Terrestrial Dissolved Organic Carbon Research Articles

High Carbon Dioxide Evasion from an Alpine Peatland Lake: The Central Role of Terrestrial Dissolved Organic Carbon Input

We measured carbon dioxide (CO2) fluxes across air-water interface with floating chambers in Lake Medo (a small, shallow lake in peatland) on the eastern Tibetan Plateau in the warm season of 2009. During the study period, mean CO2 fluxes was 488.63 +/- 1,036.17 mg CO2 m(-2) h(-1). The flux rate was high compared to those of lakes in other regions, and represented a "hotspot" of CO2 evasion. Temporal variation of CO2 flux was significant, with the peak value in the beginning and lowest point in the end of warm season. High concentration of dissolved organic carbon (DOC) in lake water (WDOC) was found to highly correlated to CO2 flux (R = 0.47, P < 0.01, n = 54). Besides, fluorescence index of WDOC showed its terrestrial origin character. In accordance with lakes in northern and boreal regions, terrestrial DOC concentration in water column was the most important regulator of CO2 flux from this lake. We suggest that large area of peatlands in catchments support high concentration of DOC in this lake, and consequently high CO2 evasion.

Influence of Dissolved Organic Carbon on Methylmercury Bioavailability across Minnesota Stream Ecosystems

Stream ecosystems are widely contaminated by mercury (Hg) via atmospheric transport and deposition in watersheds. Dissolved organic carbon (DOC) is well-known to be the dominant ligand for aqueous methylmercury (MeHg), the bioaccumulative form of Hg in aquatic food webs. However, it is less clear if and how the concentration and character (e.g., aromaticity) of DOC influences the availability of dissolved MeHg to stream food webs. In this work, we analyzed total-Hg and/or MeHg concentrations in water, seston, and macroinvertebrates (filter-feeding hydropsychid caddisflies), and other physiochemical properties in 30 streams along a south-north geographic gradient in eastern Minnesota that corresponds to substantial changes in dominant land cover (i.e., agriculture, urban, wetland, and forest). In general, MeHg concentrations in seston and hydropsychids were higher in watersheds with more forest and wetland coverage, and increased with dissolved MeHg concentration. However, we found that the efficiency of MeHg incorporation into the stream food webs (i.e., bioconcentration factors of MeHg in both seston and hydropsychids, BCF(MeHg) = solid MeHg ÷ dissolved MeHg) decreased significantly with DOC concentration and aromaticity, suggesting that MeHg bioavailability to the base of food webs was attenuated at higher levels of terrestrial DOC. Therefore, our findings suggest that there is a dual role of DOC on MeHg cycling in streams: terrestrial DOC acts as the primary carrier ligand of dissolved MeHg for transport into surface waters, yet this aromatic DOC also attenuates dissolved MeHg uptake by aquatic food webs. Thus, consideration of MeHg bioavailability and its environmental regulation could help improve predictive models of MeHg bioaccumulation in stream ecosystems.

Ultrahigh bacterial production in a eutrophic subtropical Australian river: Does viral lysis short‐circuit the microbial loop?

We studied trophic dynamics in a warm eutrophic subtropical river (Bremer River, Australia) to determine potential sources of dissolved organic carbon (DOC) and the fate of heterotrophic bacterial production. Sustained high rates of bacterial production suggested that the exogenous DOC was accessible (labile). Bacterial specific growth rates (0.2 h−1 to 1.8 h−1) were some of the highest measured for natural aquatic ecosystems, which is consistent with high respiration rates. Bacteria consumed 10 times more organic carbon than that supplied by the daily algal production, a result that implies that terrestrial sources of organic carbon were driving the high rates of bacterial production. Viruses (1011 L−1) were 10 times more abundant than bacteria; the viral to bacterial ratio ranged from 3.5 to 12 in the wet summer and 11 to 35 in the dry spring weather typical of eutrophic environments. Through a combination of high bacterial respiration and phage lysis, a continuous supply of terrestrial DOC was lost from the aquatic ecosystem in a CO2‐vented bacterial–viral loop. Bacterial processing of DOC in subtropical rivers may be contributing disproportionately large amounts of CO2 to the global carbon cycle compared to temperate freshwater ecosystems.

