Dissolved Organic C Concentrations Research Articles

Carbon Accumulation and Storage in Mineral Subsoil beneath Peat

We showed that sandy subsoils beneath peat near Ramsey Lake, MI, contained 7.5 to 8.0 kg organic C (OC) m−2 to a depth of 70 cm, compared with 1.6 to 3.6 kg m−2 in adjacent forest soils, from which the peatland had evolved through paludification. Compared with the soils beneath the forest, those beneath the peat contained similar amounts of oxalate-extractable Al (Alo) and less Fe, owing to loss of Fe under reduced leached conditions beneath the peat. Most of the extractable Al in the mineral horizons beneath peat was organically bound. Atomic pyrophosphate-extractable (Fep + Alp) Fe + Al/C ratios were smaller (<0.2:1) in the mineral horizons beneath peat than in the forest mineral subsoils (generally >0.2:1), indicating larger inputs and retention in the mineral subsoils beneath peat. Concentration of OC in subsoil samples was strongly (r2 = 0.58, p < 0.001) related to the subsoil Al, null-point dissolved organic C (DOC) and clay concentrations. A DOC sorption study using peat pore water ranging from 0 to 117 mg DOC L−1 revealed null-point DOC (DOCnp) concentrations (at which there is neither gain nor loss of OC by the soil) varied from <1 to 417 mg DOC L−1 and was primarily related to Feo (negatively), Alo (negatively), and C (positively). We suggest that the increase in mineral subsoil OC beneath peat is related to the sorption of DOC carried by pore water percolating through the mineral subsoil, combined with slow rates of mineralization of sorbed DOC under anoxic conditions.

Mobilisation of arsenic and other trace elements in fluviolacustrine aquifers of the Huhhot Basin, Inner Mongolia

Observed As concentrations in groundwater from boreholes and wells in the Huhhot Basin of Inner Mongolia, northern China, range between <1 μg l −1 and 1480 μg l −1. The aquifers are composed of Quaternary (largely Holocene) lacustrine and fluvial sediments. High concentrations are found in groundwater from both shallow and deep boreholes as well as from some dug wells (well depths ranging between <10 m and 400 m). Populations from the affected areas experience a number of As-related health problems, the most notable of which are skin lesions (keratosis, melanosis, skin cancer) but with internal cancers (lung and bladder cancer) also having been reported. In both the shallow and deep aquifers, groundwaters evolve down the flow gradient from oxidising conditions along the basin margins to reducing conditions in the low-lying central part of the basin. High As concentrations occur in anaerobic groundwaters from this low-lying area and are associated with moderately high dissolved Fe as well as high Mn, NH 4, dissolved organic C (DOC), HCO 3 and P concentrations. Many of the deep groundwaters have particularly enriched DOC concentrations (up to 30 mg l −1) and are often brown as a result of the high concentrations of organic acid. In the reducing groundwaters, inorganic As(III) constitutes typically more than 60% of the total dissolved As. The highest As concentrations tend to be found in groundwater with low SO 4 concentrations and indicate that As mobilisation occurs under strongly reducing conditions, where SO 4 reduction has been an active process. High concentrations of Fe, Mn, NH 4, HCO 3 and P are a common feature of reducing high-As groundwater provinces (e.g. Bangladesh, West Bengal). High concentrations of organic acid (humic, fulvic acid) are not a universal feature of such aquifers, but have been found in groundwaters from Taiwan and Hungary for example. The observed range of total As concentrations in sediments is 3–29 mg kg −1 ( n=12) and the concentrations correlate positively with total Fe. Up to 30% of the As is oxalate-extractable and taken to be associated largely with Fe oxides. The release of As into solution under the reducing conditions is believed to be by desorption coupled with reductive dissolution of the Fe oxide minerals. The association of dissolved As with constituents such as HCO 3, DOC and P may be a coincidence related to the prevalent reducing conditions and slow groundwater flow, but they may also be directly involved because of their competition with As for binding sites on the Fe oxides. The Huhhot groundwaters also have some high concentrations of dissolved U (up to 53 μg l −1) and F − (up to 6.8 mg l −1). In contrast to As, U occurs predominantly under the more oxidising conditions along the basin margins. Fluoride occurs dominantly in the shallow groundwaters which have Na and HCO 3 as the dominant ions. The combination of slow flow of groundwater and the young age of the aquifer sediments are also considered potentially important causes of the high dissolved As concentrations observed as the sediments are likely to contain newly-formed and reactive minerals and have not been well flushed since burial.

