Dissolved Organic Carbon Distribution Research Articles

Abundance, distribution, and fluxes of dissolved organic carbon (DOC) in four small sub-tropical rivers of the Tampa Bay Estuary (Florida, USA)

The delivery of organic carbon (C) from rivers to the coastal ocean via estuaries is recognized as an important component of the global C budget however, smaller river systems are often overlooked and modern flux estimates are not very different from historical estimates. Here, the seasonal (wet vs. dry) concentration and fluxes of dissolved organic C (DOC) were measured in five small sub-tropical rivers that drain into the Tampa Bay (FL, USA) estuary. DOC distributions were highly variable among riverine, mesohaline, and marine end-member samples in all river catchments and no significant differences were observed among or between DOC concentrations with respect to river catchment, season, or year of sampling. In general, DOC mixed non-conservatively during the wet seasons, and conservatively during the dry seasons, with the estuarine reaches of each river serving as a sink of DOC. Fluxes were strongly tied to discharge irrespective of season, and the estuaries removed 15–65% of DOC prior to export to coastal Bay and Gulf of Mexico waters. DOC concentrations were similar to others reported for low-elevation sub-tropical rivers, and a combination of elevation, residence time, and climate appear to control the abundance and variability of DOC in sub-tropical vs. tropical river systems. The characterization of DOC in small, sub-tropical rivers, which share characteristics with both their temperate and tropical counterparts, is critical for quantitatively constraining the importance of these systems in local-to-regional scale ocean C budgets. In addition to geomorphic properties, the role of past, present, and future land cover and other environmental change in small coastal rivers also exerts control on the quantity and flux of DOC in these systems.

Spatial variation of dissolved organic carbon in soils of riparian wetlands and responses to hydro-geomorphologic changes in Sanjiang Plain, China

Spatial variation of dissolved organic carbon (DOC) in soils of riparian wetlands and responses to hydro-geomorphologic changes in the Sanjiang Plain were analyzed through in situ collecting soil samples in the Naoli River and the Bielahong River. The results showed that the average contents of DOC for soil layer of 0–100 cm were 730.6 mg/kg, 250.9 mg/kg, 423.0 mg/kg and 333.1 mg/kg respectively from riverbed to river terrace along the transverse directions of the Naoli watershed. The content of the soil DOC was the highest in the riverbed, lower in the high floodplain and much lower in the river terrace, and it was the lowest in the low floodplain. The difference in the content and vertical distribution of DOC between the riverbed and the three riparian wetlands was significant, while it was not significant among the low floodplain, the high floodplain and the river terrace. The variability of soil DOC was related to the hydrological connectivity between different landscape position of the riparian wetlands and the adjacent stream. Extremely significant correlations were observed between DOC and total organic carbon (TOC), total iron (TFe), ferrous iron (Fe(II)) whose correlation coefficients were 0.819, −0.544 and −0.709 in riparian wetlands of the Naoli River. With the increase of wetland destruction, soil pH increased and soil DOC content changed. The correlation coefficients between soil DOC and TOC, TFe, Fe(II) also changed into 0.759, −0.686 and −0.575 respectively in the Bielahong River. Under the impact of drainage ditches, the correlations between soil DOC and TFe, Fe(II) were not obvious, while the soil pH was weakly alkaline and was negatively correlated with soil DOC in the previous high floodplain. It indicates that riparian hydro-geomorphology is the main factor that could well explain this spatial variability of soil DOC, and the agricultural environmental hydraulic works like ditching also must be considered.

An improved algorithm for remotely sensing marine dissolved organic carbon: Climatology in the northern South China Sea shelf-sea and adjacent waters

