Total Organic Carbon Transport Research Articles

Sediment organic carbon and nitrogen trapping in a small reservoir within a typical agricultural granite catchment

Small reservoirs are more widely distributed globally than larger reservoirs, and they have characteristic sediment total organic carbon (TOC) and total nitrogen (TN) trapping patterns that differ from those of larger reservoirs. Few studies have quantified the TOC and TN trapping in a small reservoir within a catchment underlain by a single lithology, particularly in non-karst terrain, from the perspective of environmental change. Based on the chronologies established using 137Cs and 210Pbex, we assessed the temporal patterns of sediment TOC and TN trapping in a small reservoir within a typical agricultural granite catchment (Yimeng Mountains, China) by examining depth trends in TOC and TN concentrations in three sediment cores and temporal trends in TOC and TN deposition rates in the reservoir over the past ∼60 years. The TOC and TN concentrations and C/N ratios show an overall upward increase in the three cores. The TOC and TN deposition rates were relatively high during 1954–1962, decreased to the lowest rates during 1963–1972, and generally increased thereafter. Anthropogenic sources associated with agriculture were the dominant factor in the up-core increases in TOC and TN concentrations, although natural sources were the main contributor to TOC and TN deposition. The temporal changes in TOC and TN deposition rates were mainly due to the sediment yield rather than anthropogenic sources associated with agriculture. The changes in TOC and TN concentrations with depth, and temporal changes in the TOC and TN deposition rates determined from the three cores, reflect the temporal patterns of sediment TOC and TN trapping in the small reservoir over the past ∼60 years. Sediment yield exerted a greater effect on the TOC and TN deposition in the studied reservoir, as compared with a previously reported reservoir within a small agricultural limestone catchment, and the different results of the two studies originated from the contrasting lithologies in the two catchments. Moreover, a comparison with data obtained for a previously studied large reservoir suggests that differences in TOC and TN deposition (including concentrations, specific deposition rates, and temporal trends in deposition rates) are related to catchment size, and the comparison highlights the characteristic patterns of TOC and TN trapping in the small and large reservoirs. The present findings improve our understanding of the temporal patterns of sediment TOC and TN trapping in a small reservoir within a typical agricultural granite catchment in terms of environmental changes influenced by human activity. This study also enhances our knowledge of the barrier effects of a dam on the downstream transport of TOC and TN in a small catchment in a non-karst setting, and the redistribution of sediment TOC and TN in a large river, which can help guide strategies for water resource management on the river basin-scale.

Organic carbon and planktic foraminifera radiocarbon derived Holocene sediment accumulation rates in the northern slopes of the Gulf of Mexico

In the context of climate regulation and anthropogenic waste detoxification (e.g. oil spills), estimates of deep ocean sedimentation and carbon sequestration are of the utmost importance. Radiocarbon (14C) is a common radioisotope that can be used to establish millennial scale sediment accumulation rates. The objectives of this study were to: 1) establish ages for co-occurring total organic carbon (TOC) and planktic foraminifera (carbonate) in the northeastern Gulf of Mexico (GoM), 2) use these ages to estimate accumulation rates independently, 3) identify any evidence of redistribution, and 4) examine any offset between TOC and carbonate 14C ages as a tool to potentially identify selective TOC transport. Sediment samples were collected in May 2018 from the RV Point Sur using an Ocean Instruments MC-800 multi corer. Radiocarbon measurements of both planktic foraminifera and TOC subsamples were made at the National Ocean Science Accelerator Mass Spectrometry Facility (NOSAMS). Radiocarbon ages, calibrated using the OxCal 4.4, ranged from recent to 6407 BP. Linear (LAR: 4–24 cm/kyr) and mass accumulation rates (MAR: 1.5–11.5 g/cm2/kyr) were generally consistent with those reported by other recent studies in the GoM. At two sites, C14 ages decreased from the surface to the second sampling increment which was consistent with sediment redistribution. The TOC-carbonate offsets, which are indicative of lateral advection and organic matter aging, were lower than those found in the majority of other regions, which was consistent with less lateral transport or a more oligotrophic setting. The magnitude in radiocarbon age offsets with depth could potentially be used as a relative aging or transport assessment tool in areas with little resuspension.