Investigation of a fiber optic surface plasmon spectroscopy in conjunction with conductivity as an in situ method for simultaneously monitoring changes in dissolved organic carbon and salinity in coastal waters

A combination surface plasmon resonance (SPR) and conductivity sensor array was developed and implemented to demonstrate the ability to differentiate among changes in dissolved organic carbon (DOC) and salinity in coastal water. The array is capable of achieving sufficient spatial and temporal data density to better understand the cycling and fate of terrestrial DOC in coastal areas. DOC is the second largest source of bioreactive carbon in the environment and plays a key role in mediating microbial activity and generation of atmospheric CO(2). In the coastal areas, the salinity is also an important property in many applications, such as leak detection for landfill liners, saltwater intrusion to drinking water, marine environment monitoring, and seasonal climate prediction. Conductivity sensors are the industry standard for determining salinity in ocean systems. However, both conductivity and refractive index sensors, such as SPR spectroscopy based sensors, respond to salinity and DOC levels. To demonstrate the capability of the SPR sensor and a conductivity sensor to collect complimentary data useful in discrimination of salinity and DOC in coastal zone water, conductivity, SPR, and temperature data were collected during passage from the Juan de Fuca ridge area returning to the University of Washington docks.

Nonconservative behavior of dissolved organic carbon across the Laptev and East Siberian seas

Climate change is expected to have a strong effect on the Eastern Siberian Arctic Shelf (ESAS) region, which includes 40% of the Arctic shelves and comprises the Laptev and East Siberian seas. The largest organic carbon pool, the dissolved organic carbon (DOC), may change significantly due to changes in both riverine inputs and transformation rates; however, the present DOC inventories and transformation patterns are poorly understood. Using samples from the International Siberian Shelf Study 2008, this study examines for the first time DOC removal in Arctic shelf waters with residence times that range from months to years. Removals of up to 10%–20% were found in the Lena River estuary, consistent with earlier studies in this area, where surface waters were shown to have a residence time of approximately 2 months. In contrast, the DOC concentrations showed a strong nonconservative pattern in areas with freshwater residence times of several years. The average losses of DOC were estimated to be 30%–50% during mixing along the shelf, corresponding to a first‐order removal rate constant of 0.3 yr−1. These data provide the first observational evidence for losses of DOC in the Arctic shelf seas, and the calculated DOC deficit reflects DOC losses that are higher than recent model estimates for the region. Overall, a large proportion of riverine DOC is removed from the surface waters across the Arctic shelves. Such significant losses must be included in models of the carbon cycle for the Arctic Ocean, especially since the breakdown of terrestrial DOC to CO2 in Arctic shelf seas may constitute a positive feedback mechanism for Arctic climate warming. These data also provide a baseline for considering the effects of future changes in carbon fluxes, as the vast northern carbon‐rich permafrost areas draining into the Arctic are affected by global warming.

Factors influencing the acid–base (pH) balance in the Baltic Sea: a sensitivity analysis

Using calculations based on the marine carbon system and on modelling, the sensitivity of Baltic Sea surface pH was examined. Transient long-term calculations demonstrated that the marine carbon system adjusts to lateral boundary conditions within some decades, as does salinity. Climate changes in temperature or salinity will only marginally affect the acid–base (pH) balance. Wetter or dryer climate will also play a minor role in the pH balance. The direct effect on seawater pH of acid precipitation over the Baltic Sea surface was demonstrated to be small. Acidification due to river transport of dissolved organic carbon (DOC) into the marine system seems marginal although mineralization of terrestrial DOC may cause extra marine acidification, but the effect has yet to be quantified. Increased nutrient load may increase the amplitude in the pH seasonal cycle and increase the acidification during winter time. Fossil fuel burning is likely to have both a direct and indirect effect through increased CO2 levels, altering seawater pH as well as changing the river chemistry. This may severely threaten some species in the Baltic Sea, particularly in the Northern Baltic.

