Riverine Dissolved Organic Carbon Flux Research Articles

Dissolved organic carbon (DOC) and labile organic compounds' spatial and temporal variations in coastal Indian groundwater: their bioavailability and transfer to neighboring coastal waters of India.

Submarine groundwater drainage (SGD) changes the elemental composition of the neighboring coastal ocean and impacts the biogeochemical cycles. To examine the seasonal and spatial variability in dissolved organic carbon (DOC) and labile organic compound biochemical compounds like dissolved carbohydrates (TDCHO), dissolved proteins (TDPRO), and dissolved free amino acid (TDFAA) concentrations during the dry and wet periods, groundwater samples were taken at 90 locations (180 samples) along the Indian coast. The mean DOC contents in Indian coastal groundwaters were more significant than the global mean values. DOC, TDCHO, TDPRO, and TDFAA concentrations are higher during wet than dry periods. The DOC and labile organic compound showed a substantial positive association with soil organic carbon, and respective labile compounds in soil, population, and land usage and poor relation with woodland territories, implying that soil organic compounds leaching is a source of DOC and other labile organic compounds into the groundwater. DOC and other labile compounds concentrations were linearly associated with population density, land usage, and sewage production, demonstrating that anthropogenic activities tightly regulate the formation of DOC in groundwater. During the wet and dry periods, total labile organic compounds (TDCHO, TDFAA, and TDPRO) constituted 21% and 10.5% of DOC, respectively. Compared to the wet time, more aromatic compounds accumulated during the dry season but were less bioavailable. SGD DOC flux contributed 2-7% of riverine DOC flux to the coastal ocean. The SGD flux from the Indian subcontinent to the nearby northern Indian Ocean accounts for approximately 2% of the worldwide SGD flux. The effect of DOC flux via SGD on coastal bacterial activity, the plankton food web, and the oxygen minimum zone must be studied.

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.

Riverine Dissolved Organic Carbon and Freshwater Export in the Eastern Gulf of Alaska

AbstractThe coastal zone of southeast Alaska contains thousands of streams and rivers that drain one of the wettest, carbon‐rich, and most topographically varied regions in North America. Watersheds draining temperate rainforests, peatlands, glaciers, and three large rivers that flow from the drier interior of the Yukon Territory and British Columbia discharge water and dissolved organic carbon (DOC) into southeast Alaskan coastal waters. This area, which we have designated the southeast Alaska drainage basin (SEAKDB), discharges about twice as much water as the Columbia or Yukon Rivers. An understanding of the timing, location, and source of water and DOC guides research to better understand the influence of terrestrial outputs on the adjacent marine systems. Additionally, a spatially extensive understanding of riverine DOC flux will improve our understanding of lateral losses related to terrestrial carbon cycling. We estimate 1.17 Tg C yr−1 of DOC enters the adjacent marine system along with 430 km2 of freshwater that influences estuary, shelf, and Gulf of Alaska hydrology. We estimate that 23% to 66% of the DOC entering coastal waters is bioavailable and may influence metabolism and productivity within the marine system. The combination of the large and spatially distributed water and DOC input, long and complex shoreline, large enclosed estuarine volume, and bounded nearshore coastal currents suggests that the physiographic structure of southeast Alaska may have a significant impact on the metabolism of riverine DOC in coastal marine ecosystems.

A New Remote Sensing Method to Estimate River to Ocean DOC Flux in Peatland Dominated Sarawak Coastal Regions, Borneo

We present a new remote sensing based method to estimate dissolved organic carbon (DOC) flux discharged from rivers into coastal waters off the Sarawak region in Borneo. This method comprises three steps. In the first step, we developed an algorithm for estimating DOC concentrations using the ratio of Landsat-8 Red to Green bands B4/B3 (DOC (μM C) = 89.86 ·e0.27·(B4/B3)), which showed good correlation (R = 0.88) and low mean relative error (+5.71%) between measured and predicted DOC. In the second step, we used TRMM Multisatellite Precipitation Analysis (TMPA) precipitation data to estimate river discharge for the river basins. In the final step, DOC flux for each river catchment was then estimated by combining Landsat-8 derived DOC concentrations and TMPA derived river discharge. The analysis of remote sensing derived DOC flux (April 2013 to December 2018) shows that Sarawak coastal waters off the Rajang river basin, received the highest DOC flux (72% of total) with an average of 168 Gg C per year in our study area, has seasonal variability. The whole of Sarawak represents about 0.1% of the global annual riverine and estuarine DOC flux. The results presented in this study demonstrate the ability to estimate DOC flux using satellite remotely sensed observations.