Bulk carbon and lignin fingerprinting of catchment sediments transported by mountain rivers in Nepal Himalayas

The Himalayan rivers yield the most significant flux of continental sediments into the ocean. Organic matter (OM) transported by these rivers provides a peek at the influence of diverse geological terrains, soil types, vegetation, and climate on carbon cycling within a narrow boundary. We analyzed suspended and bedload sediments from four Himalayan rivers to trace their sources, elucidate their fate during fluvial transport, and estimate the organic carbon (OC) flux. Hence, total OC (TOC), dissolved organic carbon (DOC), C:N ratios, and lignin phenols were measured. Consistent with the erosional intensity in the rivers, suspended sediment load input followed the order: Kaligandaki > Myagdikhola > Aadhikhola > Tinahukhola. C:N values in rivers from the Lesser Himalayas and Siwalik indicate sediments from mixed biogenic sources. In contrast, high TOC and C/N values in the trans-Himalaya rivers flowing through barren landscapes reflect the erosion of catchment sediments yielding petrogenic carbon. The suspended matter in rivers from the Lesser Himalayas and Siwalik has higher lignin phenol concentrations than the trans-Himalaya and Higher Himalaya rivers. The lignin phenol ratios indicate higher degradation in rivers from the trans and Higher Himalaya sections. This implies that only a small fraction of the terrestrial OM transported by these rivers deposits in the ocean sink. In contrast, rivers from the Lesser Himalayas and Siwaliks sequester a significant amount of OM bound to their bedload. As a result, these rivers transferred lower particulate OC (POC) but higher DOC than similar rivers worldwide. Rivers from Lesser Himalayas and Siwaliks transfer > 90 % of annual POC flux during monsoons. Finally, although Himalayan rivers transport less OC than other global rivers traversing densely vegetated landscapes, the sheer number of these rivers has significant implications on the fate and transport of total OC from catchments sediments.

210Pb-Derived Bioturbation Rates in Sediments Around Seamounts in the Tropical Northwest Pacific

To evaluate bioturbation coefficients (DB) and mixing depths (L), 210Pb and 226Ra activity was measured in two sediments cores (from water depths of 5,398 m and 4,428 m), which were collected from seamount areas in the Northwest Pacific. Using a steady-state diffusion mode, we estimated DB values of 16.8 and 24.1 cm2/a, higher than those in abyssal sediments and those predicted by traditional empirical equations. Corresponding L values varied between 19.3 and 23.1 cm. These high values indicate that seamounts are the area of active bioturbation. A one-dimensional model for the transport of total organic carbon (TOC) from the surface layer of sediments to the deep layer was developed using the distribution pattern of the specific activity of excess 210Pb (210Pbex) and its relationship with TOC. The model showed that the TOC flux transmitted downward by bioturbation was 0.09 mmol/(cm2⋅a) and 0.12 mmol/(cm2⋅a).

The effects of mulching with organic materials on the soil nutrient and carbon transport by runoff under simulated rainfall conditions

The aim of this study is to reveal the effects of different organic mulch materials and application at different rates on the transport of nitrate nitrogen (NO3−), ammonium nitrogen (NH4+), total nitrogen (TN), phosphate phosphorus (PO4−3) and total organic carbon (TOC) by runoff under the laboratory conditions and determine whether they affect the pH and electrical conductivity (EC) values in runoff after the application. Three different organic mulch materials (dry weed, wheat straw, and peanut hay) were applied on experimental plots prepared using Cambisol soils developed on sedimentary bedrock under laboratory conditions at four different rates (0 t/ha, 2 t/ha, 4 t/ha, and 6 t/ha). The experimental plots were placed on a smooth surface with a 9% slope, and simulated rainfall was applied at an intensity of 97 mm/h for 1 h. Runoff was collected, and NO3−, NH4+, TN, PO4−3, TOC, pH, and EC were analysed. According to the results, peanut hay and straw mulch applications reduced NH4+concentrations in runoff. The most effective mulch type in reducing NO3− concentration was straw mulch at a rate of 4 t/ha. All three mulch types increased the concentration of TN, TOC, and PO4−3 in runoff. The lowest TN transport in water was found in the plots with straw mulch at 4 t/ha. The lowest TOC transport was found in plots with mulch application at a rate of 2 t/ha. The minimum PO4−3 loss was found in dry weed mulch treatments at a rate of 2 t/ha. Mulch applications generally increased the pH and EC values of runoff. In general, straw mulch reducing nutrient and TOC transport by runoff, followed by dry weed mulch applications. The type and application rate of mulches should be chosen carefully in terms of protecting water resources.