Bioavailability of terrestrial organic carbon to lake bacteria: The case of a degrading subarctic permafrost mire complex

Permafrost degradation can result in the loss of significant amounts of carbon, through release to the atmosphere in the form of carbon dioxide and/or methane and through export downstream to lakes and rivers. The fate of this carbon in lake ecosystems is poorly understood. We investigated the capacity of lake bacteria to utilize carbon from different soils from an adjacent mire. Dark bioassays were undertaken to measure the dynamics of the bioavailability and chemical character of dissolved organic carbon (DOC). The soils ranged from already degraded minerotrophic fens to ombrotrophic active layer and soils from the permafrost layer. Our study shows that soil DOC was rapidly consumed by bacteria collected from lake water, particularly within the first 48 h (about 85% of the total consumed DOC). The mean DOC consumption by lake bacteria was 0.087 mg L−1 d−1 when supplied with lake water DOC and varied between 0.382 mg L−1 d−1 (permafrost soil) and 0.491 mg L−1 d−1 (degraded fen soil) when supplied with terrestrial DOC. Thus, the data suggest that export of DOC from degrading permafrost mires at any stage of degradation can potentially increase rates of respiration by fourfold to sevenfold and can have pronounced effects both on receiving lake ecosystems and on the land‐atmosphere carbon balance. In this study we also propose simple predictive models, incorporating weight‐averaged molecular weight and specific UV absorption in combination with other simple qualitative parameters for the estimation of potential bioavailability of soil DOC in aquatic ecosystems.

Utilisation of terrestrial carbon by osmotrophic algae

Terrestrial-derived dissolved organic carbon (DOC) contributes significantly to the energetic basis of many aquatic food webs. Although heterotrophic bacteria are generally considered to be the sole consumers of DOC, algae and cyanobacteria of various taxonomic groups are also capable of exploiting this resource. We tested the hypothesis that algae can utilise DOC in the presence of bacteria if organic resources are supplied in intervals by photolysis of recalcitrant DOC. In short-term uptake experiments, we changed irradiation in the range of minutes. As model substrates, polymers of radiolabelled coumaric acid (PCA) were used, which during photolysis are known to release aromatic compounds comparable to terrestrial-derived and refractory DOC. Three cultured freshwater algae readily assimilated PCA photoproducts equivalent to a biomass-specific uptake of 5–60% of the bacterial competitors present. Algal substrate acquisition did not depend on whether PCA was photolysed continuously or in intervals. However, the data show that photoproducts of terrestrial DOC can be a significant resource for osmotrophic algae. In long-term growth experiments, interval light was applied one hour per day. We allowed cultured Chlamydomonas to compete for ambient DOC of low concentration. We found higher abundances of Chlamydomonas when cultures were irradiated intermittently rather than continuously. These data suggest that photolysis of DOC supports algal heterotrophy, and potentially facilitates growth, when light fluctuations are large, as during the diurnal light cycle. We concluded that osmotrophic algae can efficiently convert terrestrial carbon into the biomass of larger organisms of aquatic food webs.

Flow and nutrient dynamics in a subterranean estuary (Waquoit Bay, MA, USA): Field data and reactive transport modeling

A two-dimensional (2D) reactive transport model is used to investigate the controls on nutrient ( NO 3 - , NH 4 + , PO 4) dynamics in a coastal aquifer. The model couples density-dependent flow to a reaction network which includes oxic degradation of organic matter, denitrification, iron oxide reduction, nitrification, Fe 2+ oxidation and sorption of PO 4 onto iron oxides. Porewater measurements from a well transect at Waquoit Bay, MA, USA indicate the presence of a reducing plume with high Fe 2+, NH 4 + , DOC (dissolved organic carbon) and PO 4 concentrations overlying a more oxidizing NO 3 - -rich plume. These two plumes travel nearly conservatively until they start to overlap in the intertidal coastal sediments prior to discharge into the bay. In this zone, the aeration of the surface beach sediments drives nitrification and allows the precipitation of iron oxide, which leads to the removal of PO 4 through sorption. Model simulations suggest that removal of NO 3 - through denitrification is inhibited by the limited overlap between the two freshwater plumes, as well as by the refractory nature of terrestrial DOC. Submarine groundwater discharge is a significant source of NO 3 - to the bay.

