Particulate Organic Carbon Input Research Articles

Determination of particulate organic carbon sources to the surface mixed layer of the Canada Basin, Arctic Ocean

AbstractStable isotope ratios of particulate organic carbon (POC), together with other tracers, were analyzed in samples from the Canada Basin surface mixed layer in 2008 and 2009. Sampling was conducted during the end of the 2008 melt season and at the beginning of the 2009 freezeup under a variety of surface conditions, including open water, newly formed seasonal ice, and multiyear ice. In both years, POC exhibited a wide isotopic range (δ13C‐POC −24.5 to −31.1‰), with the most isotopically depleted material generally found in the central basin. Isotopically enriched material was found on the shelves, consistent with higher biological production and strongly correlated with in situ carbon‐uptake rates. In contrast, offshore in the central basin, there was no significant relationship between δ13C‐POC distributions and either chlorophyll a or aqueous CO2 concentrations, suggesting that in situ biological production was not the dominant control. Analysis of freshwater sources suggested that the sea ice melt contribution of POC to surface waters in the central Canada Basin exerted a negligible influence on δ13C‐POC distributions, and instead isotopically depleted POC in the surface waters of the central Canada Basin were sourced externally through advective transport of riverine organic matter. We show that alkalinity and meteoric water content can be used to distinguish POC inputs from North American and Russian rivers and our analysis suggests that Russian river inputs are the predominant source of 13C‐depleted organic matter to the mixed layer of the central Canada Basin.

Sources of terrigenous inputs to surface sediments of the Colville River Delta and Simpson's Lagoon, Beaufort Sea, Alaska

AbstractThe provenance of sediments and particulate organic carbon (POC) in the Colville River delta and adjacent Simpson's Lagoon, in the Alaskan Beaufort Sea, was investigated using a variety of bulk and molecular techniques, including stable and radiocarbon isotopes, neodymium isotopes, algal pigments, and lignin‐phenols. Additionally, stable carbon isotopes and lignin‐phenols were analyzed on four different density fractions from sediments. The Colville River, the largest river in North America with a watershed exclusively located in the high‐Arctic tundra, was an important source of terrestrial POC to the western edge of the Lagoon, shown by extremely old radiocarbon ages (fraction modern of 0.165 ± 0.001 close to the river mouth up to 0.418 ± 0.002 farther away). Stations without northern protective barrier islands had large amounts of marine POC input, and evidence of benthic microalgae was found in one area of the Lagoon (chlorophyll‐a concentration 35.0 µg gOC−1 in the high‐algal biomass area compared to 1 to 7 µg gOC−1 outside of it). Stations in the middle and eastern end of the Lagoon showed significant sediment input from coastal erosion (input of peat‐indicating non‐lignin‐phenols 0.079 mg (100mgOC)−1 in the eastern lagoon compared to 0.022 mg (100mgOC)−1 near the Colville delta), and on the eastern end of the Lagoon there was evidence of input of Mackenzie River POC, shown with neodymium isotopes and also COP. POC inputs derived from rivers, coastal erosion, and marine productivity were quantified using a ternary mixing model and showed that fluvial supply and coastal erosion were the dominant carbon sources to the Lagoon. This constitutes the first study of POC delivery by the Colville River into a nearshore region and illustrates that continued warming of the high Arctic tundra will likely lead to increased riverine POC delivery to this region of the world.

New views on “old” carbon in the Amazon River: Insight from the source of organic carbon eroded from the Peruvian Andes

AbstractMountain rivers play a key role in the delivery of particulate organic carbon (POC) to large river systems and the ocean. Due to the extent of its drainage area and runoff, the Amazon River is one of Earth's most important biogeochemical systems. However, the source of POC eroded from the humid region of the Eastern Andes and the input of fossil POC from sedimentary rocks (POCfossil) remains poorly constrained. Here we collected suspended sediments from the Kosñipata River during flood events to better characterize Andean POC, measuring the nitrogen to organic carbon ratio (N/C), stable carbon isotopes (δ13Corg) and radiocarbon (Δ14Corg). Δ14Corg values ranged from −711‰ to −15‰, and significant linear trends between Δ14Corg, N/C and δ13Corg suggested that this reflects the mixing of POCfossil with very young organic matter (Δ14Corg ~ 50‰) from the terrestrial biosphere (POCnon‐fossil). Using N/C and Δ14Corg in an end‐member mixing analysis, we quantify the fraction of POCfossil (to within 0.1) and find that it contributes a constant proportion of the suspended sediment mass (0.37 ± 0.03%) and up to 80% of total POC. In contrast, the relative contribution of POCnon‐fossil was variable, being most important during the rising limb and peak discharges of flood events. The new data shed light on published measurements of “old” POC (low Δ14Corg) in Andean‐fed tributaries of the Amazon River, with their Δ14Corg and δ13Corg values consistent with variable addition of POCfossil. The findings suggest a greater persistence of Andean POC in the lowland Amazon than previously recognized.