Projecting impacts of wildfire and climate change on streamflow, sediment, and organic carbon yields in a forested watershed

Increasing temperatures and irregular precipitation associated with climate change, along with increasing frequency and severity of wildfires, contribute to increased downstream transport of sediment and total organic carbon (TOC), with potential impacts on aquatic ecosystem structure and resilience, recreational use of water bodies, and downstream drinking water treatment. Our study aimed to investigate the effects of both climate change and wildfires on water budget, sediment, and organic carbon by simulating the response of sub-catchments and in-stream processes to changes in future climate and wildfire scenarios. To achieve this, we applied a physical process-based hydrologic model, where an in-stream Organic Carbon Simulation Module was embedded within the Soil and Water Assessment Tool (SWAT-OCSM), to the Elbow River watershed in Alberta, Canada. Post-wildfire conditions of both moderate and high burn severities were replicated in the model within two burn perimeters to assess in-stream organic carbon processes related to particulate organic carbon (POC) and dissolved organic carbon (DOC) as state variables under changing climate. Results of the climate change scenarios indicated lower streamflow relative to the baseline period (1995–2014), particularly between May–August, with 25.3–46.9% less water in the near future (2015–2034) compared to 9.9–31.8% less water in the distant future (2043–2062). Sediment concentrations generally decreased, whereas TOC concentrations increased, in both the near future and distant future scenarios reflecting uncertainty in climate effects on water quality. Wildfire simulations compounded with climate change significantly changed local hydrology, increasing surface runoff, sediment, and TOC transport by over 500% in some study sub-catchments. However, at the watershed outlet, sediment yields only increased up to 6.5% and TOC yields increased up to 13.1%. Burn severity invoked a stronger watershed response than burn area, and greater relative changes were observed for wildfires occurring with the worst-case climate change scenarios. This study provided a strong basis for analyzing watershed responses to potential future wildfires. However, recommendations are provided for further model developments to account for wildfire consequences and feedbacks with hydrological and biogeochemical processes.

The nutrient and carbon losses of soils from different land cover systems under simulated rainfall conditions

Nutrient losses constitute an important issue for the protection of both soil and water resources. Organic carbon transport is also important for the global carbon cycle. In this study, it was aimed to determine the amount of nutrients and the total organic carbon which was transported by surface runoff water formed under simulated rainfall conditions from areas with different land cover systems.Accordingly, 3 cultivated areas having various product patterns under similar conditions (potato field (P), bean-cultivated area (B), abandoned farmland covered with Noaea sp. (AB)), 2 plantation areas (cedar (C) and almond (AL)) and 2 rangelands covered with Thymus sp. (T) and Stipa sp. (S) were chosen for performing applications. The applications were performed in two repetitions on 0.28 m2 circular experimental plots established in the above-mentioned areas. The collection of surface runoff was performed during land applications. The pH, electrical conductivity, nitrate (NO3−), ammonium (NH4+), total phosphorus, orthophosphate (PO43−), total organic carbon (TOC), and total nitrogen (TN) parameters of surface runoff water were measured. The land cover change affects total nitrogen, nitrate, ammonium, pH, electrical conductivity, total phosphorus, orthophosphate and TOC parameters. Nitrate transport was found to be B > P > T > AB > C > AL > S from highest to lowest. Ammonium transport was B > T > AL > C > AB > P > S. The total nitrogen transport was B > P > T > AB > C > AL. The total phosphorus transport was found to be P > B = T > AB > AL > C > S. Ortho-phosphate transport was P > B > T > AB > C > AL > S. In terms of the TOC transport, the order was found to be T > B > P > AB > AL > C from highest to lowest.