Global-scale daily riverine DOC fluxes from lands to the oceans with a generic model

Abstract The export of riverine dissolved organic carbon (DOC) to the oceans is determinant in carbon exchanges of the estuaries and oceanic food webs. Past research returned a global DOC export around 160–450 TgC.yr−1 by using complex process-based models or yearly average estimates that could have been misjudged. In this study, we try to understand the complex processes explaining daily DOC exports among 341 exoreic watersheds covering 71% of freshwater flows to the oceans. Based on a dataset of DOC concentrations among rivers at the global scale, we were able to link DOC concentrations to daily discharge, the ration between the soil organic carbon content and the amount of precipitations and the average air temperature in the watersheds. We have found a global riverine DOC flux of 131.6 TgC.yr−1 based on daily data and a generic model. Tropical and cold watersheds are the main contributors with 49.5% and 33.3% of the global riverine DOC flux on the two last decades while temperate, semi-arid and polar basins represent 15.9%, 0.7% and 0.6%, respectively. Temporal exports range from 0.1 to 0.16 TgC.day−1 in tropical areas, 0.03–0.32 TgC.day−1 in cold areas and 0.03–0.08 TgC.day−1 in temperate areas. Atlantic and Arctic oceans receive the most important fluxes (0.12–0.2 and 0.01–0.26 TgC.day−1). In a climate change context, this generic equation could be introduced in hydrological modelling tools to predict future DOC fluxes trends.

Hydro-ecological controls on riverine organic carbon dynamics in the tropical monsoon region

Transport fluxes and properties of riverine organic carbon in the tropical monsoon region were the vital parameters in the global riverine organic carbon fluxes budget. The study focused on the riverine organic carbon in the Changhuajiang River (CHJR), locating at the mid-west of the Hainan Island, China. Dissolved organic carbon (DOC) concentrations in the CHJR ranged from 0.22 mg/L to 11.75 mg/L with an average of 1.75 mg/L, which was lower than the average of global rivers and had a significantly temporal and spatial variation. Output flux of riverine DOC was calculated as 0.55 t/km2/y, which could be revised up to 1.03 t/km2/y, considering that the riverine discharge before dam construction. A linear model of riverine DOC flux suitable in CHJR basin was established, which involved the factors, such as soil organic carbon, runoff depth and slope, etc. There was a large variation of POC concentrations in the CHJR where the average POC concentration in the dry season was 2.41 times of the wet season. Riverine POC flux in CHJR basin was calculated as 1.78 t/km2/y, higher than the average of global rivers and far lower than those in other domestic larger rivers. About 8.28 × 103 t POC were exported yearly in CHJR, of which, 7.15 × 103 t originated from terrestrial ecosystem and 1.13 × 103 t stemmed from aquatic ecosystem. Meanwhile, about 87.74% of terrestrial source happened in the wet season and 12.26% in the dry season. This research revealed that the riverine organic carbon mainly stemmed from the surface erosion processes in the drainage basin during the wet season.

Seasonal Patterns of Riverine Carbon Sources and Export in NW Greenland

AbstractGlacial runoff exports large amounts of carbon (C) to the oceans, but major uncertainty remains regarding sources, seasonality, and magnitude. We apportioned C exported by five rivers from glacial and periglacial sources in northwest Greenland by monitoring discharge, water sources (δ18O), concentration and composition of dissolved organic carbon (DOC), and ages (14C) of DOC and particulate organic C over three summers (2010–2012). We found that particulate organic C (F = 1.0366–0.2506) was generally older than DOC in glacial sourced rivers and likely sourced from the physical erosion of aged C pools. Most exported DOC showed strong seasonal variations in sources and discharge. In summer, mean DOC ages ranged from modern to 4,750 cal years BP (F = 1.0022–0.6291); however, the annual C flux from glacially sourced rivers was dominated by young, plant‐derived DOC (F = 0.9667–1.002) exported during the spring freshet. The most aged DOC (F = 0.6891–0.8297) was exported in middle to late summer at lower concentrations and was glacial in origin. Scaled to the whole of Greenland using model‐estimated runoff, we estimate a total riverine DOC flux of 0.29% to 0.45% ± 20% Tg C/year. Our flux results indicate that the highest C fluxes occur during the time of year when the majority of C is modern in age. However, higher melt rates from the Greenland ice sheet and longer growing seasons could result in increasing amounts of ancient C from the Greenland ice sheet and from the periglacial landscape to the ocean.