Glacial-interglacial variability in ocean oxygen and phosphorus in a global biogeochemical model

Abstract. Increased transfer of particulate matter from continental shelves to the open ocean during glacials may have had a major impact on the biogeochemistry of the ocean. Here, we assess the response of the coupled oceanic cycles of oxygen, carbon, phosphorus, and iron to the input of particulate organic carbon and reactive phosphorus from shelves. We use a biogeochemical ocean model and specifically focus on the Last Glacial Maximum (LGM). When compared to an interglacial reference run, our glacial scenario with shelf input shows major increases in ocean productivity and phosphorus burial, while mean deep-water oxygen concentrations decline. There is a downward expansion of the oxygen minimum zones (OMZs) in the Atlantic and Indian Ocean, while the extension of the OMZ in the Pacific is slightly reduced. Oxygen concentrations below 2000 m also decline but bottom waters do not become anoxic. The model simulations show when shelf input of particulate organic matter and particulate reactive P is considered, low oxygen areas in the glacial ocean expand, but concentrations are not low enough to generate wide scale changes in sediment biogeochemistry and sedimentary phosphorus recycling. Increased reactive phosphorus burial in the open ocean during the LGM in the model is related to dust input, notably over the southwest Atlantic and northwest Pacific, whereas input of material from shelves explains higher burial fluxes in continental slope and rise regions. Our model results are in qualitative agreement with available data and reproduce the strong spatial differences in the response of phosphorus burial to glacial-interglacial change. Our model results also highlight the need for additional sediment core records from all ocean basins to allow further insight into changes in phosphorus, carbon and oxygen dynamics in the ocean on glacial-interglacial timescales.

Estimation of the global inventory of methane hydrates in marine sediments using transfer functions

Abstract. The accumulation of gas hydrates in marine sediments is essentially controlled by the accumulation of particulate organic carbon (POC) which is microbially converted into methane, the thickness of the gas hydrate stability zone (GHSZ) where methane can be trapped, the sedimentation rate (SR) that controls the time that POC and the generated methane stays within the GHSZ, and the delivery of methane from deep-seated sediments by ascending pore fluids and gas into the GHSZ. Recently, Wallmann et al. (2012) presented transfer functions to predict the gas hydrate inventory in diffusion-controlled geological systems based on SR, POC and GHSZ thickness for two different scenarios: normal and full compacting sediments. We apply these functions to global data sets of bathymetry, heat flow, seafloor temperature, POC input and SR, estimating a global mass of carbon stored in marine methane hydrates from 3 to 455 Gt of carbon (GtC) depending on the sedimentation and compaction conditions. The global sediment volume of the GHSZ in continental margins is estimated to be 60–67 × 1015 m3, with a total of 7 × 1015 m3 of pore volume (available for GH accumulation). However, seepage of methane-rich fluids is known to have a pronounced effect on gas hydrate accumulation. Therefore, we carried out a set of systematic model runs with the transport-reaction code in order to derive an extended transfer function explicitly considering upward fluid advection. Using averaged fluid velocities for active margins, which were derived from mass balance considerations, this extended transfer function predicts the enhanced gas hydrate accumulation along the continental margins worldwide. Different scenarios were investigated resulting in a global mass of sub-seafloor gas hydrates of ~ 550 GtC. Overall, our systematic approach allows to clearly and quantitatively distinguish between the effect of biogenic methane generation from POC and fluid advection on the accumulation of gas hydrate, and hence, provides a simple prognostic tool for the estimation of large-scale and global gas hydrate inventories in marine sediments.