Total organic carbon transport by the Alaknanda River, Garhwal Himalayas, India

Transport and fate of organic carbon by the fluvial system play a significant role in the global biogeochemical cycle of carbon. Previous studies show that the transportation of modern organic carbon from the Himalayan River system accounts for 10–20% of the total global flux to the oceans. Till date, no study has been published which dealt with the transport of organic carbon in the headwaters of the Ganga River. The Alaknanda River is a headwater stream of the Ganga, which flows in the Western Himalayas of India. Water and freshly deposited channel sediment samples were collected during the months of March 2014 and August 2014 and analysed for dissolved organic carbon (DOC), particulate organic carbon (POC) and channel organic carbon (COC). The observed variability of organic carbon concentration was correlated with factors such as discharge, physiography and suspended sediment concentration (SSC). The results show that seasonal erosivity in the basin influences its DOC concentration and physiography, thus acting as a key parameter which controls transportation, oxidation and residence time of the organic matter. The allochthonous input of sediments from the erosional activities is the major source of organic carbon. At Devprayag, Alaknanda contributes 66% of the total DOC flux carried by the Ganga River. The comparison with the previously published values indicate that due to differences in physiography and chemical weathering rate, the Ganga River transports organic carbon mainly as a dissolved load in its upstream and predominantly as POC down the Himalayan foothills.

Coastal–offshore exchange of organic matter across the Cape Ghir filament (NW Africa) during moderate upwelling

The net coastal–ocean export of particulate organic carbon (POC), dissolved organic carbon (DOC) and chlorophyll a is studied in August 2009 at the Cape Ghir filament, a recurrent feature located within the NW African upwelling system. The estimated flux of excess total organic carbon (the non-refractory pools of DOC and POC) is about 2.1×109kgCy−1. DOC represents ~70% of the excess organic carbon in August 2009, during moderate upwelling. Assuming that this flux is representative of the range within a typical year, the yearly offshore net transport of total organic carbon would represent at least 29% of the primary production in this area. Since the Cape Ghir filament may extend hundreds of kilometers offshore, the associated seaward flux of organic carbon would contribute to the high microbial respiration rates reported from the nearby oligotrophic open ocean region. Our results illustrate that, when considering the regional carbon budgets of eastern boundary regions, it is imperative to take account of the offshore transport of organic matter in the numerous and recurrent upwelling filaments.

Temporal variation and fluxes of dissolved and particulate organic carbon in the Apure, Caura and Orinoco rivers, Venezuela

The concentrations of total suspended sediments (TSS), dissolved organic carbon (DOC) and particulate organic carbon (POC) were measured in water samples taken monthly in the Apure, Caura and Orinoco rivers during a hydrological cycle (between Sept. 2007 and Aug. 2008). The DOC concentration values ranged between 1.5 and 6.8 mgC l−1 in the Apure River; 2.07 and 4.9 mgC l−1 in the Caura River and 1.66 and 5.35 mgC l−1 in the Orinoco River. The mean concentration of DOC was 3.9 mgC l−1 in the Apure River, 3.24 mgC l−1 in the Caura River and 2.92 mgC l−1 in the Orinoco River at Puerto Ordaz. The three rivers showed a similar temporal pattern in the concentrations of DOC, with higher DOC values during the increasing branch of the hydrograph due to wash-out processes of the organic material stored in soils. The mean concentration values of POC were 1.33 mgC l−1; 0.77 mgC l−1 and 0.91 mgC l−1 in the Apure, Caura and Orinoco rivers, respectively. The inverse relationship found between the percentage in weight of the POC and the concentrations of TSS in the three rivers fits a logarithmic model, as it has been previously reported for other worldwide rivers. The POC concentrations in the Orinoco River showed a positive relationship with the TSS, suggesting that the POC in the Orinoco is the result of terrestrially organic matter. Although the fluxes of organic carbon (OC) in the three studied rivers are dependent on the values of water discharge, the fluxes of DOC during the increasing branch of the hydrograph are higher than those found during the decreasing stage, due to the yield of organic material accumulated in soils during the preceding dry season. The mean annual flux of total organic carbon (TOC) of the Orinoco River at Puerto Ordaz was about 4.27 × 106 TonC yr−1. Of this, 3.28 × 106 TonC yr−1 (77%) represents the flux of DOC and about 0.99 TonC yr−1 (23%) represents the flux of POC. The mean annual input of TOC from the Apure River to the Orinoco River was about 4.92 × 105 TonC yr−1 (11.5%), while the contribution of TOC from the Caura River to the Orinoco River was estimated at 3.05 × 105 TonC yr−1 (7.1%).The values of annual transport of TOC calculated for the Apure, Caura and Orinoco rivers were lower than those reported twenty years ago. This could be related to interannual variations of precipitation in the Orinoco Basin, due to runoff variations can have a strong effect on the fluxes of OC from land to rivers.

