and exporting it through downstream transport and gas exchange with the atmosphere. Although their importance is now widely recognized, aquatic carbon fluxes in headwater streams remain poorly characterized. In this study, aquatic carbon fluxes were measured in 15 mountain headwater streams and were used in a geostatistical analysis to determine how landscape characteristics influence aquatic carbon fluxes. In‐stream sensors were used to measure fluorescent dissolved organic matter (fDOM) (a surrogate for dissolved organic carbon (DOC)) at a subset of sites to characterize dynamic responses to hydroclimatic events. Wetlands have a positive influence on aquatic carbon fluxes, whereas perennial snow/ice has the opposite effect, reflecting differences in soil organic matter content in these landscapes. Mean annual temperature (MAT) has a complex influence on DOC, with peak DOC exports in basins with MAT of 0–2°C. Precipitation has a strong positive influence on aquatic carbon fluxes, and declining snowpacks in the western United States may reduce future aquatic carbon exports. fDOM (and by implication DOC) and showed strong dynamic responses to snowmelt and rain events, with fDOM increasing and decreasing during events. Combining results from this study with those from a companion study on CO2 exchange yielded total aquatic carbon fluxes of 7.2–15.7 g C m−2 yr−1 (median = 12.22), similar to those from forests and peatlands. Given net ecosystem production (NEP) of similar magnitude, NEP calculations that do not account for losses via the aquatic pathway can substantially overestimate terrestrial carbon sequestration.

Export of Dissolved Organic Carbon (DOC) compared to the particulate and active fluxes near South Georgia, Southern Ocean

Quantifying the relative contributions of the export of particulate organic carbon (POC), dissolved organic carbon (DOC) and active fluxes by migrating organisms is essential to understand the functioning and vulnerability of the ocean's biological pump. However, these fluxes are rarely measured at the same time. Here we provide a first simultaneous comparison of these biological pump components in the region of South Georgia. We use a combination of in-situ data and an inverse model to calculate the DOC export and the suspended POC export and compare them to the sinking POC and active export. We find that, in this region, the DOC total export contributes about 6.6% (23.0–37.5 mg C m−2 day−1) to the total export flux, the active flux has no discernible contribution, and the sinking POC flux is dominant with a mean value of 409 mg C m−2 day−1. Diapycnal fluxes of DOC obtained from the cruise data constitute only a minor fraction (0.05–1.28 mg C m−2 day−1) of the total DOC export estimated by the inverse model and are exceeded on average by the diapycnal flux of suspended POC. Our results also indicate that the total export of DOC is driven by isopycnal transport. Future fieldwork in the region of South Georgia should focus on quantifying the isopycnal flux of DOC. Future measurement campaigns should also aim to simultaneously measure the particulate, dissolved and active components of the biological pump at contrasting locations and at different times to resolve the variability of their relative contribution.

Geomorphic complexity influences coarse particulate organic matter transport and storage in headwater streams

Coarse particulate organic matter (CPOM; organic matter 1–100 mm in diameter, excluding small wood) stored in streams provides an important energy source for aquatic ecosystems, and CPOM transport provides downstream energy subsidies and is a pathway for watershed carbon export. However, we lack understanding of the magnitude of and processes influencing CPOM storage and transport in headwater streams. We assessed how geomorphic complexity and hydrologic regime influence CPOM transport and storage in the Colorado Front Range, USA. We compared CPOM transport during snowmelt in a stream reach with high retentive feature (e.g., wood, cobbles, and other features) frequency to a reach with low retentive feature frequency, assessing how within-a-reach geomorphic context influences CPOM transport. We also compared CPOM transport in reaches with differing valley geometry (two confined reaches versus a wide, multi-thread river bead) to assess the influence of geomorphic variations occurring over larger spatial extents. Additionally, we compared CPOM storage in accumulations in reaches (n = 14) with flowing water or dry conditions in late summer and investigated how small pieces of organic matter [e.g., woody CPOM and small wood (>1 min length and 0.05–1 min diameter or 0.5–1 min length and >0.1 min diameter)] influence CPOM storage. We found that within-a-reach retentive feature frequency did not influence CPOM transport. However, valley geometry influenced CPOM transport, with a higher CPOM transport rate (median: 1.53 g min−1) downstream of a confined stream reach and a lower CPOM transport rate (median: 0.13 g min−1) downstream of a low gradient, multi-thread river bead. Additionally, we found that particulate organic carbon (POC) export (0.063 Mg C) in the form of CPOM was substantially lower than dissolved organic carbon (DOC) export (12.3 Mg C) in one of these headwater streams during the 2022 water year. Dry reaches stored a higher volume of CPOM (mean = 29.18 m3 ha−1) compared to reaches with flowing water (15.75 m3 ha−1), and woody CPOM pieces trapped 37% of CPOM accumulations. Our results demonstrate that the influence of geomorphic context on CPOM transport depends on the scale and type of geomorphic complexity, POC may be lower than DOC export in some headwater streams, and small woody organic material is important for trapping CPOM small streams.

