Surface Water Dissolved Organic Carbon Research Articles

Combined neutral sugars as indicators of the diagenetic state of dissolved organic matter in the Arctic Ocean

Dissolved organic matter (DOM) was sampled by tangential flow ultrafiltration from different basins in the central Arctic Ocean, the Greenland, Iceland and Norwegian (GIN) Seas, and the estuaries of the Ob and Yenisei Rivers. Dissolved organic carbon (DOC) concentrations ranged from more than 500 μM C in the rivers to about 50 μM C in the deep basins. Arctic Mediterranean Sea (AMS) surface water DOC concentrations ranged from 66 to 137 μM C. High concentrations in surface waters were due mainly to river discharge of terrigenous DOC, while elevated concentrations in AMS deep water likely reflect downwelling of DOM in this system. Total hydrolyzable neutral sugar concentrations were ∼7-fold higher in ultrafiltered DOM (UDOM) from estuaries (1200 nM) than in Arctic ocean surface waters (160 nM), but carbon-normalized neutral sugar yields (% OC) were lower in the estuaries indicating that marine DOM sources were relatively rich in neutral sugars. Depth profiles of neutral sugar concentrations and carbon-normalized yields indicated that carbohydrates were produced and consumed largely in surface waters of the AMS. These observations were consistent with rapid bacterial degradation of neutral sugars during decomposition experiments. The molecular composition of neutral sugars was very similar among AMS surface samples and showed only minimal differences between terrestrial and marine UDOM, indicating that the neutral sugar composition is not useful for distinguishing these sources. It appears diagenetic processes rather than sources are primarily responsible for determining neutral sugar compositions in UDOM. Neutral sugar compositions in all of the UDOM samples analyzed in this study were indicative of heteropolysaccharides. The strong relationship between neutral sugars and the diagenetic state of DOM was best represented by neutral sugar yields (% OC), which were used as a quantitative indicator for the amount of labile DOM in the AMS. Based on the low average neutral sugar yield (5.5% OC) in UDOM from AMS surface waters, we estimated that only ∼2.2% of the DOC was of labile nature. This value was lower than for surface waters of the Ross Sea and Atlantic and Pacific Oceans suggesting DOM is less bioavailable and the microbial loop is less active in AMS surface waters.

VARIATION IN BIOAVAILABILITY OF DISSOLVED ORGANIC CARBON AMONG STREAM HYPORHEIC FLOWPATHS

Dissolved organic carbon (DOC) dominates the flux of organic matter in most stream ecosystems, but the proportion susceptible to microbial degradation is often presumed to be low. The fraction of bulk DOC contributing to microbial metabolism was assessed in five streams representing the regional range in surface-water DOC concentration in eastern New York State, USA (range 0.5–7.7 mg/L; n = 82). Transects of shallow wells along two hyporheic flowpaths (i.e., saturated sediments found below and lateral to the open-stream channel in active exchange with surface waters) in each of five streams were sampled monthly at baseflow to determine changes in subsurface DOC and dissolved oxygen concentrations. Hyporheic DOC concentrations ranged from 50% to 100% of surface-water concentrations and decreased along hyporheic flowpaths in four of five streams. Dissolved oxygen losses along hyporheic flowpaths paralleled DOC loss, and bacterial activity on tiles incubated at points along the flowpaths generally declined as hyporheic DOC was depleted. DOC losses along natural flowpaths exceeded the quantity of DOC lost during laboratory bottle incubations, even when samples were amended with inorganic nutrients. Hyporheic mesocosms were used to examine the fate of stream-derived DOC along replicated flowpaths under controlled hydrologic conditions. The overall patterns of DOC losses along mesocosm flowpaths supplied with water from previously studied streams were similar to DOC losses along natural flowpaths. DOC declines were paralleled by declines in bacterial activity and dissolved oxygen. Mesocosm results indicated that variation in percentage of DOC loss along natural flowpaths was not a function of dilution, residence time, or initial DOC concentration and that subsurface DOC dynamics were linked to variation in microbial metabolism. The fraction of total DOC available for metabolism varied markedly among regional streams and was independent of initial DOC concentration. DOC near the end of hyporheic flowpaths was not subject to further degradation, regardless of the bioavailability of surface-water DOC entering these flowpaths. Hence, in streams with significant hyporheic exchange, the amount and bioavailability of DOC transported to downstream ecosystems may be affected by subsurface metabolism. DOC depletion during hyporheic transport may provide a general in situ measure of bioavailable DOC in surface water and be a powerful predictor of rates of heterotrophic activity in sediments at the reach level.