Modeling Global Riverine DOC Flux Dynamics From 1951 to 2015

AbstractClimate change has a profound impact on the global carbon cycle, including effects on riverine carbon pools, which connect terrestrial, oceanic, and atmospheric carbon pools. Until now, terrestrial ecosystem models have rarely incorporated riverine carbon components into global carbon budgets. Here we developed a new process‐based model, TRIPLEX‐HYDRA (TRIPLEX‐hydrological routing algorithm), that considers the production, consumption, and transport processes of nonanthropogenic dissolved organic carbon (DOC) from soil to river ecosystems. After the parameter calibration, model results explained more than 50% of temporal variations in all but three rivers. Validation results suggested that DOC yield simulated by TRIPLEX‐HYDRA has a good fit (R2 = 0.61, n = 71, p < 0.001) with global river observations. And then, we applied this model for global rivers. We found that mean DOC yield of global river approximately 1.08 g C/m2 year, where most high DOC yield appeared in the rivers from high northern or tropic regions. Furthermore, our results suggested that global riverine DOC flux appeared a significant decrease trend (average rate: 0.38 Pg C/year) from 1951 to 2015, although the variation patterns of DOC fluxes in global rivers are diverse. A decreasing trend in riverine DOC flux appeared in the middle and high northern latitude regions (30–90°N), which could be attributable to an increased flow path and DOC degradation during the transport process. Furthermore, increasing trend of DOC fluxes is found in rivers from tropical regions (30°S–30°N), which might be related to an increase in terrestrial organic carbon input. Many other rivers (e.g., Mississippi, Yangtze, and Lena rivers) experienced no significant changes under a changing environment.

The dynamic observation of dissolved organic matter in the Zhujiang (Pearl River) Estuary in China from space

The distributions of estuarine colored dissolved organic matter (CDOM) are the combined results of physicalbiogeochemical processes. Remote sensing is needed to monitor highly dynamically estuarine CDOM. Using in situ data from four seasonal cruises, an algorithm is developed to estimate CDOM absorption coefficient at 400 nm (aCDOM(400)) in the Zhujiang (Pearl River) Estuary (ZJE). The algorithm uses band ratios of Rrs(667)/Rrs(443) and Rrs(748)/Rrs(412). By applying it to moderate resolution imaging spectroradiometer onboard Aqua satellite (MODIS/Aqua) data from 2002 to 2014, seasonal climatology aCDOM(400) in the ZJE is calculated. CDOM distributions are majorly influenced by water discharge from the Zhujiang River and underwater topography. Along the section vertical to a water depth gradient, the seasonal aCDOM(400) exponentially decreased (y=aebx, b<0), but with great differences among seasons. Riverine fresh water is the primary source of CDOM in the ZJE. Fulvic acid fraction decreases with increasing salinity. Using developed algorithms, conservative CDOM mixing equation, and river discharge, effective riverine end-member concentration and flux of dissolved organic carbon (DOC) in summer and winter from 2002 to 2014 are first estimated from the MODIS/Aqua data. Both effective riverine end-member DOC concentration and flux are positively related to the river discharge, significantly in summer with R2 of 0.698 for concentration and 0.965 7 for flux.