Efficient transport of terrestrial particulate carbon in a tectonically-active marginal sea off southwestern Taiwan

This study investigates the fluvial and marine burial fluxes of particulate carbon in the source-to-sink system of the mountainous Gaoping (GP) river system to elucidate the efficient transport of terrestrial particulate carbon via a submarine canyon in an active margin. The river fluxes of total suspended matter (TSM), particulate organic carbon (POC) and particulate inorganic carbon (PIC) were much higher in the wet season than in the dry season, mainly because of extremely high sediment yield associated with typhoons in the summer. The TSM, POC and PIC were carried mainly in <63μm particles and showed difference in size distribution between the dry and wet seasons for TSM and POC but not for PIC. The river transported around 226GgCyr−1 POC (64% particulate carbon (PC)) and 125GgCyr−1 PIC (36% PC) into the Gaoping coastal sea during the study period. The POC flux accounts for only 0.074−0.164% of the global river flux, but the POC yield (69.2tonCkm−2yr−1) is about one order of magnitude higher than the global average, and may be the second highest among global small mountainous rivers. If integrated over an area of 3045km2 around the Gaoping Canyon with a maximum water depth of 1500m, the mean burial fluxes of total organic carbon (TOC) and inorganic carbon (TIC) were 13.0gCm−2yr−1 and 9.5gCm−2yr−1, respectively. The buried PC comprised ~58% TOC (39.6GgCTOCyr−1) and ~40% TIC (28.9GgCTICyr−1). From the isotopic composition (δ13C) of TOC in the surface sediments, around 62% of the TOC was estimated to be sourced from the river. The diagenetic and benthic flux of dissolved carbon (DIC+DOC) may account for 18.1% of the deposited TOC flux or 12.9% of the deposited total carbon (TOC+TIC) flux. The burial efficiency of terrestrial TOC was therefore estimated to be only 10.9% of the riverine POC input. Despite the lack of direct evidence, the ratio of buried terrestrial TIC to riverine PIC input may be roughly equal to that of terrestrial TOC, as river POC/PIC and sedimentary TOC/TIC ratios did not differ significantly from each other. The low accumulation of terrestrial sediment and particulate carbon suggests that most POC and PIC that were derived from the Gaoping River may have been recycled and/or moved out of the study area. The active margin associated with the canyon appears to act as an efficient conduit for the transfer of terrestrial POC and PIC into the deep ocean.

Terrestrial subsidies to lake food webs: an experimental approach

Cross-ecosystem movements of material and energy are ubiquitous. Aquatic ecosystems typically receive material that also includes organic matter from the surrounding catchment. Terrestrial-derived (allochthonous) organic matter can enter aquatic ecosystems in dissolved or particulate form. Several studies have highlighted the importance of dissolved organic carbon to aquatic consumers, but less is known about allochthonous particulate organic carbon (POC). Similarly, most studies showing the effects of allochthonous organic carbon (OC) on aquatic consumers have investigated pelagic habitats; the effects of allochthonous OC on benthic communities are less well studied. Allochthonous inputs might further decrease primary production through light reduction, thereby potentially affecting autotrophic resource availability to consumers. Here, an enclosure experiment was carried out to test the importance of POC input and light availability on the resource use in a benthic food web of a clear-water lake. Corn starch (a C(4) plant) was used as a POC source due to its insoluble nature and its distinct carbon stable isotope value (δ(13)C). The starch carbon was closely dispersed over the bottom of the enclosures to study the fate of a POC source exclusively available to sediment biota. The addition of starch carbon resulted in a clear shift in the isotopic signature of surface-dwelling herbivorous and predatory invertebrates. Although the starch carbon was added solely to the sediment surface, the carbon originating from the starch reached zooplankton. We suggest that allochthonous POC can subsidize benthic food webs directly and can be further transferred to pelagic systems, thereby highlighting the importance of benthic pathways for pelagic habitats.

Food quality of anemophilous plant pollen for zooplankton

Pollen rain can lead to considerable allochthonous particulate organic carbon input into freshwater systems. However, the importance of allochthonous pollen deposition for zooplankton production has not so far been considered. Here, standardized growth experiments were performed to assess the quality of pollen from three species (Alnus sp., Populus sp., and Cedrus sp.) as a food source for two cladocerans (Daphnia longispina and Simocephalus vetulus). Although lipid analysis revealed high polyunsaturated fatty acid (PUFA) contents in pollen, both cladoceran species exhibited suboptimal development when directly fed on pollen (ranging from 0.2 d−1 to 0.3 d−1). The low food value of pollen was attributed to the presence of a refractory wall reducing its digestibility. In a second set of experiments, cladocerans were fed on a mixture of heterotrophic microorganisms (bacteria, chytrids, and protozoa) that had grown on pollen grains (Cedrus sp.). The introduction of microorganisms as an intermediate trophic level resulted in cladoceran growth rates that were about double those obtained on pollen alone. Hence, our findings suggest that pollen carbon could sustain zooplankton growth indirectly, and highlight the key role of microorganisms, and especially of chytrids, in transferring and upgrading pollen PUFA to higher trophic levels.