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.

Permafrost Wetlands Are Sources of Dissolved Iron and Dissolved Organic Carbon to the Amur‐Mid Rivers in Summer

AbstractDissolved iron (dFe) transported by the Amur River greatly contributes to phytoplankton growth in the Sea of Okhotsk. Nevertheless, there has been little research on the source of dFe to rivers, especially in the Amur‐Mid Basin, which is situated in a sporadic permafrost area. In the Amur‐Mid Basin, permafrost generally exists in wetlands in flat valleys, and these permafrost wetlands could be a source of dFe to rivers. To assess the importance of permafrost wetlands for dFe export, we conducted a local survey on land and soil characteristics of wetlands, and moreover analyzed the chemical composition (dFe, dissolved organic carbon [DOC], pH, and electrical conductivity [EC]) of 24 rivers with different watershed sizes in summer. As a result of local survey, the thickly accumulated peat soils in the permafrost wetland were almost saturated and rich in organic matter from the surface to a greater depth near the permafrost table. In addition, the coverage of such permafrost wetlands in watersheds showed significant positive correlations with dFe (r2 = 0.67, p = 1.7e−6) and DOC (r2 = 0.48, p = 1.8e−4) and a negative correlation with EC (r2 = 0.52, p = 7.7e−5). The dFe concentration was also correlated well with DOC concentration (r2 = 0.68, p = 7.3e−7) but not correlated with pH and watershed area. These findings are the first to indicate that permafrost wetlands in the Amur‐Mid Basin considerably contribute to dFe and DOC export to rivers, and their coverage primarily determines riverine dFe and DOC concentrations in summer.

Watershed scale modeling of Dissolved organic carbon export from variable source areas

Dissolved organic carbon (DOC) in surface water influences the global carbon cycle, ecosystem productivity, and water quality in potable water supply systems. Few physically based watershed models can simulate carbon cycling and predict DOC in surface waters under the influence of natural and anthropogenic drivers. In this work we transform the SWAT-Carbon (C) model to simulate DOC from variable source runoff areas in a humid forested watershed in the northeastern United States. Remotely sensed data were used to parameterize and simulate forest growth and evapotranspiration. The calibrated model accurately simulated streamflow and DOC flux at six sites across the watershed compared to measured data. The DOC predictions across sites showed model sensitivity to soil properties, particularly soil depth and available water capacity. The spatial distribution of DOC export across the watershed followed the pattern of surface runoff from variable source areas. Model sensitivity of DOC flux showed greater sensitivity to changes in precipitation than temperature. The overall good performance of the model makes it a valuable tool for watershed-scale modeling of DOC to understand the influence of climate and watershed management towards developing mitigation strategies. The methods presented in this study can be used in forested watersheds in regions where runoff from variable source areas is important for water quality predictions.

Changes in carbon metabolism and dissolved organic carbon fluxes on seagrass patches (Halodule wrightii) with different ages in Southern Gulf of California

Seagrass meadows are highly dynamic, particularly in sites where extreme climatological events may produce a mosaic of patches with different ages. This study evaluates the community carbon metabolism, dissolved organic carbon (DOC) fluxes and associated fauna in patches of Halodule wrightii with different ages since establishment. Net community production declined as patch age increased, probably due to the increase in non-photosynthetic tissues, higher respiration rates of the community assemblage and a likely increase in self-shading of the canopy. The export of DOC was significantly higher in the youngest patches, mainly as a consequence of the lower seagrass net production recorded in older meadows. We concluded that ‘colonizers’ seagrass species may show higher production rates and DOC release during the first stages of colonization, which suggest that, the production, organic carbon exportation and their role as relevant blue carbon communities may be higher than expected.