The carbon flux of global rivers: A re-evaluation of amount and spatial patterns

Global rivers connect three large carbon reservoirs in the world: soil, atmosphere, and ocean. The amount and spatial pattern of riverine carbon flux are essential for the global carbon budget but are still not well understood. Therefore, three linear regression models for riverine DOC (dissolved organic carbon), POC (particulate organic carbon), and DIC (dissolved inorganic carbon) fluxes were established with related generating and transfer factors based on an updated global database. The three models then were applied to simulate the spatial distribution of riverine DOC, POC, and DIC fluxes and to estimate the total global riverine carbon flux. The major conclusions of this study are as follows: the correlation analysis showed that riverine DOC flux is significantly related to discharge (r2=0.93, n=109) and soil organic carbon amount (r2=0.60), POC flux increases with discharge (r2=0.55, n=98) and amount of soil erosion (r2=0.48), and DIC flux is strongly linked to CO2 consumption by rock weathering (r2=0.66, n=111) and discharge (r2=0.63). In addition, Asia exports more DOC and POC than other continents and North America exports more DIC. The Atlantic Ocean accepts the major portion of riverine DOC, POC, and DIC fluxes of all the oceans. The highest riverine DOC flux occurs in the 0–30°S zone, and the highest riverine POC and DIC fluxes appear in the 30–60°N zone. Furthermore, re-estimation revealed that global rivers export approximately 1.06PgC to oceans every year, including 0.24Pg DOC, 0.24Pg POC, 0.41Pg DIC, and 0.17Pg PIC.

Variation of dissolved organic carbon transported by two Chinese rivers: The Changjiang River and Yellow River

Real-time monitoring of riverine dissolved organic carbon (DOC) and the associated controlling factors is essential to coastal ocean management. This study was the first to simulate the monthly DOC concentrations at the Datong Hydrometric Station for the Changjiang River and at the Lijin Hydrometric Station for the Yellow River from 2000 to 2013 using a multilayer back-propagation neural network (MBPNN), along with basin remote-sensing products and river in situ data. The average absolute error between the modeled values and in situ values was 9.98% for the Changjiang River and 10.84% for the Yellow River. As an effect of water dilution, the variations of DOC concentrations in the two rivers were significantly negatively affected by discharge, with lower values reported during the wet season. Moreover, vegetation growth status and agricultural activities, represented by the gross primary product (GPP) and cropland area percent (CropPer) in the river basin, respectively, also significantly affected the DOC concentration in the Changjiang River, but not the Yellow River. The monthly riverine DOC flux was calculated using modeled DOC concentrations. In particular, the riverine DOC fluxes were affected by discharge, with 71.06% being reported for the Changjiang River and 90.71% for the Yellow River. Over the past decade, both DOC concentration and flux in the two rivers have not shown significant changes.

Dissolved Organic Carbon in Upland Forested Watersheds Underlain by Continuous Permafrost in Central Siberia

Hydroclimatic variability and plant species ecology cause mosaics in forested watersheds in permafrost zones. Measurements of organic matter accumulation, stock of dissolved organic carbon (DOC), DOC concentrations in litter leachates, subsurface flow, stream and seasonal and annual export were made in two contrasting slopes and valleys in the northern taiga of Central Siberia. Increased organic carbon accumulation in litter was found in poor hydroclimatic conditions of the north-facing slope and bottom valleys. In contrast, DOC contents and its export to soil were almost two-fold higher in warmer well-drained sites of the south-facing slope. The overall DOC flux to mineral soil from the beginning of June to mid-September was 17 g C m−2 in the south-facing slope sites and only 9 g C m−2 in the north-facing slope sites. DOC export was positively correlated with precipitation stimulating leaching of mobilizable organic matter. Accordingly intra-seasonal and inter-annual variability of the DOC fluxes was tightly coupled with water input. Meanwhile DOC export in Sphagnum and feathermoss sites showed different behavior in dry and wet years. The presence of permafrost preventing deep seepage of organic solutes results in higher stream DOC fluxes compared to permafrost-free or island permafrost regions. However, thawing of seasonally frozen soil layer during the growing season led to the decreasing concentrations of DOC in the stream from June to September. For two seasons of continuous stream water sampling (June and August–September), the riverine DOC flux constituted about 14% of DOC entering mineral soil on both slopes. The ratio of hydrologic DOC loss to NPP of larch forests of the region was estimated to be 1.1%.

Distribution of dissolved organic matter in the eastern Bering Sea, Chukchi Sea (Barrow Canyon) and Beaufort Sea

The distribution of dissolved organic carbon (DOC) in the western Arctic Ocean is greatly influenced by conservative riverine DOC flux into the northern Bering Sea, Bering Strait and Beaufort Sea, as indicated by an inverse correlation with salinity. Based on the relations between DOC, salinity and seawater temperature, several water masses could be identified. These included riverine water, ice‐melt water, surface mixed water, cold and saline shelf water, and Atlantic Ocean water. High concentrations of chlorophyll a and DOC were found in some parcels of dense shelf water in Barrow Canyon. In addition, labile DOC compounds, such as polyunsaturated fatty acids (PUFA), were found in the dense shelf water, suggesting the dense shelf water contains a product (marine organic matter) of the biological CO2 pump.