Impact of Hydropower Regulation on River Water Composition in Northern Sweden

Using hydrogeochemical analysis of two large boreal rivers (pristine Kalix and hydropower regulated Lule) discharging into the Gulf of Bothnia, the major impacts of regulation on water discharge, element transport and their seasonal redistribution have been assessed. The pre-regulation hydrogeochemical features were assumed to be similar for the two rivers. For the Lule River, the average maximum runoff was almost halved, while the average minimum was tripled as a result of the regulation. The fraction of winter transport of total organic carbon (TOC), Fe, Si, suspended Mn and P in the Lule River was, according to a conservative estimate, two to three times higher than in the pristine river. Longer residence time in the Lule River delayed arrival of the suspended Mn peak and dissolved Si decline to the river mouth. During summer, the suspended C/N ratio in the regulated river was 10–20 compared to <10 for the pristine, suggesting presence of predominantly old organic material. This was supported by a virtually constant suspended P/Fe ratio throughout the year in the Lule River, indicating low abundance of phytoplankton. TOC varied irregularly in the Lule River suggesting temporal disconnection between the river and the upper riparian zone. The disappearance of the spring flow maximum, a shift of element transport from spring to winter and supply of mainly old organic material during the vegetation growth season may have a pronounced impact on the ecosystem of the Gulf of Bothnia and the river itself.

Examining organic carbon transport by the Orinoco River using SeaWiFS imagery

The Orinoco River is the fourth largest in the world in terms of water discharge and organic carbon export to the ocean. River export of organic carbon is a key component of the carbon cycle and the global carbon budget. Here, we examined the seasonal transport of organic carbon by the Orinoco River into the eastern Caribbean using the conservative relationship of colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) in low salinity coastal waters influenced by river plumes. In situ measurements of CDOM absorption, DOC, and salinity were used to develop an empirical model for DOC concentration at the Orinoco River Plume. Satellite remote sensing reflectances were used with empirical models to determine DOC and Particulate organic carbon (POC) river transport. Our estimates of CDOM and DOC significantly correlated with in situ measurements and were within the expected ranges for the river. Total organic carbon transport by the Orinoco River during the period of 1998 to 2010 was 7.10 ×1012 g C y−1, from 5.29 × 1012 g C y−1 of DOC and 1.81 × 1012 g C y−1 of POC, representing ∼6% increase to previous published estimates. The variability in organic carbon transport responded to the seasonality in river flow more than to changes in organic carbon concentration in the river. Our results corroborate that is possible to estimate organic carbon transport using ocean color data at global scales. This is needed to reduce the uncertainties of land–ocean carbon fluxes.