By using field in situ observations, data from the NOMAD database, and remotely sensed data from MODIS-Aqua, an improved algorithm has been developed for estimating the surface concentrations of dissolved organic carbon (DOC) in the Northern South China Sea Shelf-sea (NoSoCS) and adjacent waters through a combination of the remotely sensed sea-surface temperature (SST) and the absorption coefficient of colored dissolved organic matter (CDOM). Unlike previously reported algorithms, which were based on only one of these two parameters, this algorithm does not require seasonal tuning and is applicable to both coastal and pelagic waters. The uncertainty in the remotely sensed concentrations of DOC is about ±4–10µM. The climatological distributions of the satellite-derived surface DOC in the NoSoCS and adjacent waters between 2002 and 2012 followed a general trend of increasing concentrations landward, from about 58 and 73μM in the open South China Sea (SCS) in January and July, respectively, to >100μM at the mouth of the Pearl River. The intra-annual changes in the monthly average concentration of DOC in the NoSoCS follow a distinct seasonal cycle, with the lowest and highest concentrations, 56μM and 75μM, in February and July, respectively. The seasonal cycle of DOC was in phase with that of SST but out of phase with that of primary production, suggesting that biological activity was not likely its primary control. Instead, it was consistent with the entrainment of the cold and less DOC-rich sub-surface water in the surface water by enhanced vertical mixing during winter, and the high inflow of organic-rich river water to the NoSoCS in summer. Sub-regionally, lower concentrations of DOC were found in the known upwelling zones in the NoSoCS, where vertical mixing was enhanced. In the inner shelf, the effect of riverine input was indicated by higher concentrations of DOC in the plume of water at the mouth of the Pearl River. This work represents the first attempt to characterize the regional distribution of surface concentration of DOC in the NoSoCS and adjacent waters.

Spatial Distribution and Fluorescence Properties of Soil Dissolved Organic Carbon Across a Riparian Buffer Wetland in Chongming Island, China

The migration of soil dissolved organic carbon (DOC) from terrestrial to aquatic environments has important impacts on the adjacent water quality and the transport of organic and inorganic contaminants. However, few studies have investigated the sources and properties of DOC in riparian zones. A total of 34 soil samples were collected across four riparian buffer zones (Zones A–D) on Chongming Island, China. The vertical distributions of soil organic carbon (SOC) and DOC, fluorescence excitation-emission matrix (EEM) spectra of DOC and the optical indices, including fluorescence index (FI), index of recent autochthonous contribution (BIX), and humification index (HIX), were measured across the riparian environment to investigate the sources and fluorescence properties of DOC. The results showed that SOC stored in the surface soil (0–30 cm) accounted for 40% of the total soil profile SOC. The DOC accumulated in Zones A–C, which accounted for 5% of the SOC. The fluorescence EEM spectra of DOC showed that DOC contained humic-like and protein-like substances, which were mainly derived from recent plant debris by microbes. A large amount of humic-like substances were sorbed to minerals in the surface soil (0–30 cm). In addition, the riparian topography and soil physico-chemical properties (pH, EC and moisture) dictated the transformation and transport of DOC. The results suggested that EEMs could reveal the source of DOC in riparian soil systems, and that optical indices were complementary tools that revealed the characteristics of soil DOC and provided supplemental evidence about DOC sources.

CDOM-DOC relationship in contrasted coastal waters: implication for DOC retrieval from ocean color remote sensing observation.

Increasing our knowledge on dissolved organic carbon (DOC) spatio-temporal distribution in the coastal ocean represents a crucial challenge for better understanding the role of these ecosystems in the global oceanic carbon cycle. The assessment of DOC concentration from the absorption properties of the colored part of the dissolved organic matter (a(cdom)) was investigated from an extensive data set covering a variety of coastal environments. Our results confirmed that variation in the a(cdom)(412) to DOC ratio (a*(cdom)(412)) can be depicted from the CDOM spectral slope in the UV domain (S(275-295)). They also evidenced that regional first order variation in both a*(cdom)(412) and S(275-295) are highly correlated to variation in a(cdom)(412). From these observations, generalized relationships for estimating a*(cdom)(412) from S(275-295) or a(cdom)(412) were parameterized from our development sites (N = 158; English Channel, French Guiana, Hai Phong Bay) and tested against an independent data set covering others coastal regions (N = 223; French Polynesia, Rhone River estuary, Gulf of Maine, Chesapeake Bay, Southern Middle Atlantic Bight) demonstrating the possibility to derive DOC estimates from in situ CDOM optical properties with an average accuracy of ~16% over very contrasted coastal environments (with DOC ranging from 50 to 250 µmol.L(-1)). The applicability of these generalized approaches was evaluated in the context of ocean color remote sensing observation emphasizing the limits of S(275-295)-based formulations and the potential for a(cdom)-based approaches to represent a compelling alternative for assessing synoptic DOC distribution.