Relationship between sedimentation rates and benthic impact on Maërl beds derived from fish farming in the Mediterranean

The aim of this work was to study the dispersion of particulate wastes derived from marine fish farming and correlate the data with the impact on the seabed. Carbon and nutrients were correlated with the physico-chemical parameters of the sediment and the benthic community structure. The sedimentation rates in the benthic system were 1.09, 0.09 and 0.13 g m −2 day −1 for particulate organic carbon (POC), particulate organic nitrogen (PON) and total phosphorus (TP), respectively. TP was a reliable parameter for establishing the spatial extent of the fish farm particulate wastes. Fish farming was seen to influence not only physico-chemical and biological parameters but also the functioning of the ecosystem from a trophic point of view, particularly affecting the grazers and the balance among the trophic groups. POC, PON and TP sedimentation dynamics reflected the physico-chemical status of the sediment along the distance gradient studied, while their impact on the benthic community extended further. Therefore, the level of fish farm impact on the benthic community might be underestimated if it is assessed by merely taking into account data obtained from waste dispersion rates. The benthic habitat beneath the fish farm, Maërl bed, was seen to be very sensitive to aquaculture impact compared with other unvegetated benthic habitats, with an estimated POC-carrying capacity to maintain current diversity of 0.087 g C m −2 day −1 (only 36% greater than the basal POC input). Environmental protection agencies should define different aquaculture waste load thresholds for different benthic communities affected by finfish farming, according to their particular degree of sensitivity, in order to maintain natural ecosystem functions.

Is there a distinct continental slope fauna in the Antarctic?

The Antarctic continental slope spans the depths from the shelf break (usually between 500 and 1000 m) to ∼3000 m, is very steep, overlain by ‘warm’ (2–2.5 °C) Circumpolar Deep Water (CDW), and life there is poorly studied. This study investigates whether life on Antarctica’s continental slope is essentially an extension of the shelf or the abyssal fauna, a transition zone between these or clearly distinct in its own right. Using data from several cruises to the Weddell Sea and Scotia Sea, including the ANDEEP (ANtarctic benthic DEEP-sea biodiversity, colonisation history and recent community patterns) I–III, BIOPEARL (BIOdiversity, Phylogeny, Evolution and Adaptive Radiation of Life in Antarctica) 1 and EASIZ (Ecology of the Antarctic Sea Ice Zone) II cruises as well as current databases (SOMBASE, SCAR-MarBIN), four different taxa were selected (i.e. cheilostome bryozoans, isopod and ostracod crustaceans and echinoid echinoderms) and two areas, the Weddell Sea and the Scotia Sea, to examine faunal composition, richness and affinities. The answer has important ramifications to the link between physical oceanography and ecology, and the potential of the slope to act as a refuge and resupply zone to the shelf during glaciations. Benthic samples were collected using Agassiz trawl, epibenthic sledge and Rauschert sled. By bathymetric definition, these data suggest that despite eurybathy in some of the groups examined and apparent similarity of physical conditions in the Antarctic, the shelf, slope and abyssal faunas were clearly separated in the Weddell Sea. However, no such separation of faunas was apparent in the Scotia Sea (except in echinoids). Using a geomorphological definition of the slope, shelf-slope-abyss similarity only changed significantly in the bryozoans. Our results did not support the presence of a homogenous and unique Antarctic slope fauna despite a high number of species being restricted to the slope. However, it remains the case that there may be a unique Antarctic slope fauna, but the paucity of our samples could not demonstrate this in the Scotia Sea. It is very likely that various ecological and evolutionary factors (such as topography, water-mass and sediment characteristics, input of particulate organic carbon (POC) and glaciological history) drive slope distinctness. Isopods showed greatest species richness at slope depths, whereas bryozoans and ostracods were more speciose at shelf depths; however, significance varied across Weddell Sea and Scotia Sea and depending on bathymetric vs. geomorphological definitions. Whilst the slope may harbour some source populations for localised shelf recolonisation, the absence of many shelf species, genera and even families (in a poorly dispersing taxon) from the continental slope indicate that it was not a universal refuge for Antarctic shelf fauna.