Bubbles: An estimate of their role in the global oceanic flux of carbon

The amount of particulate organic carbon (POC) formed annually via the scavenging of dissolved organic carbon (DOC) by bubbles is calculated to be 1.9 Gt C yr−1. In arriving at this estimate use was made of (1) published descriptions of the bubble spectrum associated with the alpha plumes beneath spilling wave crests, (2) the published relationship of stage A whitecap coverage to wind speed, (3) data on the scale depth of alpha plumes and on the rise velocity of bubbles coated with organic film, (4) published bubble scavenging efficiencies, and (5) recorded concentrations of DOC in oceanic surface waters. This rate of DOC scavenging by bubbles represents a 1.7% yr−1 turnover of the global oceanic DOC pool, and each year produces an amount of POC equal to 17% of the POC pool in the uppermost 300 m of the ocean. Bubble scavenging turns over the DOC present in the shallow layer of bubble influence once every 67 days. Moreover, within the depth of influence of large bubbles (uppermost 1.4 m of the water column), bubble scavenging represents an annual threefold turnover of the DOC pool and a POC production equivalent to thirtyfold of the POC pool. It is noted that the amount of DOC transformed into POC each year via bubble scavenging is equal to the amount of carbon present in the carbon dioxide estimated to be absorbed by the ocean each year from the atmosphere.

The relationship between soil heterotrophic activity, soil dissolved organic carbon (DOC) leachate, and catchment‐scale DOC export in headwater catchments

Dissolved organic carbon (DOC) from terrestrial sources forms the major component of the annual carbon budget in many headwater streams. In high‐elevation catchments in the Rocky Mountains, DOC originates in the upper soil horizons and is flushed to the stream primarily during spring snowmelt. To identify controls on the size of the mobile soil DOC pool available to be transported during the annual melt event, we measured soil DOC production across a range of vegetation communities and soil types together with catchment DOC export in paired watersheds in Summit County, Colorado. Both surface water DOC concentrations and watershed DOC export were lower in areas where pyrite weathering resulted in lower soil pH. Similarly, the amount of DOC leached from organic soils was significantly smaller (p < 0.01) at sites having low soil pH. Scaling point source measurements of DOC production and leaching to the two basins and assuming only vegetated areas contribute to DOC production, we calculated that the amount of mobile DOC available to be leached to surface water during melt was 20.3 g C m−2 in the circumneutral basin and 17.8 g C m−2 in the catchment characterized by pyrite weathering. The significant (r2=0.91 and p < 0.05), linear relationship between over‐winter CO2 flux and the amount of DOC leached from upper soil horizons during snowmelt suggests that the mechanism for the difference in production of mobile DOC was heterotrophic processing of soil carbon in snow‐covered soil. Furthermore, this strong relationship between over‐winter heterotrophic activity and the size of the mobile DOC pool present in a range of soil and vegetation types provides a likely mechanism for explaining the interannual variability of DOC export observed in high‐elevation catchments.

Effect of landscape form on export of dissolved organic carbon, iron, and phosphorus from forested stream catchments

Predicting the effects of climate change and atmospheric deposition on water quality requires predicting the effects of landscape form on export of substances downstream. In this paper, we present dissolved organic carbon (DOC), total phosphorus (TP), and iron (Fe) export data (1980–1992) for 20 relatively undisturbed, forested catchments draining into seven lakes in central Ontario and develop regression models of chemical export as functions of landscape composition. The extent of wetlands was correlated with export of DOC and TP; the proportion of the catchment covered by peatlands accounted for 78% of the variance in a regression model of long‐term average DOC export and 76% of the variance in a model of color “export.” Peatland coverage and Fe export together explained 76% of the variance in a long‐term average TP export model, which is consistent with published experimental evidence that Fe facilitates P complexation with DOC in surface waters. The long‐term average Fe export model was not significant when all 20 catchments were included. However, Fe export from the 14 catchments with thin tills was a function only of peatland coverage (R2 = 0.71), suggesting that Fe export is dependent to a large extent upon either the export of organic material or the reducing conditions. The long‐term export models worked well when export of a substance was dominated by peatlands but was not very sensitive to the influence of mineral soils. The long‐term average TP/DOC ratio was remarkably constant among most whole‐lake catchments, ranging from 1.4 to 2.0 mg P/g C, the exception being the catchment of anthropogenically acidified Plastic Lake with a ratio of 0.8. Fe export to Plastic lake was also enriched relative to TP export compared with the other lakes. Therefore TP export to Plastic Lake may be limited by some mechanism related to acidification.