Estimating dissolved organic carbon inventories in the East China Sea using remote‐sensing data

AbstractThe coastal ocean is characterized by a high dissolved organic carbon (DOC) concentration due to large terrestrial inputs and high primary production. Monitoring the dynamic variation of DOC inventories in coastal oceans provide more information on carbon flux, but is very challenging in practice. We propose a method of estimating DOC inventories in the East China Sea (ECS) by integrating the surface DOC distribution with a vertical model of the DOC profile via satellite data. Surface DOC concentration was retrieved from satellite‐derived chromophoric dissolved organic matter and chlorophyll concentration. Two vertical DOC profile models (uniform model and stratified model) were established based on water‐density profiles, and the usage of these two models was corresponded to water mass classification determined by a water mass index. The average bias of the satellite‐derived DOC inventory, in the euphotic layer was 23.8% in spring, 24.8% in autumn and 14.7% in winter, and in the water column was 15.6% in spring, 12.3% in autumn, and 10.2% in winter. The total satellite‐derived DOC inventory integrated in the water column was about 31.84, 31.96, 28.59, and 31.18 Tg in four seasons in the ECS (5.84 × 105 km2). Sensitivity analysis indicated that the remote‐sensing method of estimating DOC inventory is stable and reliable. The available long‐term and large‐scale satellite‐derived DOC inventories in marginal sea would provide us basic information on carbon cycle, and the difference between the seasonal DOC inventories would help to understand the DOC export and relative biogeochemical processes in the ECS.

Modeling the contribution of dissolved organic carbon to carbon sequestration during the last glacial maximum

Dissolved organic carbon (DOC) is a carbon reservoir that is as large as the atmospheric CO2 pool, and its contribution to the global carbon cycle is gaining attention. As DOC is a dissolved tracer, its distribution can serve to trace the mixing of water masses and the pathways of ocean circulation. Published proxy and model reconstructions have revealed that, during the last glacial maximum (LGM), the pattern of deep ocean circulation differed from that of the modern ocean, whereby additional carbon is assumed to have been sequestered in stratified LGM deep water. The aim of this study is to explore the distribution of DOC and its production/removal rate during the LGM using the Grid ENabled Integrated Earth system model (GENIE). Modeled results reveal that increased salinity of bottom waters in the Southern Ocean is associated with stronger stratification and oxygen depletion. The stratified LGM deep ocean traps more nutrients, resulting in a decrease in the DOC reservoir size that, in turn, causes a negative feedback for carbon sequestration. This finding requires an increase in DOC lifetime to compensate for the negative feedback. The upper limit of DOC lifetime is assumed to be 20,000 years. Modeled results derive an increase (decrease) in DOC reservoir by 100 Pg C leading to an atmospheric CO2 decrease (increase) of 9.1 ppm and a dissolved inorganic carbon δ13C increase (decrease) of 0.06‰. The DOC removal rate is estimated to be 39.5 Tg C year–1 in the deep sea during the LGM. The contribution of DOC to the LGM carbon cycle elucidates potential carbon sink-increasing strategies.

Review and suggestions for estimating particulate organic carbon and dissolved organic carbon inventories in the ocean using remote sensing data

Dissolved organic carbon (DOC) and particulate organic carbon (POC) are basic variables for the ocean carbon cycle. Knowledge of the distribution and inventory of these variables is important for a better estimation and understanding of the global carbon cycle. Owing to its considerable advantages in spatial and temporal coverage, remote sensing data provide estimates of DOC and POC inventories, which are able to give a synthetic view for the distribution and transportation of carbon pools. To estimate organic carbon inventories using remote sensing involves integration of the surface concentration and vertical profile models, and the development of these models is critical to the accuracy of estimates. Hence, the distribution and control factors of DOC and POC in the ocean first are briefly summarized, and then studies of DOC and POC inventories and flux estimations are reviewed, most of which are based on field data and few of which consider the vertical distributions of POC or DOC. There is some research on the estimation of POC inventory by remote sensing, mainly in the open ocean, in which three kinds of vertical profile models have been proposed: the uniform, exponential decay, and Gauss models. However, research on remote-sensing estimation of the DOC inventory remains lacking. A synthetic review of approaches used to estimate the organic carbon inventories is offered and the future development of methods is discussed for such estimates using remote sensing data in coastal waters.