Dissolved organic carbon is released from sealings and glues of pore-water samplers

To quantify solute concentrations in soil pore waters, their alteration on contact with sampler components should be negligible compared with naturally occurring concentrations. We hypothezise that elastomer sealings and glues are potential sources of dissolved organic C (DOC) in pore water samples. We immersed different sealing materials and filled assembled samplers with water and determined the release of DOC from glues, elastomers, and other components of suction cups and plates into water samples. Between 0.8 and 63 mg DOC L -1 were released on contact with sampler parts. We conclude that soil solution samplers should be designed without glues or elastomers, if DOC concentrations <10 mg L -1 are the targets of investigations and expected flux rates are low. Glass suction plates entirely made from borosilicate glass avoided the contamination of samples with sealing derived DOC.

Factors controlling concentration, export, and decomposition of dissolved organic nutrients in the Everglades of Florida

Water draining from the Everglades marshes of southern Florida contains high concentrations of dissolved organic C (DOC), N (DON), and in some locations, P (DOP). These dissolved organic nutrients carry over 90% of the N and organic C, and about 25% of the P exported downstream in the Everglades. Our objectives were to describe the most important aspects of the origin and fate of dis- solved organic matter (DOM) in the Everglades, and to describe the processes controlling its concen- tration and export. Concentrations of dissolved organic nutrients are influenced by local plant produc- tion, decomposition, and sorption equilibrium with peat. The drained peat soils of the Everglades Agricultural Area and the more productive marshes of the northern Everglades produce some of the highest concentrations of DOC and DON in the Everglades watershed. In portions of the marshes of the northern Everglades, P enrichment was correlated with higher local DOC and DON concentrations and greater production of soluble plant matter. Microbial degradation of Everglades DOM was very slow; less than 10% of the DOC was lost after 6 months of incubation in the laboratory and supplements of inorganic nutrients failed to speed the decomposition. Exposure to solar radiation increased the subse- quent decay rate of the remaining DOC (25% in 6 mo.). Solar radiation alone mineralized 20.5% of the DOC, 7% of the DON, and degraded about 50% of the humic substances over 21 days in sterile pore- water samples and thus degraded DOM faster than microbial degradation. The humic substances ap- peared to inhibit biodegradation of the other fractions of the DOC since hydrophilic organic acids de- composed faster when isolated from the humic substances. The fate of DOC and DON is closely linked as indicated by a generally narrow range of C/N ratios. In contrast, high concentrations of DOP were associated with P enrichment (at least in pore water). The DOC was composed of about 50% humic substances, 33% hydrophilic acids, and 15% hydrophilic neutral substances, typical of DOC from other environments, despite the fact that it originates from a neutral to slightly alkaline peatland. Despite high exports of DON (3.9 g m2 y1 from one area), the marshes of the northern Everglades are a sink for DON on a landscape scale. The agricultural fields of the Everglades Agricultural Area, however, ex- ported net quantities of DON. High concentrations of DOC desorbed from the agricultural soils when water with no DOC was added. Sorption experiments indicated that high concentrations of dissolved organic matter flowing into the marshes from the Everglades Agricultural Area could suppress the fur- ther desorption of additional soluble organic matter through physicochemical mechanisms. While bio- logical factors, plant production and microbial decomposition are important in producing potentially soluble organic nutrients, physicochemical sorption equilibria, hydrology, and degradation by solar ra- diation are also likely to control the export of this material on the landscape scale.