Bulk organic δ 13C and C/N as indicators for sediment sources in the Pearl River delta and estuary, southern China

Preservation of organic matter in estuarine and coastal areas is an important process in the global carbon cycle. This paper presents bulk δ 13C and C/N of organic matter from source to sink in the Pearl River catchment, delta and estuary, and discusses the applicability of δ 13C and C/N as indicators for sources of organic matter in deltaic and estuarine sediments. In addition to the 91 surface sediment samples, other materials collected in this study cover the main sources of organic material to estuarine sediment. These are: terrestrial organic matter (TOM), including plants and soil samples from the catchment; estuarine and marine suspended particulate organic carbon (POC) from both summer and winter. Results show that the average δ 13C of estuarine surface sediment increases from −25.0 ± 1.3‰ in the freshwater environment to −21.0 ± 0.2‰ in the marine environment, with C/N decreasing from 15.2 ± 3.3 to 6.8 ± 0.2. In the source areas, C 3 plants have lower δ 13C than C 4 plants (−29.0 ± 1.8‰ and −13.1 ± 0.5‰ respectively). δ 13C increases from −28.3 ± 0.8‰ in the forest soil to around −24.1‰ in both riverbank soil and mangrove soil due to increasing proportion of C 4 grasses. The δ 13C POC increases from −27.6 ± 0.8‰ in the freshwater areas to −22.4 ± 0.5‰ in the marine-brackish-water areas in winter, and ranges between −24.0‰ in freshwater areas and −25.4‰ in brackish-water areas in summer. Comparison of the δ 13C and C/N between the sources and sink indicates a weakening TOM and freshwater POC input in the surface sedimentary organic matter seawards, and a strengthening contribution from the marine organic matter. Thus we suggest that bulk organic δ 13C and C/N analysis can be used to indicate sources of sedimentary organic matter in estuarine environments. Organic carbon in surface sediments derived from anthropogenic sources such as human waste and organic pollutants from industrial and agricultural activities accounts for less than 10% of the total organic carbon (TOC). Although results also indicate elevated δ 13C of sedimentary organic matter due to some agricultural products such as sugarcane, C 3 plants are still the dominant vegetation type in this area, and the bulk organic δ 13C and C/N is still an effective indicator for sources of organic matter in estuarine sediments.

Mineralization of organic matter and nitrogen transformations in the Gotland Sea deep water

Measurements of the concentrations of total CO 2 (C T), O 2, H 2S, NO 2 −, NO 3 −, and NH 4 + were performed between 2003 and 2007 during intervals of 2–3 months in the deep water of the Gotland Basin (Baltic Sea). The data were used to determine mineralization and denitrification rates in four sublayers below 150 m during a period of stagnation. To account for the vertical mixing at the boundaries of the sublayers, mixing coefficients were determined based on the temporal changes of the salinity profiles. Mass balances were established for each measured variable and yielded the sink/source rates in the individual sublayers for each time interval. The seasonality of mineralization rates was confined to water layers that were in close contact with the sediment surface, indicating that the mineralization took place mainly at the sediment surface. Moreover, it was shown that the mineralization rates did not depend on the redox conditions in the water column. The total mineralization rate in the basin was found to be 2.0 mol m –2 yr –1, thereby implying that about 80% of the particulate organic carbon input occurred by lateral transport. A redox balance indicated that manganese dioxide and possibly iron oxides contributed to the oxidation of organic matter by about 13%. Intense denitrification was observed during the transition from oxic to anoxic conditions. A value of 456 mmol–N m –2 yr –1 was found, which included a possible anammox contribution of about 5%.