Dissolved organic matter cycling in the confluence of the Atlantic and Indian oceans south of Africa

The boundary between the Atlantic and Indian sectors of the Southern Ocean is a key spot of the thermohaline circulation, where the following water masses mix up: Indian Central water (ICW), South Atlantic Central Water (SACW), Antarctic Intermediate Water (AAIW), Circumpolar Deep Water (CDW), North Atlantic Deep Water (NADW), Weddell Sea Deep Water (WSDW) and Antarctic Winter Water (WW). An optimum multiparameter analysis based on the distributions of potential temperature, salinity, NO (=O2+9.3×NO3) and silicate during the GoodHope 2004 (GH04) cruise allowed us to (i) define the realms of these water masses; (ii) obtain the water mass proportion weighted-average (archetypal) apparent oxygen utilization (AOU) and dissolved organic carbon (DOC) concentrations of each water mass; and (iii) estimate the contribution of DOC to the oxygen demand of the study area. WW represented only 5.2% of the water volume sampled during GH04, followed by WSDW with 10.8%, NADW with 12.7%, SACW with 15.3%, AAIW with 23.1% and CDW with 32.8%. The distributions of DOC and AOU were mainly explained by the mixing of archetypal concentrations of these variables, 75±5% and 65±3% respectively, which retained the variability due to the basin-scale mineralization from the formation area to the barycentre of each water mass along the GH04 line. DOC accounted for 26±2% and 12±5% of the oxygen demand of the meso- and bathypelagic ocean, respectively. Conversely, local mineralization processes, retained by the residuals of the archetypal concentrations of DOC and AOU, did not contribute to improve significantly the mixing model of DOC.

Spatial distribution and mobility of organic carbon (POC and DOC) in a coastal Mediterranean environment (Saronikos Gulf, Greece) during 2007–2009 period

Particulate (POC) and dissolved organic carbon (DOC) is an important parameter for the pollution assessment of coastal marine systems, especially those affected by anthropogenic, domestic, and industrial activities. In the present paper, a similar marine system (Saronikos Gulf) located in the west-central Aegean Sea (eastern Mediterranean Sea) was examined, in terms of the temporal and spatial distribution of organic carbon (POC and DOC), with respect to marine sources and pathways. POC was maximum in winter in the Saronikos Gulf, due to the bloom of phytoplankton, whereas in the Elefsis Bay (located in the north side of the Saronikos Gulf) in summer, since phytoplankton grazes in the Bay in the end of summer (except for winter). Approximately 60 % of the bulk DOC of the water column was estimated as non-refractory (labile and semi-labile), due to the major anthropogenic, domestic, and industrial effects of the region and the shallow depths. The spatial distribution of POC and DOC mainly affects the northeastern section of the Gulf, since that region has been accepted major organic discharges for a long time period, in connection to the relatively long renewal times of its waters.

Factors controlling the C:N:P stoichiometry of dissolved organic matter in the N-limited, cyanobacteria-dominated East/Japan Sea

The vertical and horizontal distributions of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP), together with dissolved inorganic nitrogen (DIN), phosphorus (DIP), silicate (DSi), and photosynthetic pigments, were measured in the euphotic layer (0–100m) along the south–north transect of the East/Japan Sea (EJS) in May 2007. In the mixed layer, the DIN concentrations were lower than 3μM, and the DIN:DIP ratios were lower than 10. Under this DIN-limited condition, cyanobacteria (20–65%) and diatoms (20–50%) dominated the phytoplankton community. The concentrations of DOC (60–83μM) and DOP (0.1–0.4μM) in the EJS fell into a range similar to those found in the major oceans, whereas the concentration of DON (2–7μM) was lower than that in the Pacific Ocean (7–13μM). The correlation analyses showed that the distributions of DOC, DON, and DOP in the study region were affected mainly by the physical characteristics of water masses. The lower DON concentrations, higher DOC:DON ratios, and lower DON:DOP ratios in the EJS, relative to the major oceans, together with a strong negative correlation between the concentrations DON and zeaxanthin, suggest that DON is preferentially taken up by cyanobacteria in this environment. Thus, our results show that the C:N:P stoichiometry of dissolved organic matter (DOM) in the EJS is controlled mainly by the physical and biological characteristics of water masses.