Hydrographic controls on net community production and total organic carbon distributions in the eastern Bering Sea

In order to assess spatial and temporal variability of net community production (NCP) in shelf areas of the eastern Bering Sea, seawater samples for dissolved inorganic carbon (DIC) and total organic carbon (TOC) were collected during BEST-BSIERP cruises in the spring, summer, and fall of 2009 and compared to prior measurements made in 2008. DIC and TOC data were used to estimate seasonal changes in rates of NCP and the balance of net autotrophy versus heterotrophy in different shelf areas. In 2009, springtime surface layer DIC concentrations were generally uniform across the shelf and averaged ∼2100μmolkg−1, although concentrations in northern shelf areas (under sea-ice cover) were slightly higher (∼2130μmolkg−1). Subsequently, surface layer DIC (∼1950μmolkg−1) decreased significantly by summertime with the largest drawdown of DIC observed in the Middle Domain between 57° and 61°N. In this area, high NCP rates of up to 92mmolCm−2d−1 were observed and were higher than those reported in 2008. Comparing 2008 and 2009, the shelfwide average drawdown of DIC in the upper 30m between spring and summer was greater by ∼16μmolkg−1. In both spring and summer of 2008 and 2009, concentrations of TOC generally decreased from the coast. TOC concentrations were tightly coupled to salinity, particularly in spring, and largely influenced by the discharge of the Yukon and Kuskokwim Rivers. TOC accumulation between spring and summer was relatively small. In nearshore regions of the shelf, negative rates of NCP observed in 2009 were indicative of net heterotrophy with remineralization of labile organic carbon from rivers likely contributing to the observed net respiration signal in this region. In contrast, net heterotrophy was not observed in 2008, when river discharge rates were 30% lower (likely with lower river transport of TOC). While 2009 rates of production were higher outside the coastal domain than those observed in 2008, integrated annual production over the shelf was fairly comparable between the two years (2008: 103TgCyr−1; 2009: 97.2TgCyr−1). DOC accumulation in the surface layer was also equivalent between the two years (∼12μmolkg−1), and in both years shelfwide export production was estimated to be ∼75% of total NCP.

Transport of fresh and resuspended particulate organic material in the Baltic Sea — a model study

A fully coupled high-resolution 3-dimensional biogeochemical–physical ocean model including an empirical wave model was used to investigate the long-term average (1970–2007) distributions and transports of resuspended matter and other types of suspended organic matter in the Baltic Sea. Modelled bottom types were compared to observations and the results showed that the model successfully managed to capture the horizontal, as well as the vertical, distribution of the different bottom types: accumulation, transport and erosion bottoms. The model also captured well the nutrient element contents in the sediments. On average the largest contribution of resuspended organic carbon to the transport of total organic carbon is found at erosion and transport bottoms. Although the relative transport of resuspended organic carbon at deeper accumulation bottoms in general is low (< 10% of total), the central parts of the sub-basins act on average as sinks that import organic matter while the more shallow areas and the coastal regions acts as sources of organic carbon in the water column. This indicates that the particulate organic matter produced in erosion and transport areas might be kept in suspension long enough to be transported and settle in less energetic areas, i.e. on accumulation bottoms.

Dynamics of stream water TOC concentrations in a boreal headwater catchment: Controlling factors and implications for climate scenarios

Two different but complementary modelling approaches for reproducing the observed dynamics of total organic carbon (TOC) in a boreal stream are presented. One is based on a regression analysis, while the other is based on riparian soil conditions using a convolution of flow and concentration. Both approaches are relatively simple to establish and help to identify gaps in the process understanding of the TOC transport from soils to catchments runoff. The largest part of the temporal variation of stream TOC concentrations (4–46 mg L−1) in a forested headwater stream in the boreal zone in northern Sweden may be described using a four-parameter regression equation that has runoff and transformed air temperature as sole input variables. Runoff is assumed to be a proxy for soil wetness conditions and changing flow pathways which in turn caused most of the stream TOC variation. Temperature explained a significant part of the observed inter-annual variability. Long-term riparian hydrochemistry in soil solutions within 4 m of the stream also captures a surprisingly large part of the observed variation of stream TOC and highlights the importance of riparian soils. The riparian zone was used to reproduce stream TOC with the help of a convolution model based on flow and average riparian chemistry as input variables. There is a significant effect of wetting of the riparian soil that translates into a memory effect for subsequent episodes and thus contributes to controlling stream TOC concentrations. Situations with high flow introduce a large amount of variability into stream water TOC that may be related to memory effects, rapid groundwater fluctuations and other processes not identified so far. Two different climate scenarios for the region based on the IPCC scenarios were applied to the regression equation to test what effect the expected increase in precipitation and temperature and resulting changes in runoff would have on stream TOC concentrations assuming that the soil conditions remain unchanged. Both scenarios resulted in a mean increase of stream TOC concentrations of between 1.5 and 2.5 mg L−1 during the snow free season, which amounts to approximately 15% more TOC export compared to present conditions. Wetter and warmer conditions in the late autumn led to a difference of monthly average TOC of up to 5 mg L−1, suggesting that stream TOC may be particularly susceptible to climate variability during this season.