Quantification of Discharge‐Specific Effects on Dissolved Organic Matter Export From Major Arctic Rivers From 1982 Through 2019

AbstractLong‐term increases in Arctic river discharge have been well documented, and observations in the six largest Arctic rivers show strong positive correlations between dissolved organic carbon (DOC) concentration, river discharge, and chromophoric dissolved organic matter (CDOM) content. Here, observations of DOC and CDOM collected from 2009 to 2019 by the Arctic Great Rivers Observatory were used to estimate chromophoric DOC (CDOC) concentrations in the Kolyma, Lena, Mackenzie, Ob', Yenisey, and Yukon Rivers. All rivers except the Mackenzie showed significant positive correlations between annual watershed runoff and the proportion of the DOC that is chromophoric. Historical estimates of DOC and CDOC export were calculated for 1982–2019 by extrapolating the DOC and CDOC concentration—discharge relationships from 2009 to 2019 as a hindcast modeled estimate. For the six rivers combined, modeled DOC and CDOC exports increased, but CDOC increased faster than total DOC. The Lena and Ob' Rivers showed significant increases in DOC export individually, with annual trends of 39.1 and 20.4 Gg C yr−1 respectively. November–April (winter) DOC and CDOC exports increased in all rivers but the Yenisey, with the hindcast winter Kolyma export increasing by more than 20% per decade. There were no significant trends in discharge or associated DOC and CDOC fluxes during the observational period from 2009 to 2019; only when hindcasted values driven by changes in river discharge were analyzed did trends in DOC and CDOC emerge. This demonstrates how shifting seasonal distributions and increases in discharge can drive changes in DOC and CDOC concentrations and exports independent of other environmental factors.

Using 224Ra/228Th disequilibrium to quantify carbon transformation and export from intertidal sandy and muddy sediments

The intertidal zone is an important pathway for the transport of dissolved carbon across the land–ocean interface. Quantifying carbon transformation and export from the intertidal zone remains large uncertainties due to the natural heterogeneity and various driving mechanisms. Here, we used the 224Ra/228Th disequilibrium method to quantify porewater exchange and dissolved carbon export from intertidal sandy and muddy sediments, two main intertidal wetland types in the coast. Porewater exchange fluxes from sandy sediments ranged from 1.4 ± 0.1 to 1084 ± 53 L m−2 h−1, which gradually increased in the landward direction. Porewater exchange fluxes from mangrove muddy sediments ranged from 0 to 4.4 ± 0.7 L m−2 h−1. Advective porewater exchange was recognized as the dominant solute transport process in sandy sediments, whereas bioturbation may be the key process in mangrove muddy sediments. Furthermore, marine dissolved organic carbon (DOC) was removed in sandy sediments, indicating that the sandy sediment acted as a sink of DOC, likely via aerobic remineralization. DOC consumption fluxes ranged from 3.7 ± 0.2 to 206 ± 9 mmol C m−2 d−1. The total consumption flux of DOC in the entire sandy beach was one order of magnitude higher than the globally maximum DOC removal flux in the upper ocean. Porewater-derived dissolved inorganic carbon (DIC) export fluxes from sandy sediments ranged from 5.6 ± 0.3 to 883 ± 43 mmol C m−2 d−1. Nearly 50% of DIC export originated from the consumption of DOC. The rate constants for DOC transformation ranged from 0.009 to 0.155 h−1. In comparison, porewater-derived DOC and DIC export fluxes from muddy sediments were 25 ± 4 and 206 ± 30 mmol C m−2 d−1, respectively. Porewater-derived DIC export from sandy sediments was comparable or even higher than those from muddy sediments. These results showed that sandy sediments acted as a fast lane for marine DOC remineralization. Overall, both organic-poor sandy and organic-rich mangrove muddy sediments play important roles in the delivery of dissolved carbon to the ocean.