Concentration and characterization of dissolved organic matter in the surface microlayer and subsurface water of the Bohai Sea, China

A total of 19 sea-surface microlayer and corresponding subsurface samples collected from the Bohai Sea, China in April 2010 were analyzed for chlorophyll a, dissolved organic carbon (DOC) and its major compound classes including total dissolved carbohydrates (TDCHO, including monosaccharides, MCHO, and polysaccharides, PCHO) and total hydrolysable amino acids (THAA, including dissolved free, DFAA, and combined fraction, DCAA). The concentrations of DOC in the subsurface water ranged from 130.2 to 407.7μM C, with an average of 225.9±75.4μM C, while those in the surface microlayer varied between 140.1 and 330.9μM C, with an average of 217.8±56.8μM C. The concentrations of chlorophyll a, DOC, TDCHO and THAA in the microlayer were, respectively correlated with their subsurface water concentrations, implying that there was a strong exchange effect between the microlayer and subsurface water. The concentrations of DOC and TDCHO were negatively correlated with salinity, respectively, indicating that water mixing might play an important role in controlling the distribution of DOC and TDCHO in the water column. Major constituents of DCAA and DFAA present in the study area were glycine, alanine, glutamic acid, serine and histidine. Principal component analysis (PCA) was applied to examine the complex compositional differences that existed among the sampling sites. Our results showed that DFAA had higher mole percentages of glycine, valine and serine in the microlayer than in the subsurface water, while DCAA tended to have higher mole percentages of glutamic acid, aspartic acid, threonine, arginine, alanine, tyrosine, phenylalanine and leucine in the microlayer. The yields of TDCHO and THAA exhibited similar trends between the microlayer and subsurface water. Carbohydrate species displayed significant enrichment in the microlayer, whereas the DFAA and DCAA exhibited non-uniform enrichment in the microlayer.

Properties of sediment NOM collected from a drinking water reservoir in South China, and its association with THMs and HAAs formation

The natural organic matter (NOM) from the sediment elutriate, which was obtained from a drinking water reservoir in South China, was fractionated into four fractions by using XAD-8 and XAD-4 resin. Dissolved organic carbon (DOC) as well as UV absorbance at 254 nm (UV254) and specific UV254 (SUV254) were used to quantify and characterize the NOM of raw sediment elutriate and the fractions. Formation of disinfection by-products (DBPs), i.e. trihalomethanes (THMs) and haloacetic acids (HAAs) were also investigated. Results showed that lots of organic matter can be liberated from the sediment during suspension. Hydrophilic organic matter (HiM) accounted for 50.7% of the DOC, followed by hydrophobic acid (HoA) (21.3%), transphilic organic matter (TPH) (15.2%) and other hydrophobic substances (HoS) (12.9%). Different from DOC distribution, the highest UV254 and SUV254 were detected in HoA, while the least were observed in HiM. All the organic fractions were good DBPs precursors. The hydrophobic fractions (HoS and HoA) were more important precursors of CHCl3 and TCAA as compared to other fractions, and UV254 was a good indicator for trichloroacetic acid (TCAA) formation. While for the hydrophilic fraction, relative higher dichloroacetic acid (DCAA) yields were observed, and DOC were good index for its formation.

Effect of benthic-pelagic coupling on dissolved organic carbon concentrations in permeable sediments and water column in the northeastern Gulf of Mexico

Large areas of the continental shelf are covered by permeable sand beds that filter substantial volumes of coastal water. This study investigated the temporal changes in and coupling between dissolved organic carbon (DOC) concentrations in the water column and pore water of nearshore permeable sediments in the northeastern Gulf of Mexico. Time series samples, collected in the nearshore environments at an exposed and a nearby protected site, showed very similar patterns of sedimentary DOC concentrations at both sites indicating that large-scale, seasonal, and weather-related processes controlled these distributions. A summer situation, with the sediment surface layer depleted in DOC and upward diffusion of recalcitrant DOC from deeper layers, was separated by a fall transition phase, where increasing winds and waves caused a mixing of the sedimentary DOC, resulting in relatively even concentrations over the investigated sediment depth (0–12cm). The subsequent winter situation was characterized by increased DOC in the surface layer caused by filtration of degradable organic material into the sands and subsurface removal of degradable DOC. A second transition phase in early spring marked the end of the winter situation, with strong winds and waves and thorough pore water mixing, leading again to even DOC distribution in the investigated sediment layer. This transition phase initiated the next summer situation with depletion of DOC in the sediment surface layers. Our results indicate that DOC in the upper layer (≤12cm) of the shallow sands is controlled by benthic-pelagic coupling facilitated by relatively rapid solute and particle exchange in the highly permeable sediments at our study sites. The prompt responses of the surface layer DOC concentrations to the changes in the overlying water underline the dynamic character of substrate supply in the permeable sediments, setting them apart from fine-grained cohesive beds characterized by relatively stable DOC distributions.