Towards a sediment and organic carbon budget for Hudson Bay

A preliminary sediment and organic carbon budget is constructed for the Hudson Bay marine system based on published literature and new data collected from nine rivers and 13 sediment boxcores. The budget considers the main inputs of terrestrial and marine components of sediment and organic carbon and the main sinks (sediment burial) and losses (oxidation, export to Hudson Strait). Sedimentation rates (0.032–0.23 g cm − 2 yr − 1 ) were estimated from 210Pb profiles and a sediment advection–diffusion model in our cores and in two cores collected previously by Lockhart et al. [Lockhart, W.L. et al., 1998. Fluxes of mercury to lake sediments in central and northern Canada inferred from dated sediment cores. Biogeochemistry 40, 163–173]. Sedimentation rates implied by the 210Pb profiles were verified against profiles of 137Cs and, for some of the cores, contaminant Pb. The sediment sink in Hudson Bay was estimated at 138 (± 64) × 10 6 t a − 1 by applying the average sedimentation rate to the area of active sedimentation in the Bay (~ 15% of the seafloor), calculated according to a regional sedimentation map developed from seismic data [Josenhans, H.W. et al., 1988. Preliminary seismostratigraphic and geomorphic interpretations of the Quaternary sediments of Hudson Bay, Natural Resources Canada]. The known sediment sources to Hudson Bay include river input (11.6 (± 6) × 10 6 t a − 1 ), subaerial coastal erosion (8 (± 4) × 10 6 t a − 1 ), marine primary production (24.2 (± 17) × 10 6 t a − 1 ), and atmospheric deposition (0.5 × 10 6 t a − 1 ). Based on the available data, the modern sediment input (44.3 (± 18) × 10 6 t a − 1 ) represents only about one-third of the apparent sediment sink in Hudson Bay. Resuspension and lateral transport of shallow-water deposits (primarily glacigenic) and winnowing from topographic highs, likely consequences of the exceptional, continuous relative sea-level (RSL) fall in Hudson Bay, provides a plausible source for the ~ 116 × 10 6 t a − 1 imbalance. Using organic carbon and δ 13C values in the cores, we estimate the burial of particulate organic carbon (POC) in sediments at 1.3 (± 0.6) × 10 6 t C a − 1 , of which ~ 80% is marine. The importance of reworked glaciomarine carbon to the burial flux is difficult to estimate because modern (autochthonous) primary production provides a large source of POC to the system (16.1 (± 7) × 10 6 t C a − 1 ) and its degradation is not well-constrained. The 0.58 × 10 6 t C a − 1 terrestrial carbon, estimated to be transported as part of the redistributed sediment load, accounts for almost one-half the total terrigenous POC inputs to the Bay. If sediment and OC supply and burial in the Hudson Bay system are presently responding to the history of isostatic rebound, as proposed here, the task of predicting and measuring the additional consequences of river diversions and climate change (e.g., sea ice, permafrost, runoff) is rendered considerably more difficult.

Lateral POC transport and consumption in surface and deep waters of the Canary Current region: A box model study

We have estimated the lateral transport and consumption, from surface to 3000 m, of suspended particulate organic carbon (POC), through a box model approach, in the Canary Current region (subtropical northeast Atlantic). Our results show that lateral POC fluxes are up to 3 orders of magnitude higher than vertical fluxes. In the mesopelagic ocean, the central waters (100–700 m) presented a net carbon consumption of 8.51 × 108 mol C d−1 with the highest POC entering through the more coastal section. This lateral flux accounted for 28–59% of the total mesopelagic respiration (R), on the basis of lower and upper case scenarios of vertical POC inputs and dissolved organic carbon contribution to R. We suggest that boundary currents may support higher lateral export of coastally produced POC than previously assumed. A large fraction of this POC would, however, be remineralized in the upper 1000 m instead of being transported to the ocean interior.

Benthic remineralization at the land–ocean interface: A case study of the Rhône River (NW Mediterranean Sea)

Biogeochemical processes in sediments under the influence of the Rhône River plume were studied using both in situ microelectrodes and ex situ sediment core incubations. Organic carbon (OC) and total nitrogen (TN) content as well as stable carbon isotopic composition of OC ( δ 13C OC) were analysed in 19 surface sediments to determine the distribution and sources of organic matter in the Rhône delta system. Large spatial variations were observed in both the total O 2 uptake (5.2 to 29.3 mmol m −2 d −1) and NH 4 + release (−0.1 to −3.5 mmol m −2 d −1) rates at the sediment–water interface. The highest fluxes were measured near the Rhône River mouth where sedimentary OC and TN contents reached 1.81% and 0.23% respectively. Values of δ 13C OC ranged from −26.83‰ to −23.88‰ with a significant seawards enrichment tracing the dispersal of terrestrial organic matter on the continental shelf. The amount of terrestrial-derived OC reaches 85% in sediments close to the Rhône mouth decreasing down to 25% in continental shelf sediments. On the prodelta, high terrestrial OC accumulation rates support high oxygen uptake rates and thus indicating that a significant fraction of terrestrial OC is remineralized. A particulate organic carbon (POC) mass balance indicates that only 3% of the deposited POC is remineralized in prodelta sediments while 96% is recycled on the continental shelf. It was calculated that a large proportion of the Rhône POC input is either buried (∼52%) or remineralized (∼8%), mostly on the prodelta area. The remaining fraction (∼40%) is either mineralized in the water or exported outside the Rhône delta system in dissolved or particulate forms.