Links between Terrestrial Primary Production and Bacterial Production and Respiration in Lakes in a Climate Gradient in Subarctic Sweden

We compared terrestrial net primary production (NPP) and terrestrial export of dissolved organic carbon (DOC) with lake water heterotrophic bacterial activity in 12 headwater lake catchments along an altitude gradient in subarctic Sweden. Modelled NPP declined strongly with altitude and annual air temperature decreases along the altitude gradient (6°C between the warmest and the coldest catchment). Estimated terrestrial DOC export to the lakes was closely correlated to NPP. Heterotrophic bacterial production (BP) and respiration (BR) were mainly based on terrestrial organic carbon and strongly correlated with the terrestrial DOC export. Excess respiration over PP of the pelagic system was similar to net emission of CO2 in the lakes. BR and CO2 emission made up considerably higher shares of the terrestrial DOC input in warm lakes than in cold lakes, implying that respiration and the degree of net heterotrophy in the lakes were dependant not only on terrestrial export of DOC, but also on characteristics in the lakes which changed along the gradient and affected the bacterial metabolization of allochthonous DOC. The study showed close links between terrestrial primary production, terrestrial DOC export and bacterial activity in lakes and how these relationships were dependant on air temperature. Increases in air temperature in high latitude unproductive systems might have considerable consequences for lake water productivity and release of CO2 to the atmosphere, which are ultimately determined by terrestrial primary production.

Dissolved Organic Carbon in Alaskan Boreal Forest: Sources, Chemical Characteristics, and Biodegradability

The fate of terrestrially-derived dissolved organic carbon (DOC) is important to carbon (C) cycling in both terrestrial and aquatic environments, and recent evidence suggests that climate warming is influencing DOC dynamics in northern ecosystems. To understand what determines the fate of terrestrial DOC, it is essential to quantify the chemical nature and potential biodegradability of this DOC. We examined DOC chemical characteristics and biodegradability collected from soil pore waters and dominant vegetation species in four boreal black spruce forest sites in Alaska spanning a range of hydrologic regimes and permafrost extents (Well Drained, Moderately Well Drained, Poorly Drained, and Thermokarst Wetlands). DOC chemistry was characterized using fractionation, UV–Vis absorbance, and fluorescence measurements. Potential biodegradability was assessed by incubating the samples and measuring CO2 production over 1 month. Soil pore water DOC from all sites was dominated by hydrophobic acids and was highly aromatic, whereas the chemical composition of vegetation leachate DOC varied significantly with species. There was no seasonal variability in soil pore water DOC chemical characteristics or biodegradability; however, DOC collected from the Poorly Drained site was significantly less biodegradable than DOC from the other three sites (6% loss vs. 13–15% loss). The biodegradability of vegetation-derived DOC ranged from 10 to 90% loss, and was strongly correlated with hydrophilic DOC content. Vegetation such as Sphagnum moss and feathermosses yielded DOC that was quickly metabolized and respired. In contrast, the DOC leached from vegetation such as black spruce was moderately recalcitrant. Changes in DOC chemical characteristics that occurred during microbial metabolism of DOC were quantified using fractionation and fluorescence. The chemical characteristics and biodegradability of DOC in soil pore waters were most similar to the moderately recalcitrant vegetation leachates, and to the microbially altered DOC from all vegetation leachates.