Dissolved organic carbon is released from sealings and glues of pore-water samplers

To quantify solute concentrations in soil pore waters, their alteration on contact with sampler components should be negligible compared with naturally occurring concentrations. We hypothezise that elastomer sealings and glues are potential sources of dissolved organic C (DOC) in pore water samples. We immersed different sealing materials and filled assembled samplers with water and determined the release of DOC from glues, elastomers, and other components of suction cups and plates into water samples. Between 0.8 and 63 mg DOC L−1 were released on contact with sampler parts. We conclude that soil solution samplers should be designed without glues or elastomers, if DOC concentrations <10 mg L−1 are the targets of investigations and expected flux rates are low. Glass suction plates entirely made from borosilicate glass avoided the contamination of samples with sealing derived DOC.

Contribution of plant-derived carbon to soil microbial biomass dynamics in a paddy rice microcosm

An understanding of the microbial biomass dynamics in rice paddies is essential for managing their nutrient and C cycling. Our objectives were to determine whether the seasonal dynamics of microbial biomass C (MBC) was related to the release of organic substance from rice roots. MBC and the dissolved organic C (DOC) in soil solutions were measured over a growing period of rice plants in a pot experiment. The 13C pulse labelling (through supplying rice plants with 13CO2 for 6 h) was performed at different growth stages of rice to estimate the contribution of photosynthesized C to MBC. DOC concentrations increased with plant growth, reflecting the release of soluble root exudates from rice roots. MBC declined in the early period, and then rapidly increased from the maximum tillering stage to the heading stage of rice. During this period of increase, MBC was positively correlated to the DOC concentration and root biomass. About 0.15–0.94% (mean 0.54%) of photosynthesized 13C was incorporated into MBC immediately after pulse labelling, and 0.18–0.75% (mean 0.41%) still remained at the end of the season. The estimated total contribution of photosynthates to MBC amounted to 91 mg C plant–1, corresponding to 28% of total MBC at the end of the season or a 100% increase in MBC over the growing season. The results suggest that MBC dynamics in rice soil are largely controlled by organic substances released from rice roots.

Dissolved Organic Carbon Retention in Soils

Interactions between dissolved organic C (DOC) and mineral soils were examined in a laboratory column study. Specific emphasis was given to comparisons of soil and solution measurements of organic C (OC) retention in four soil parent materials. Soil columns were leached with one of three concentrations of forest floor (FF) leachate or distilled water every third day for 1 yr. The concentration of DOC in solution was analyzed before and after percolation through soil columns to assess net release or retention of DOC. Changes in soil chemical properties (pH, loss‐on‐ignition [LOI], extractable Al and Fe) following leaching were consistent with podzolization. Micromorphological investigation of the soil materials following leaching showed increased accumulation of organo‐metallic coatings in materials treated with high concentrations of DOC relative to distilled water or low concentrations of DOC. The most dramatic changes in soil chemical properties were found in soils leached with high concentrations of DOC. Comparison of solution and soil measures of OC retention indicates that only ∼50% of the DOC lost from solution was measured as soil OC at the end of the experiment. This finding suggests that microbial decomposition is a significant factor regulating OC concentrations in soils and that soil solution data alone do not yield a complete picture of soil OC dynamics.