A major biopolymeric component to dissolved organic carbon in surface sea water

Organic carbon dissolved in sea water is an important component of the global carbon cycle1. Concentrations of dissolved organic carbon (DOC) in the ocean's surface mixed layer are at least twice those in the deep sea2,3, because of the production of soluble carbon compounds by marine algae in the euphotic zone4,5. But very little is known about the chemical composition of DOC, and the connection between photosynthetic production and DOC accumulation is not well understood6,7. Here we report the chemical characterization of macromolecular DOC at several sites in the Atlantic and Pacific oceans. Neutral sugars, acetate and lipids show similar distributions, suggesting that these constituents are linked together in a common macromolecular structure. Chemical linkage patterns between the oligosaccharide portions of dissolved organic matter subjected to ultrafiltration are highly specific, with little variation between ocean basins. We show that laboratory culture experiments on the decomposition of algal exudate produce macromolecular organic matter with similar compositions and linkage characteristics. We propose that a significant fraction of DOC in sea surface water consists of structurally related and biosynthetically derived acyl oligosaccharides that persist after more labile organic matter has been degraded.

Stoichiometry of dissolved organic matter dynamics on the continental shelf of the northeastern U.S.A.

Dissolved organic carbon (DOC), nitrogen (DON), phosphorus (DOP), inorganic nitrogen and inorganic phosphorus were measured from the Georges Bank region during spring 1993 and summer 1994. Concentrations of DOC in surface waters ranged from 65–92 μM C and decreased with depth to about 50 μM in shelf slope water at a depth of 1500 m. DON and DOP concentrations showed a similar pattern with depth, being higher in surface water and lower in deep water (∼5 to 3 μM N and 0.17 to 0.02 μM P). There was an inverse relation between inorganic nutrient concentrations and DON and DOP. While DOC concentrations were elevated in regions of highest primary production, there was little evidence of corresponding elevations for DON and DOP. Slopes of regression lines relating total dissolved N and P to inorganic N and P indicated that 19 and 15% of remineralized N and P that accumulates in deeper water is derived from the export and decomposition of dissolved organic material (DOM). While overall inorganic N and P dynamics followed Redfield, A. C. (1958) ( American Scientist, 46, 205–221) stoichiometry (C:N:P, 106:16:1), the C:N:P stoichiometry of dissolved organic matter deviated substantially. Near-surface stoichiometry of DOM was 400–800:1 for DOC:DOP, 11–15:1 for DOC:DON and 24–55:1 for DON:DOP. Relationships between DOC, DON and DOP indicated that N and P were preferentially remineralized relative to carbon resulting in C:N: P ratios for deeper water DOM that deviated even further from Redfield. Decomposition studies revealed that DOC decomposition rate was related to initial DOC concentration. The higher the DOC concentration the more quickly it was mineralized. DOC first order decay coefficients ranged from undetectable for deep slope water with an initial DOC concentration of 50 μM to 0.0024 day −1 for water collected from the Georges Bank shoals with an initial DOC concentration of 92 μM. DON and DOP decomposition occurred concomitantly with DOC remineralization. Loss rates ranged from undetectable to 0.00064 day −1 for DON and from undetectable to 0.0023 day −1 for DOP. During DOM decomposition we noted increases in the C:N, C:P and N:P ratios, which indicated preferential remineralization of N and P relative to C. Vertical concentration gradients and slow DOM turnover times relative to water residence times in the Georges Bank continental shelf region indicate the potential for export of DOM to deeper water. DOM export to depth and preferential remineralization of N and P relative to C may be important mechanisms for pumping C to depth in the ocean.

Photochemistry of natural dissolved organic matter in lake and wetland waters—production of carbon monoxide

The photochemical formation of carbon monoxide (CO) in various lake and wetland waters was found to be correlated with the concentration of dissolved organic carbon (DOC), as well as light absorbance and fluorescence of the waters, suggesting that dissolved organic matter is a major precursor for CO photoproduction. Rates of CO formation also increased nonlinearly with increasing concentration of dissolved iron. For the lake and wetland waters studied, CO production rates were between 13 and 910 nmol 1 −1 h −1 using June solar noon sunlight irradiations. Field experiments in a subtropical pond demonstrated that much of photochemically formed CO in the surface water was effluxed to the atmosphere. Considering the globally-averaged effective sunlight flux, we estimate that photoformation of CO leads to a turnover time of 2–10 yr for DOC in sunlit surface waters.

Accumulation of degradable DOC in surface waters: Is it caused by a malfunctioning microbialloop?

Recent literature indicates that dissolved organic carbon (DOC) may accumulate in productive surface waters. Such accumulation will allow export of DOC to the aphotic zone by diffusion and downwelling. As an alternative to models based on low degradability, we here propose a mechanism where bacterial carbon consumption is restricted due to food web mechanisms controlling both growth and biomass of the bacteria: growth rate is kept low by bacteria‐phytoplankton competition for mineral nutrients, and biomass is kept low by bacterial predators. With such a mechanism, otherwise degradable material may accumulate and become subject both to chemical transformation and vertical transport. The steady‐states of a model describing the interactions between heterotrophic bacteria, phytoplankton, and bacterivorous protozoa is used to explore how the balance between DOC production and consumption shifts along a gradient from oligotrophy to eutrophy.