Importance of seasonality and small streams for the landscape regulation of dissolved organic carbon export

The regulation of the spatial and seasonal variation in terrestrial dissolved organic carbon (DOC) exports was studied in a 68 km2 boreal stream system in northern Sweden. A total of 1213 DOC samples were collected in 15 subcatchments over a 3 year period (2003–2005). The mean annual DOC exports from the 15 subcatchments (0.03–21.72 km2) ranged from 14.8 to 99.1 kg ha−1 yr−1. Many catchment characteristics determined the spatial variation in DOC exports. The relative importance of the different catchment characteristics varied greatly between seasons because of differing hydrological conditions. During winter base flow the spatial variation was linked to patterns in wetland coverage. During snowmelt in spring the spatial variation was connected to characteristics describing size and location, i.e., median stream size, silty sediment distribution, stream order, altitude, and proportion of catchment above highest postglacial coastline (HC). During the snow‐free season the spatial variation in DOC exports was regulated by the amount of wetlands and forests, particularly forests made up of Norway spruce (Picea abies). Median stream size also influenced the exports during this season. A striking result in this study was the effect of size implying that small headwaters may be the largest contributor to the terrestrial DOC export, per unit area.

Interactions between Elevated CO2and Warming Could Amplify DOC Exports from Peatland Catchments

Peatlands export more dissolved organic carbon (DOC) than any other biome, contributing 20% of all terrestrial DOC exported to the oceans. Both warming and elevated atmospheric CO2 (eCO2) can increase DOC exports, but their interaction is poorly understood. Peat monoliths were, therefore, exposed to eCO2, warming and eCO2 + warming (combined). The combined treatment produced a synergistic (i.e., significant interaction) rise in DOC concentrations available for export (119% higher than the control, interaction P < 0.05) and enriched this pool with phenolic compounds (284%). We attribute this to increased plant inputs, coupled with impaired microbial degradation induced by competition with the vegetation for nutrients and inhibitory phenolics. Root biomass showed a synergistic increase (407% relative to the control, P < 0.1 only), while exudate inputs increased additively. Phenol oxidase was suppressed synergistically (58%, interaction P < 0.1 only) and beta-glucosidase (27%) additively, while microbial nutritional stress increased (51%) additively. Such results suggest intensified carbon exports from peatlands, with potentially widespread ramifications for aquatic processes in the receiving waters.

Carbon isotopic characterization of dissolved organic carbon in rainwater: Terrestrial and marine influences

This study presents the first published 14C and 13C isotopic data for dissolved organic carbon (DOC) in rainwater. The 14C fraction modern ( Fm) values for rain DOC ranged from 0.9051 to 1.0569. Between 4% and 24% of the rainwater DOC was of fossil fuel origin based on a two end member calculation assuming biogenic carbon has the same 14C content as the contemporary atmosphere ( Fm=1.11–1.19) and fossil fuel carbon is devoid of 14C. The δ 13C content of rain DOC ranged from −21.8‰ to −28.2‰ spanning the terrestrial and fossil fuels average (∼−26‰) to marine (−18‰ to −20‰) values. Rain events with more continental influences had δ 13C values closer to the terrestrial and fossil fuels signal and higher concentrations of fossil fuel derived DOC. Rain events with stronger oceanic influences had δ 13C values shifting towards those typical of marine systems and had lower concentrations of fossil fuel derived DOC. The influence air mass back trajectory has on the terrestrial and marine characteristics of rainwater DOC identifies air mass recent history as an important factor controlling the sources of DOC in precipitation. It also indicates water soluble volatile organic carbon (VOC's) have relatively short residence times on the order of days with minimal global transport. However, residence times of days, and the presence of both marine and terrestrial DOC in rainwater, suggests that precipitation represents an important exchange of carbon between land and the oceans especially in coastal regions.