Fate of dung-applied copper in a British grassland soil

Copper is commonly used as a fodder amendment in cattle nutrition, but little is known about the relation between dung turnover and environmental fate of Cu. We devised a Cu budget for a common British grassland soil (Typic Epiaquept) for the initial 70 days after the application of 5.33 kg m −2 of dry dung mass in 0.045 m 2 patches. Three replicated treatments: (i) no dung control, (ii) Cu-amended dung (3.2 g Cu m −2), and (iii) unamended dung (i.e. no extra Cu in the animal feed, 1.0 g Cu m −2) were established. Total Cu concentrations were determined in the 0–1, 1–5, and 5–10 cm depth layers 14, 28, 42, and 70 days after dung application. We also determined Cu, pH, and dissolved organic C (DOC) concentrations in the soil solution at 30 cm depth. Initially, Cu concentrations ranged between 24 and 26 mg kg −1 at all depths in all plots and did not change in the control during 70 days at any depth. The recovery of the applied Cu in the remaining dung, soil, and leachate after 70 days was 5%, 10%, and 2% for the Cu-dung treatment and 18%, 8%, and 2% for the unamended dung treatment leaving >70% of the dung-derived Cu unaccounted for in both dung treatments. In soil, applied Cu was sorbed preferentially to aggregate surfaces where we detected 30 mg Cu kg −1 whilst the aggregate interior contained 26 mg Cu kg −1. However, the differences between aggregate interior and exterior disappeared 28 days after application. The Cu concentrations in soil solution were closely related with DOC concentrations but not with pH values. The percentage of dung-derived Cu and C lost by leaching were almost identical. Our results suggest that (i) dung-applied Cu is mainly distributed and lost laterally, (ii) aggregate turnover in a grassland soil may occur within one month, and (iii) the fate of applied dung Cu is quantitatively linked to the fate of dung C.

Evidence that polysaccharide and humic and fulvic acids are co-extracted during analysis but have different roles in the chemistry of natural waters

Reaction with periodic acid-Schiff reagent (PAS), a modified histochemical staining procedure, was used as an index of carbohydrate content of natural waters. Material derived from the standard method for extraction of soluble humic and fulvic acids (HFA) from natural waters, namely adsorption onto and subsequent elution from DEAE cellulose and XAD resin, showed 5–31% of its C content to be carbohydrate. On passage of pure polysaccharide through a DEAE column, 88% of the loading was retained and 50% was subsequently eluted into what would be classified as the HA fraction. Clearly DEAE extraction cannot be regarded as specific for HFA. Similarly, samples were analysed from 62 lentic (non-flowing) waters of varied chemistry and trophic status in NW England. 22%±5% (mean±S.D.) of dissolved organic C (DOC) was PAS+. The proportion of the DOC that reacted positively to the PAS test exceeded 30% of total DOC in sites where DOC concentration was low (<7 mg/l). There was no correlation in the concentrations of DOC and PAS+C suggesting the components of the former have independent sources. Aluminium and Si concentration correlated negatively with PAS+ concentration whilst exhibiting no correlation with DOC. Consequently, prediction of metal speciation in natural waters requires separate consideration of metal interaction with PAS+ and other components of DOC, such as HFA.

Dynamics of Dissolved Organic Carbon and Methane Emissions in a Flooded Rice Soil

Limited information is available on the dynamics of dissolved organic C (DOC) and its relationship with CH4 emissions in flooded rice (Oryza sativa L.) soils as affected by rice cultivar. Greenhouse and laboratory experiments were conducted to determine root C release in culture solution, DOC and dissolved CH4 concentration in soil solution, and CH4 emission in a flooded soil planted with three rice cultivars. Soil solutions were sampled in the root zone (soil surrounding rice roots) and the non‐root zone (soil outside the root zone). The release of root exudates increased in the order: IR65598 (new plant type) &lt; IR72 (modern cultivar) &lt; Dular (a traditional cultivar). Correspondingly, DOC concentrations in the root zone and CH4 emission rates increased. The dynamics of DOC and dissolved CH4 differed greatly between the root zone and the non‐root zone. Dissolved organic C in the root zone increased with plant growth and reached maximum (13–24 mmol C L−1) between rice flowering and maturation (Week 11–13), whereas DOC in the non‐root zone remained low (1–5 mmol C L−1) throughout the growing season. Similarly, dissolved CH4 concentrations in the root zone increased sooner and were greater (mean 138 μmol CH4 L−1) than those in the non‐root zone (mean 97 μmol CH4 L−1). The seasonal patterns of CH4 emissions closely followed the dynamics of DOC concentrations in the root zone. The results suggest that (i) DOC pool in the root zone of rice plants is enriched by root‐derived C; (ii) the rates of CH4 emissions are positively correlated with the dynamics of DOC in the root zone; (iii) the intercultivar difference in root C releases is responsible for the intercultivar difference in DOC production, and consequently in CH4 flux.