Flux and bioavailability of Cape Fear River and rainwater dissolved organic carbon to Long Bay, southeastern United States

This study presents the first comparison of riverine and rainwater dissolved organic carbon (DOC) flux to a coastal region, and the first examination of rainwater DOC bioavailability. The bioavailability of DOC from these two sources was determined because its impact on coastal waters depends on whether DOC is labile or refractory. The study site, Long Bay, off the North Carolina and South Carolina coasts in the southeastern United States, receives external sources of DOC from the Cape Fear River system and rainwater. Conservative mixing of estuarine DOC, and a consistent DOC value for the river sources entering the Cape Fear Estuary (Cape Fear and Black rivers combined: 937 ± 178 μM, n = 12; Northeast Cape Fear River: 1266 ± 196 μM, n = 8) allowed for a calculation of DOC flux using river DOC values and U.S.G.S. river flow data. Rainwater DOC flux was determined by a deposition calculation at a rainwater collection station in Wilmington, N. C., 40 km from Long Bay. Riverine DOC flux was determined to be (77 × 109 gCyr−1) or roughly 4x the rainwater flux (21 × 109 gCyr−1) on an annual basis. The bioavailability of the riverine DOC (9.0 ± 4.5%) was much less than that of rainwater (63 ± 14%). Therefore, taking into account only bioavailable DOC, rainwater provides more labile DOC (13 × 109 gCyr−1) to Long Bay than riverwater (7 × 109 gCyr−1) meaning that rainwater is important for fueling secondary productivity whereas river DOC is important for long‐term storage of carbon.

Soil C:N ratio as a predictor of annual riverine DOC flux at local and global scales

Dissolved organic carbon (DOC) is important in a wide variety of chemical, physical, and biological processes in surface waters. We examined the relationship between DOC flux and soil C:N ratio on a biome basis. DOC fluxes for 164 rivers were subdivided into 15 biome types including tropical rain forest, coniferous forests, peatland, deciduous forests, mixed forests, and grasslands. A database of soil C:N ratios was constructed and subdivided into biome types. At a global scale, mean soil C:N ratio of a biome accounts for 99.2% of the variance in annual riverine DOC flux among biomes. The relationship between soil C:N ratio and DOC flux at the biome scale was used to predict annual riverine DOC flux at the watershed scale for three test watersheds not included in the original model. Predicted flux of each watershed was within 4.5% of the actual DOC flux. Using the C:N model, we estimated the total export of carbon from land to the oceans to be 3.6×1014 g yr−1. This empirical model should be useful in predicting changes in DOC flux under changing climatic conditions.

Fluxes of dissolved organic carbon from California continental margin sediments

Fluxes of dissolved organic carbon (DOC) from marine sediments represent a poorly constrained component of the oceanic carbon cycle that may affect the concentration and composition of DOC in the ocean. Here we report the first in situ measurements of DOC fluxes from continental margin sediments (water depths ranging from 95 to 3,700 m), and compare these fluxes with measured benthic fluxes from 20 other coastal and continental margin sediments. With this combined data set data we have estimated that benthic DOC fluxes are less than ∼10% of sediment carbon oxidation rates, and that the integrated DOC flux from sediments in water depths less than 2,000 m is ∼180 Tg C/yr. These fluxes are roughly equivalent to the riverine DOC flux, and the organic carbon burial rate in marine sediments. Benthic DOC fluxes therefore represent an important net source of DOC to the oceans. We also note that: (1) benthic DOC fluxes represent a loss of organic carbon from sediments; (2) in many sediments these fluxes appear to be controlled by molecular diffusion (i.e., by pore water concentration gradients); (3) pore water DOC may be an important intermediate in sediment carbon burial and preservation. These observations therefore suggest a linkage between benthic DOC fluxes and sediment carbon preservation that may be mediated by pore water DOC concentrations and cycling. The magnitude and fate of DOC effluxing from marine sediments is thus important to understanding carbon cycles and budgets in the marine environment.