Assessing the spatial and temporal variability of dissolved organic matter in Liverpool Bay using excitation–emission matrix fluorescence and parallel factor analysis

The spatial and diurnal tidal variability of dissolved organic carbon (DOC) concentrations and the composition of dissolved organic matter (DOM), as evaluated by high-temperature catalytic oxidation and excitation–emission matrix combined with parallel factor analysis (EEM–PARAFAC), respectively, were determined in Liverpool Bay. EEM–PARAFAC modeling resulted in six fluorescent components characterized as terrestrial humic-like (two), microbial humic-like (two), and protein-like (two). The spatial distributions of DOC and the four humic-like components were negatively correlated with salinity in the high-salinity waters observed in this study (30.41–33.75), suggesting that terrestrial DOM was conservatively distributed. The spatial patterns of protein-like components were largely different from those of DOC, humic-like components, and chlorophyll a, suggesting that these distributions were the combined result of production and degradation in the bay in addition to river inputs. These findings suggest that the DOM dynamics in Liverpool Bay are strongly controlled by river-dominated allochthonous DOM inputs with some less significant contributions of autochthonous DOM within the bay. In addition, the temporal variations of DOM associated with the diurnal tidal cycles were determined at one inshore (31.34–32.24 salinity) and one offshore (33.64–33.75 salinity) station in the bay. Negative linear relationships between salinity and DOM characteristics, i.e., DOC, humic-like, and protein-like components, were observed at the inshore station. In contrast, no relationship was observed at the offshore station, suggesting that the export of DOM through rivers and possibly tidal flats have a noticeable influence on DOM concentration and composition up to a relatively elevated salinity of around 33 in Liverpool Bay.

Influences of dissolved organic carbon on stream water chemistry in two forested catchments in central Sweden

Stream water chemistry in two headwater streams draining two small, adjacent catchments in Bispgarden, central Sweden was studied during the 2003 and 2004 growing seasons. The two catchments closely resemble each other in regard to size, shape and drainage density, with the major difference found in the area of wetland lining the streambeds. The emphasis of the study was to investigate the stream water chemistry of these closely resembling catchments, regarding the quality and quantity of dissolved organic carbon (DOC) and its influence on the concentrations of di- and trivalent cations. The streams showed significant differences in the content and size distribution of DOC and in the distribution of cations between the different size fractions. For both streams the high flow events induced by precipitation influenced the chemistry of the streams through increase of organic matter and its associated cations. Fanbergsbacken, with relatively low pH and high DOC concentration, had a greater amount of high molecular mass (HMM) DOC to which approximately 75% of Al and Fe and about 50% of Ca and Mg were associated. Grabergsbacken, with a higher pH and lower DOC level, had approximately 65% of Al, 40% of Fe and 30% of Ca and Mg associated to its HMM DOC fraction. Sixteen different low molecular mass organic acids were found in the stream water, of which oxalic and lactic acid were present in the highest concentrations.

DOC dynamics in the meso and bathypelagic layers of the Mediterranean Sea

Abstract Seven years (2001–2008) of dissolved organic carbon (DOC) vertical profiles were examined in order to assess the main processes determining DOC concentration and distribution in the meso- and bathypelagic layers of the Mediterranean Sea. As expected, DOC showed high and highly variable concentrations in the surface layer of 57–68 μM (average values between 0 and 100 m), with a decrease to 44–53 μM between 200 and 500 m. Deep DOC distribution was strongly affected by deep-water formation, with a significant increase to values of 76 μM in recently ventilated deep waters, and low concentrations, comparable to those observed in the open oceanic waters (34–45 μM), where the oldest, deep waters occurred. In winter 2004/2005 a deep-water formation event was observed and the consequent DOC export at depth was estimated to range between 0.76–3.02 Tg C month–1. In the intermediate layer, the main path of the Levantine Intermediate Water (LIW) was followed in order to estimate the DOC consumption rate in its core. Multiple regression between DOC, apparent oxygen utilization (AOU), and salinity indicated that 38% of the oxygen consumption was related to DOC mineralization when the effect of mixing was removed. In deep waters of the southern Adriatic Sea a DOC decrease of 6 μM, together with an AOU increase of 9 μM, was observed between the end of January 2008 and the end of June 2008 (5 months). These data indicate a rate of microbial utilization of DOC of about 1.2 μM C month−1, with 92% of the oxygen consumption due to DOC mineralization. These values are surprisingly high for the deep sea and represent a peculiarity of the Mediterranean Sea.