Distribution and cycling of total organic carbon across the Almeria‐Oran Front in the Mediterranean Sea: Implications for carbon cycling in the western basin

The dynamics of the total organic carbon (TOC) pool were studied during winter 1997–1998 in the Almeria‐Oran jet‐front (AOF) system. This system includes the modified Atlantic Jet, which spreads into the Mediterranean Sea from the Gibraltar Strait, its associated gyre, and the front between the Mediterranean and Atlantic waters. We determined TOC concentrations, bacterial production (BP), and primary production (PP) during field work, and labile dissolved organic carbon (l‐DOC) and bacterial growth efficiency (BGE), which were calculated from biodegradation experiments. Our results showed that the geostrophic Atlantic Jet, which is the most dynamic area (horizontal speed of 80 cm s−1 in the upper 100 m flowing eastward), was characterized by low TOC stocks integrated in the first 100 m (6330–6990 mmol C m−2), a proportion of l‐DOC of 5 ± 0.5%, a BGE of 15 ± 2% and moderate residence times of excess‐TOC (42 ± 7 days). Higher TOC stocks were found in the surrounding areas, including the Mediterranean (7298–7400 mmol C m−2) and gyre waters (6718–8315 mmol C m−2) whereas l‐DOC averaged 6 ± 0.9% and 15 ± 2%, respectively. BGE averaged 7 ± 1% in Mediterranean waters and 21 ± 3% in the gyre giving rise to slightly different excess‐TOC residence times (28 ± 1 days in Mediterranean waters and 109 ± 30 days in the gyre). We estimated that the transport of TOC and excess‐TOC within the Atlantic Jet averaged 8.04 ± 0.32 × 104 and 1.68 ± 0.32 × 104 mol C s−1, respectively.

Export flux in the western and central equatorial Pacific: zonal and temporal variability

Particulate organic carbon export fluxes were measured along the equator to resolve the zonal extent of high productivity in the equatorial Pacific during two cruises: the French JGOFS FLUPAC study aboard the R/V l'Atalante in October 1994 and the Zonal Flux study aboard the R/V Thomas G. Thompson in April 1996. Both cruise tracks went along the equator from 165°E to 150°W. The cruises took place under different seasonal and El Niño-Southern Oscillation (ENSO) conditions: FLUPAC during a strong El Niño in the boreal fall and Zonal Flux during a mild La Niña in the boreal spring. Drifting sediment traps were deployed at the base of the euphotic zone and calibrated using 234 Th . These traps showed over-trapping by 2.7±1.5 times during FLUPAC and 1.5±0.7 times during Zonal Flux. During the FLUPAC time-series at 167°E, the upper euphotic zone was devoid of nitrate, and particulate organic carbon export was low (6±1 mmol m −2 d −1). The FLUPAC time-series at 150°W had abundant nitrate and much higher particulate organic carbon export (12±1 mmol m −2 d −1). Similarly high levels of particulate organic carbon export were observed all along the equator during the Zonal Flux cruise (10±2 mmol m −2 d −1), when cold tongue, high nitrate conditions extended west of 165°E. Synthesis of this data with results from the US Joint Global Ocean Flux Study (JGOFS) equatorial Pacific (EqPac) program allowed a detailed evaluation of equatorial production variability. Data from the TOGA-TAO array illustrated that both Kelvin Waves and tropical instability waves (TIW) were present during the FLUPAC cruise, while neither wave type was present during Zonal Flux. Comparison with results from the US JGOFS EqPac cruises suggested that the ubiquity of super-μM nitrate was the major forcing for new production and particle export near the equator, accounting for a doubling of production over areas with only subsurface nitrate. Within the high nitrate zone, new production and particle export were both found to be enhanced during TIW activity and diminished during Kelvin Wave activity. While the geographical extent of surface nutrients and associated enhanced production is clearly a strong function of season and ENSO, we suggest that equatorially trapped waves — rather than long-term variability in upwelling velocity — are the dominant sources of variability within the equatorial upwelling zone. Comparison of new production and particle export and regressions between nitrate and total organic carbon (TOC) suggest that accumulation and transport of TOC accounts for 17–27% of new production.