Small‐Scale Variability of Metal Concentrations in Soil Leachates

Soil tests often use composite soil samples to assess metal bioavailability, Composite soil samples cannot address small-scale soil heterogeneity. In this study, the concentrations of Al, Cd, Cu, Mn, Ni, Pb, Zn, dissolved organic C (DOC), and pH in soil leachates were examined as an index of small-scale soil heterogeneity. Ten undisturbed soil cores (0-4 cm, 100 cm 3 ) from a 1-m 2 area of a Lithic Haplumbrept (pH 5.2, 3 g CaCO 3 kg -1 ) and a Typic Hapludoll (pH 4.3) under forest canopy were equilibrated with deionized water. The soil cores were then leached with a mock soil solution (pH 4.0, 6.8-11.4 mg L -1 DOC, 0.001 M CaCl 2 ). In the Haplumbrept, the pH of the first 50-mL fraction of the leachates (deionized water extract) was 4.2 to 7.4, DOC concentrations were 11.4 to 38.9 mg L -1 . Aluminum, Cd, Mn, and Ni concentrations were significantly correlated with pH (r = 0.88, 0.93, 0.69, 0.78, respectively; P < 0.05). In the Hapludoll, the pH (4.1-4.6) varied little in the first 50-mL fractions; Cr, Cu, and Pb concentrations were correlated with DOC concentrations (9.6-36.3 mg L -1 ). The variability in metal concentrations of the first 50-mL fractions (coefficients of variation, CV = 25-91%) was comparable in both soils and did not change with increasing leachate volume (mock soil solution) except for Zn in the Haplumbrept (CV up to 174%). In all leachate fractions, variability was markedly higher than those reported for salt extracts of composite soil samples (CV = 1-18%). Thus, the analysis of composite samples may be insufficient to address metal bioavailability in soils.

Seasonal variation of CDOM and DOC in the Middle Atlantic Bight: Terrestrial inputs and photooxidation

Extensive surveys of the fluorescence and absorption of chromophore‐containing dissolved organic matter (CDOM), dissolved organic C (DOC) concentration, chlorophyll fluorescence, and salinity were performed during August and November 1993 and March and April 1994 along a cruise line extending from the mouth of Delaware Bay southeast to the Sargasso Sea. With shallow stratification in August, photobleaching dramatically altered the optical properties of the surface waters, with ∼70% of the CDOM absorption and fluorescence lost through photooxidation in the waters at the outer shelf. S, the slope of the log‐linearized absorption spectrum of CDOM, increased offshore and seemed to increase with photodegradation. The increase in S combined with the seasonal variation in the relationship between Chl and CDOM underscores the difficulty in developing algorithms to predict Chl concentrations in turbid coastal waters with ocean color data. Despite the photooxidation of CDOM, the seasonal variation in the CDOM fluorescence‐absorption relationship and fluorescence quantum yields was &lt;15%. When using appropriate methods, the airborne lidar approach for remote determination of CDOM absorption coefficients seems to be a very robust technique. The photooxidation of CDOM in August also affected the relationship between CDOM and DOC concentration in the surface waters, although for the rest of the year the relationship was reasonably linear. The results of a simple model suggest ∼10% of the DOC in the mixed layer was directly converted photochemically to dissolved inorganic C (DIC).