Particulate organic carbon in the estuarine turbidity maxima of the Gironde, Loire and Seine estuaries: origin and lability

A study of the particulate organic carbon (POC) in the estuarine turbidity maxima (ETMs) of the three major French macrotidal estuaries shows that the average contents are 1.5, 3.3 and 3.1% (expressed in % of dry suspended sediment) in the Gironde, Loire and Seine Estuaries, respectively. There is no seasonal variation of POC contents in the Gironde Estuary, whereas, they often increase in the Loire and the Seine Estuaries in spring and summer. The lability of the estuarine particulate organic matter was estimated by two analyses: 1/labile organic matter was measured as the organic carbon loss during incubation tests over one month; 2/ the hydrolysable organic fraction was determined after 6N HCl digestion. The organic fractions of the ETMs are mainly refractory. Any increase in the amount of POC as compared to the background levels (cited above) is always correlated to an increase of organic matter lability. The yearly average fluvial contributions by various particulate organic pools (soil and litter organic matter; organic matter of phytoplanktonic and human origin) that enter the three estuaries were quantified. In the Garonne River, soil and litter are the major (90%) POC sources. In the Loire system, due to the eutrophication of the river water, phytoplankton contributes up to 50% of the total POC load. In the Seine river, soil and litter contribute 70% of the total POC input; POC of human origin is also significant (10%), due to the impact of the City of Paris (10 million inhabitants). The lability of the different types of organic matter ranks as follows: phytoplankton ∼litter > human-origin organic matter >> soil. By combining the POC budgets and the lability of each type of organic fraction, it was possible to explain why the POC of the three ETMs is different and characterizes its refractory vs. labile nature.

Bulk particulate organic carbon in the East China Sea: Tidal influence and bottom transport

Five oceanographic surveys were carried out in the East China Sea (ECS) and Yellow Sea from 1999 to 2003. In all, seven different sections were surveyed, but one section (the PN section) was observed on every cruise. Two time-series stations were also surveyed, one located at the Changjiang River mouth, the other over the continental shelf in the PN section. We identified biogeochemical characteristics for waters close to the Changjiang Estuary and in the Kuroshio waters (KW), respectively. Resuspension is a strong feature near bottom over the ECS continental shelf, with suspended matter values 13 times higher than that for the surface. A model of particulate organic carbon (POC) dynamics based on a rectangle equation reveals that POC concentration close to the Changjiang Estuary varies with a semidiurnal period of ∼13 h, coinciding with the tidal period. The upper limit for POC residence times in the seasons we covered over the shelf are estimated to be on the order of weeks and generally increase seaward from near the Changjiang Estuary to the KW. Short POC residence times suggest that POC in the ECS is rapidly exported from euphotic waters. A nepheloid layer, observed as elevated suspended matter in near the bottom of the water column, is important in particle transfer over the shelf, especially in winter when the residual current flows mainly eastward. Cross-shelf transport of POC via the nepheloid layer is estimated to be 0.22 × 10 12 g yr −1. Comparison with other work indicates that POC transport is ∼2% of the Changjiang POC input.

Contribution of small mountainous rivers to particulate organic carbon input in the Bay of Biscay

The Nivelle River, a typical Pyrenean mountainous watershed reaching the Bay of Biscay (Atlantic Ocean), was sampled with high resolution during 1996. The particulate organic carbon (POC) contents during successive floods shows that there is a graduated impoverishment of the organic fraction of suspended particulate matter (SPM) from the first flood to the next ones, reaching a threshold value (3%) attributed to allochtonous fraction (soil). On the basis of the high frequency data of water discharge and POC concentration, an annual POC flux was established: 845 tons, corresponding to a specific POC flux of 5.3 tC km−2 yr−1. This value was obtained during a dry period and must be considered as a minimum value for longer time scale. The POC originated mostly from soil (55%) and riparian/litter (~40%) with a very minor (<5%) contribution of autochthonous POC. Thirty-two percent of the annual POC flux was carried in 1% of time and 66% in 10% of time. The specific POC yield, 5.3 tC km−2 yr−1, if extended to the whole mountainous area of the southern coast of the Bay of Biscay (19,000 km2), leads to an estimated POC flux around 100,000 t yr−1. Although small Cantabrian mountainous rivers contributed to only 28% of the freshwater discharge in the Bay of Biscay, their POC load was estimated to account for 70% of the total POC inputs in the Bay.