Relative importance of CO2 recycling and CH4 pathways in lake food webs along a dissolved organic carbon gradient

Terrestrial ecosystems export large quantities of dissolved organic carbon (DOC) to aquatic ecosystems. This DOC can serve as a resource for heterotrophic bacteria and influence whether lakes function as sources or sinks of atmospheric CO2. However, it remains unclear as to how terrestrial carbon moves through lake food webs. We addressed this topic by conducting a comparative lake survey in the northeastern U.S. along a gradient of terrestrial‐derived DOC. We used naturally occurring carbon stable isotopes of CO2, particulate organic matter (POM), and crustacean zooplankton, as well as gas measurements and culture‐independent assessments of microbial community composition to make inferences about the flow of terrestrial carbon in lake food webs. Stable isotope ratios of POM and zooplankton decreased with DOC and were often depleted in 13C relative to terrestrial carbon, suggesting the importance of an isotopically light carbon source. It has been proposed that the incorporation of biogenic methane (CH4) into plankton food webs would account for such trends in stable isotope ratios, but we found weak evidence for this hypothesis, on the basis of relationships of CH4, methanogenic archaebacteria, and methanotrophic bacteria in our lakes. Instead, our results are consistent with the view that phytoplankton increase their use of heterotrophically respired CO2 with increasing concentrations of terrestrialderived DOC. The effect of this CO2 recycling can be detected in the stable isotope composition of crustacean zooplankton, suggesting that the direct transfer of terrestrial DOC inputs to higher trophic levels may be relatively inefficient.

Sources and transport of dissolved and particulate organic carbon in the Mississippi River estuary and adjacent coastal waters of the northern Gulf of Mexico

Dissolved organic carbon (DOC), stable carbon isotopic ( δ 13C) compositions of DOC and particulate organic carbon (POC), and elemental C/N ratios of POC were measured for samples collected from the lower Mississippi and Atchafalaya rivers and adjacent coastal waters in the northern Gulf of Mexico during the low flow season in June 2000 and high flow season in April 2001. These isotopic and C/N results combined with DOC measurements were used to assess the sources and transport of terrestrial organic matter from the Mississippi and Atchafalaya rivers to the coastal region in the northern Gulf of Mexico. δ 13C values of both POC (−23.8‰ to −26.8‰) and DOC (−25.0‰ to −29.0‰) carried by the two rivers were more depleted than the values measured for the samples collected in the offshore waters. Strong seasonal variations in δ 13C distributions were observed for both POC and DOC in the surface waters of the region. Fresh water discharge and horizontal mixing played important roles in the distribution and transport of terrestrial POC and DOC offshore. Our results indicate that both POC and DOC exhibited non-conservative behavior during the mixing especially in the mid-salinity range. Based on a simple two end-member mixing model, the comparison of the measured DOC- δ 13C with the calculated conservative isotopic mixing curve indicated that there was a significant in situ production of marine-derived DOC in the mid- to high-salinity waters consistent with our in situ chlorophyll- a measurements. Our DOC- δ 13C data suggest that a removal of terrestrial DOC mainly occurred in the high-salinity (>25) waters during the mixing. Our study indicates that the mid- to high- (10–30) salinity range was the most dynamic zone for organic carbon transport and cycling in the Mississippi River estuary. Variability in isotopic and elemental compositions along with variability in DOC and POC concentrations suggest that autochthonous production, bacterial utilization, and photo-oxidation could all play important roles in regulating and removing terrestrial DOC in the northern Gulf of Mexico and further study of these individual processes is warranted.

Use of Keeling plots to determine sources of dissolved organic carbon in nearshore and open ocean systems

We apply a Keeling mixing model commonly used in atmospheric sciences to oceanic dissolved organic carbon (DOC) 14C and concentrations to demonstrate that DOC distribution across spatial transects or depth profiles can be modeled as two‐component mixtures; one component consists of an old refractory pool to which a newly synthesized component pool is added. We then use the Keeling approach to determine the Δ14C signature of excess DOC added to background concentrations in nearshore and open ocean systems. Our analysis indicates enrichment in 14C of excess DOC in coastal waters relative to offshore waters, particularly in spring when river discharge increases. These findings are consistent with the export of 14C‐enriched terrestrial DOC to the ocean and inconsistent with the idea of selective degradation of young terrestrial DOC in estuaries, resulting in export of old refractory component to the ocean. In the middle Atlantic Bight region, during the spring, excess DOC has a 14C signature similar to that of DOC in the major rivers, whereas during the peak primary productivity of the summer season, excess DOC has a signature more similar to that of dissolved inorganic carbon. Variability in the Δ14C of excess DOC in the open ocean was lower than that of the coastal ocean. Minima in Δ14C values of excess DOC in the open ocean were measured during periods of high particulate organic matter (POC) flux, indicating that POC can interact with DOC and selectively remove 14C‐enriched components from the surface ocean. The Keeling plot provides a powerful approach for determining the isotopic composition of excess DOC from an entire DOC profile or transect.