Dissolved organic carbon export and subsequent remineralization in the mesopelagic and bathypelagic realms of the North Atlantic basin

Dissolved organic carbon (DOC) data are presented from three meridional transects conducted in the North Atlantic as part of the US Climate Variability (CLIVAR) Repeat Hydrography program in 2003. The hydrographic sections covered a latitudinal range of 61 St o 631N along longitudes 201W (CLIVAR line A16), 521W (A20) and 661W (A22). Over 3700 individual measurements reveal unprecedented detail in the DOC distribution and systematic variations in the mesopelagic and bathypelagic zones of the North Atlantic basin. Latitudinal gradients in DOC concentrations combined with published estimates of ventilation rates for the main thermocline and North Atlantic Deep Water (NADW) indicate a net DOC export rate of 0.081 Pg C yr � 1 from the epipelagic zone into the mesopelagic and bathypelagic zones.

Distribution, degradation and dynamics of dissolved organic carbon and its major compound classes in the Pearl River estuary, China

We investigated the distribution, degradation and dynamics of organic carbon and its major compound classes, carbohydrates and amino acids, based upon a cruise in the Pearl River estuary in April 2007. Dissolved oxygen (DO), nutrients, particulate organic carbon (POC), chlorophyll a (Chl a), dissolved organic carbon (DOC), total dissolved carbohydrates (TCHO, including monosaccharides, MCHO, and polysaccharides, PCHO) as well as total dissolved amino acids (TAA, both dissolved free, DFAA, and combined components, DCAA) were measured along a salinity gradient. Community respiration and biodegradable DOC were also determined via both short term (within 3 days) and long term (lasting 30 days) incubation. DOC, MCHO, TCHO, DFAA and TAA concentrations were high in the upper reach of the Pearl River estuary and decreased rapidly downstream. Anthropogenic sewage input appeared to be an important source of the DOC pool in the upper estuary. DOC distribution was non-conservative during the estuarine mixing, showing a net consumption of DOC in the upper reach and in the low salinity ( S < 20) region of the Pearl River estuary. Changes in the relative compositions of carbohydrates (MCHO vs. PCHO) and amino acids (DFAA vs. DCAA) along the salinity gradient further indicated that different processes (biodegradation, flocculation, and phytoplankton production) had different influences on distributions of organic compound classes in this estuarine system. Our one-month incubation experiment further revealed that a substantial portion (15–45%) of DOC from the estuary was biodegradable. Bacterial respiration rates were much higher (0.12–5.8 μmol O 2 L − 1 h − 1 ) than the DOC consumption rates, suggesting that there were other oxygen consumption processes, such as nitrification besides the aerobic respiration of organic matter in the Pearl River estuary, as inferred by the distribution of NH 4 + and NO 3 −. We estimated that 5.3 × 10 8 g C d − 1 of DOC can be exported out from the Lingdingyang Bay (a major subestuary of the Pearl River estuary) to the continental shelf of the South China Sea during this low flow season.

FRACTIONATION OF SOLUBLE MICROBIAL PRODUCTS (SMP) AND SOLUBLE EXTRACELLULAR POLYMERIC SUBSTANCES (EPS) FROM WASTEWATER SLUDGE

Dissolved organic matter (DOM) in soluble microbial products (SMP), loosely bound (LBEPS) and tightly bound extracellular polymeric substances (TBEPS) were extracted from wastewater sludge, fractionated by size exclusion chromatography and characterized by ultraviolet absorbance at 230, 254 and 280 nm, the dissolved organic carbon (DOC) content and excitation‐emission‐matrix (EEM) spectra. The DOM of molecular weight (MW) > 20000 Da accounted for 33% of SMP, including proteins of high aromaticity, and 22% of both LBEPS and TBEPS, which were composed of organic acid‐like substances and proteins, respectively. Polyaluminum chloride (PACl) coagulation effectively removed this DOM fraction. Conversely, the DOM of MW < 1000 Da were mostly non‐aromatic organic acids (and possibly polysaccharides) of low aromaticity for SMP and both EPS, and removal by PAC1 coagulation was minimal. The findings indicated that the chemical characteristics of DOM in SMP and LBEPS were similar but not identical. The TBEPS and LBEPS exhibited significantly different DOM based on DOC distribution or EEM fingerprint.