Organic carbon budget at the sediment–water interface on the Gulf of Lions continental margin

Oxygen uptake rates were measured at the sediment–water interface along a shelf-slope transect in the western Gulf of Lions (northwestern Mediterranean Sea). Values ranged from 0.6 to 48.0 mmol O 2 m −2 d −1, and decreased from inshore to the continental slope. The overall mean was 36.8±11.2 mmol O 2 m −2 d −1 for the coastal zone, 11.2±1.4 mmol O 2 m −2 d −1 for the shelf and 2.4±1.8 mmol O 2 m −2 d −1 for the slope. Slope stations exhibited significantly different values according to their location in the canyon axis (average 3.7±2.1 mmol O 2 m −2 d −1) or open slope sites (average 1.5±0.9 mmol O 2 m −2 d −1). Independent on the location, sediment oxygen uptake reached maximum rates in late spring, when fluxes were up to 4 times higher than in other seasons. Rates were significantly related ( r=0.92, n=16, p<0.001) to meiofauna density and consistent with the spatial patterns of bacterial potential respiration (Tholosan, 1999). This suggests that the rates of sediment oxygen uptake recorded during our experiment at least partially resulted from spatial and seasonal variability of the biological components (micro- and meiobenthos). In this study we attempt to construct an organic carbon (OC) budget for the different investigated sites, with the “input” term represented by the arrival of particulate organic carbon (POC) in proximity to the seafloor and the “output” term represented by the sum of the OC accumulated in the sediment plus the POC remineralized at the sediment–water interface. Input and output terms are approximately balanced at the slope stations, where, according to our estimation, about 50% of the deposited POC is accumulated in the sediment, while the remaining 50% is remineralized. Conversely, on the shelf, a lower percentage of the POC input (about 31%) is able to accumulate in the surficial sediment, while a higher proportion (about 69%) is remineralized at the sediment–water interface.

Ten-year observation of the Gironde tributary fluvial system: fluxes of suspended matter, particulate organic carbon and cadmium

A program of long-term observation of suspended solids (TSS), particulate organic carbon (POC) and cadmium transported into the Gironde estuary (France) by its major tributaries has been carried out between 1990 and 1999. This decade included contrasting hydrologic cycles and appears representative of a much longer period (1959–1999). The Garonne and the Dordogne river systems are the main tributaries of the Gironde estuary and derive their waters from drainage basins with different geological, industrial and agricultural features. To better understand their respective contributions, they have been observed separately and compared. Water and TSS fluxes of the Garonne River show greater temporal variations and discharge is more related to the hydrology of the drainage basin (e.g. wet/dry years, local flood events etc.). As POC and particulate Cd concentrations in suspended matter are much less variable than turbidity, their fluxes are mainly controlled by the TSS transport. A major part of annual fluxes of TSS and associated pollutants may occur within few flood days (depending on various parameters, e.g. intensity, duration, season, etc.), and also the succession of dry and wet years has an important influence on annual fluxes. The presented data allow calculating fluvial inputs into the Gironde as the sum of fluxes transported by its major tributaries, the Garonne and the Dordogne river systems. Mean annual fluxes into the Gironde observed in 1990–1999 are about 34×109 m3 year−1 for river water, 3.24×106 t year−1 for suspended solids (TSS) and 9.88×109 mol year−1 for particulate organic carbon (POC). Generally, these fluxes are dominated by the contributions of the Garonne River. However, in dry years, the mean contribution of the Dordogne river system (including Dronne and Isle rivers) to the POC input into the estuary exceeded that of the Garonne. This reflects significant differences in vegetation and soil due to natural properties and land management of the basins. Mean Cd fluxes into the estuary are about 110×103 mol year−1 of which 19.6×103 mol year−1 are transported in the dissolved and 90.8×103 mol year−1 in the particulate phases, respectively. In 1991 (dry year), the net (dissolved) Cd flux towards the ocean exceeded the gross fluvial input of total Cd, suggesting the release of Cd from an important stock in the maximum turbidity zone (MTZ) or the fluid mud of the Gironde estuary.