Fate of terrigenous dissolved organic matter (DOM) in estuaries: Aggregation and bioavailability

Abstract When dissolved organic matter (DOM) from terrestrial and freshwater sources is mixed with estuarine waters at the land-sea interface, the change in salinity has been suggested to cause fast aggregation and an increase in the bioavailability of dissolved organic carbon (DOC) and nitrogen (DON). These processes were investigated in different Danish freshwaters. Aggregation of DOC in short-term (hours) mixing events at increasing salinity was low. In one stream with forest and wetland runoff and in a humic lake, the decrease of DOC over a 0 to 25 ppt salinity gradient was 2 to 5%. Optical analyses by absorption and fluorescence, revealed changes in the composition of the humic components due to salt. The bioavailability of terrestrial DOC was also investigated and found not to change moving from limnic to estuarine conditions. Although the yield of freshwater bacteria cells was about twice the yield of estuarine bacteria, the utilization of DOC was identical and not influenced by the bacterial commu...

Formula omitted] composition of marine dissolved organic carbon

The stable isotopic composition of marine dissolved organic carbon (DOC) was measured with a high temperature, sealed tube combustion technique. The δ 13 C values of DOC ( DO 13 C) were approximately −22‰ for all samples that were collected (1) in a monthly survey conducted in Woods Hole Harbor, (2) in a spatial survey across Georges Bank during a spring bloom period, and (3) in depth profiles near Bermuda, Georges Bank and in the eastern Equatorial Pacific. The DO 13 C values were uniform in waters of varying DOC content, including low DOC deep waters and high DOC surface waters, indicating little overall isotopic fractionation during formation and degradation of marine DOC. The −22‰ DO 13 C values are similar to those observed for average marine particulate and sedimentary organic material, and exclude a large contribution from −27‰ terrestrial DOC to ocean pools.

Distribution of Dissolved Organic Carbon in the Continental Margin off Northern Taiwan

Analytical precision of dissolved organic carbon (DOC) in seawater using the high-temperature catalytic method (HTCO) and distributions of DOC in the continental margin off northern Taiwan were investigated. The DOC concentration of Kuroshio water off northeastern Taiwan varied from 122-136μM C at the surface to 65-74μM C at the bottom. The concentration decreased sharply downward in the upper 200m of the water column and then decreased slightly toward the bottom. The pattern showed neither seasonal variation nor a significant difference from that observed in the water off southeastern Taiwan. The concentration of DOC was found to be inversely correlated with apparent oxygen utilization (AOU) in the Kuroshio water. The slope, however, was smaller than that predicted from the Redfield stoichiometry, indicating that only a small fraction (＜25%) of AOU was derived from the oxidation of DOC. Distributions of DOC were found to vary, both temporally and spatially in the continental shelf and slope waters. Although the distributions were relatively patchy in the inner and middle shelves, they were influenced apparently by the upwelled Kuroshio subsurface water in the outer shelf and slope regions. DOC may have been diluted in the intensive upwelling regime but enhanced on the margin of the up- welling center. Terrestrial DOC was largely confined to areas close to the continent. DOC was higher in near bottom waters, indicating that it diffused from sediments and/or was released from resuspended sediments. Thus, DOC varied non-conservatively across the continental margin through various blogeochemical processes. The horizontal gradient of surface DOC decreasing from the southern East China Sea (ECS) to the N. Pacific may reflect a similar trend of surface biological productivity and suggest the potential export of DOC out of the ECS continental margin as well.