Transport of Particulate Organic Carbon by the Mississippi River and Its Fate in the Gulf of Mexico

This study was designed to determine the amount of particulate organic carbon (POC) introduced to the Gulf of Mexico by the Mississippi River and assess the influence of POC inputs on the development of hypoxia and burial of organic carbon on the Louisiana continental shelf. Samples of suspended sediment and supporting hydrographic data were collected from the river and >50 sites on the adjacent shelf. Suspended particles collected in the river averaged 1.8±0.3% organic carbon. Because of this uniformity, POC values (in μmol l−1) correlated well with concentrations of total suspended matter. Net transport of total organic carbon by the Mississippi-Atchafalaya River system averaged 0.48×1012 moles y−1 with 66% of the total organic carbon carried as POC. Concentrations of POC decreased from as high as 600 μmol l−1 in the river to 35% along the shelf at ≥10 km from the river mouth. River flow was a dominant factor in controlling particle and POC distributions; however, time-series data showed that tides and weather fronts can influence particle movement and POC concentrations. Values for apparent oxygen utilization (AOU) increased from ∼60 μmol l−1 to >200 μmol l−1 along the shelf on approach to the region of chronic hypoxia. Short-term increases in AOU were related to transport of more particle-rich waters. Sediments buried on the shelf contained less organic carbon than incoming river particles. Orgamic carbon and δ13C values for shelf sediments indicated 3 that large amounts of both terrigenous and marine organic carbon are being decomposed in shelf waters and sediments to fuel observed hypoxia.

Transport of organic carbon to the oceans by rivers of North America: a synthesis of existing data

Transport of organic carbon in rivers of North America to the oceans was assessed by compiling and analyzing data from 82 North American rivers draining 60% of Canada and the United States. These data were collected by the U.S. Geological Survey and the Water Resources Branch of Canada's Inland Waters Directorate. Mean annual concentrations of total organic carbon showed considerably less variation than did annual specific export (gC m -2 yr -1 ). Regional variation in annual specific export of total organic carbon was attributed primarily to differences in annual runoff. Transport of total organic carbon to coastal regions by North American rivers totalled about 35 × 10 12 ± 5 × 10 12 gC yr -1 in 1977 and 1978. The total organic carbon flux to the oceans associated with the long-term average annual flow in North American rivers was computed to be about 40 × 10 12 gC yr -1 , slightly larger than that in 1977 and 1978 because of the lower-than-average runoff in some regions those years. DOI: 10.1111/j.2153-3490.1982.tb01805.x

Transport of organic carbon to the oceans by rivers of North America: a synthesis of existing data

Transport of organic carbon in rivers of North America to the oceans was assessed by compiling and analyzing data from 82 North American rivers draining 60% of Canada and the United States. These data were collected by the U.S. Geological Survey and the Water Resources Branch of Canada's Inland Waters Directorate. Mean annual concentrations of total organic carbon showed considerably less variation than did annual specific export (gC m^-2 yr-1). Regional variation in annual specific export of total organic carbon was attributed primarily to differences in annual runoff. Transport of total organic carbon to coastal regions by North American rivers totalled about 35 × 10¹² ± 5 × 10¹² gC yr^-1 in 1977 and 1978. The total organic carbon flux to the oceans associated with the long-term average annual flow in North American rivers was computed to be about 40 × 1012 gC yr^-1, slightly larger than that in 1977 and 1978 because of the lower-than-average runoff in some regions those years.