Small, Coastal Temperate Rainforest Watersheds Dominate Dissolved Organic Carbon Transport to the Northeast Pacific Ocean

AbstractThe northeast Pacific Coastal Temperate Rainforest (NPCTR) extending from southeast Alaska to northern California is characterized by high precipitation and large stores of recently fixed biological carbon. We show that 3.5 Tg‐C yr−1 as dissolved organic carbon (DOC) is exported from the NPCTR drainage basin to the coastal ocean. More than 56% of this riverine DOC flux originates from thousands of small (mean = 118 km2), coastal watersheds comprising 22% of the NPCTR drainage basin. The average DOC yield from NPCTR coastal watersheds (6.20 g‐C m−2 yr−1) exceeds that from Earth's tropical regions by roughly a factor of three. The highest yields occur in small, coastal watersheds in the central NPCTR due to the balance of moderate temperature, high precipitation, and high soil organic carbon stocks. These findings indicate DOC export from NPCTR watersheds may play an important role in regional‐scale heterotrophy within near‐shore marine ecosystems in the northeast Pacific.

Increasing riverine export of dissolved organic carbon from China.

River transport of dissolved organic carbon (DOC) to the ocean is a crucial but poorly quantified regional carbon cycle component. Large uncertainties remaining on the riverine DOC export from China, as well as its trend and drivers of change, have challenged the reconciliation between atmosphere-based and land-based estimates of China's land carbon sink. Here, we harmonized a large database of riverine in-situ measurements and applied a random forest model, to quantify riverine DOC fluxes (FDOC ) and DOC concentrations (CDOC ) in rivers across China. This study proposes the first DOC modeling effort capable of reproducing well the magnitude of riverine CDOC and FDOC , as well as its trends, on a monthly scale and with a much wider spatial distribution over China compared to previous studies that mainly focused on annual-scale estimates and large rivers. Results show that over the period 2001-2015, the average CDOC was 2.25 ± 0.45 mg/L and average FDOC was 4.04 ± 1.02 Tg/year. Simultaneously, we found a significant increase in FDOC (+0.044 Tg/year2 , p = .01), but little change in CDOC (-0.001 mg/L/year, p > .10). Although the trend in CDOC is not significant at the country scale, it is significantly increasing in the Yangtze River Basin and Huaihe River Basin (0.005 and 0.013 mg/L/year, p < .05) while significantly decreasing in the Yellow River Basin and Southwest Rivers Basin (-0.043 and -0.014 mg/L/year, p = .01). Changes in hydrology, play a stronger role than direct impacts of anthropogenic activities in determining the spatio-temporal variability of FDOC and CDOC across China. However, and in contrast with other basins, the significant increase in CDOC in the Yangtze River Basin and Huaihe River Basin is attributable to direct anthropogenic activities. Given the dominance of hydrology in driving FDOC , the increase in FDOC is likely to continue under the projected increase in river discharge over China resulting from a future wetter climate.

Marine nutrient subsidies promote biogeochemical hotspots in undisturbed, highly humic estuaries

AbstractThe land‐ocean dissolved organic carbon (DOC) flux represents a significant term within the global carbon budget, with peatland‐dominated regions representing the most intense sources of terrestrial DOC export. As the interface between freshwater and marine systems, estuaries have the potential to act as a filter of the land‐ocean carbon flux, removing terrestrially derived DOC, which is present at low concentrations in the oceans, via a combination of physicochemical and biological processes. However, the fate of peat‐derived DOC within estuaries remains poorly quantified, partly due to the complicating influences of heterogeneous soils, land‐use, point sources, and upstream modification of organic matter. To minimize these modifying factors, we studied DOC and inorganic nutrients in four small, peat‐dominated, minimally disturbed, and oligotrophic Falkland Island estuaries. Contrary to expectations, we found limited evidence of physicochemical estuarine DOC removal, and instead observed apparent “hot zones” of biogeochemical activity, where terrestrially‐derived silicate mixed with inorganic nitrogen and phosphorus entering the estuaries from the nutrient‐rich marine ecosystem. In two estuaries, this coincided with apparent in situ DOC production. We suggest that the observed phenomena of marine nutrient subsidy of estuarine productivity, and flexible utilization of multiple nutrients within the oligotrophic system, may once have been widespread in temperate estuaries. However, this function has been lost in many ecosystems due to catchment eutrophication by agricultural and urban development. We conclude that the estuaries of the Falkland Islands provide a valuable pre‐disturbance analogue for natural biogeochemical functioning in temperate estuaries receiving high organic matter inputs.