Effects of lake size, water clarity, and climatic variability on mixing depths in Canadian Shield lakes

The depth of the summer mixed (Ed) of 21 Canadian Shield lakes in northwestern Ontario was examined in relation to lake size (surface area, Ao) and water clarity (extinction coefficient, Kd) for periods ranging from 2 to 23 yr (n = 1,408). The lakes range in Ao from 4 to 4.9 × 105 ha, and midsummer (mid‐June through mid‐August mean) Kd varied from &lt;0.2 to &gt;2.5 m‒1 with greatest variation in small lakes that were experimentally eutrophicated or acidified or whose terrestrial basins were burned. Over the full spectrum of lake sizes, Ao was the primary determinant of Ed; transparency significantly modified this relationship but only in small lakes (Ao &lt;500 ha). In noneutrophic shield lakes, transparency is controlled by the concentration of dissolved organic C (DOC). Lower DOC concentrations (a likely consequence of 2 × CO2 climate change) would cause transparency to increase, resulting in 1–2‐m‐deeper epilimnia in small lakes; there would be no similar effect in large lakes.

Ultraviolet radiation in North American lakes: Attenuation estimates from DOC measurements and implications for plankton communities

Climate warming in North America is likely to be accompanied by changes in other environmental stresses such as UV‐B radiation. We apply an empirical model to available DOC (dissolved organic C) data to estimate the depths to which 1% of surface UV‐B and UV‐A radiation penetrate for several major regions of North America. UV attenuation depths are also estimated from DOC data collected from treatment and reference basins during the experimental acidification of Little Rock Lake, Wisconsin.In some regions of North America 25% of the lakes have 1% attenuation depths for UV‐B radiation on the order of 4 m or more (western and northwestern U.S., Newfoundland). In other regions, 75% of the lakes have 1% attenuation depths for UV‐B shallower than 0.5 m (Florida, upper midwestern U.S., northwestern Ontario, Quebec, and Nova Scotia). Attenuation depths for UV‐A radiation are ∼2.5 times as deep as those for UV‐B. Experimental acidification approximately doubled the estimated 1% attenuation depths for UV radiation in Little Rock Lake.The strong dependence of 1% attenuation depth on DOC below the 1–2 mg liter‒1 DOC range suggests that UV attenuation in low DOC lakes is highly sensitive to even very small changes in DOC. We conclude that changes in climate, lake hydrology, acid deposition, and other environmental factors that alter DOC concentrations in lakes may be more important than stratospheric ozone depletion in controlling future UV environments in lakes.

The biogeochemical cycling of Zn, Cu, Fe, Mn, and dissolved organic C in a seasonally anoxic lake

Detailed time‐depth distributions of dissolved Zn, Cu, Fe, Mn, and dissolved organic C (DOC), together with pH, were obtained for the water column and near‐surface sediment pore waters of a seasonally anoxic lake (Esthwaite Water, U.K.) over a 1‐yr period. Dissolved Fe and Mn foll owed well‐known patterns linked to redox recycling. The variation in DOC resulted mostly from its coupling with Fe, DOC being coprecipitated with Fe oxide and released into solution on the reductive remobilization of the oxide. This process produced a &gt; 7‐fold seasonal variation in DOC concentrations in the pore waters, with a maximum of ∼7 mg liter−1. Dissolved Zn and Cu exhibited coherent and similar distribution patterns unrelated in any simple way to the distributions of dissolved Fe and Mn, pH, and DOC. The most ubiquitous feature of the distribution of dissolved Zn, and to a lesser degree that of dissolved Cu, was a concentration maximum at the sediment‐water interface, which disappeared completely only when anoxia was maximal in mid‐ and late summer. The interfacial maxima were attributed to rapid release from freshly deposited particles allied to sulfide precipitation in the sediments. The timing of events, associated with an increase in interfacial dissolved Cu and Zn concentrations before and during a period of ice cover, indicated that the Cu was derived from Fe and possibly Mn oxides and Zn from planktonic algae. The cumulative downward diffusive fluxes of Zn and Cu from the interfacial maxima were unimportant 1% of total) compared to total metal sedimentation rates. Chemical equilibrium calculations indicated that Zn‐ and Cu‐sulfide complexes dominated in the pore waters under reducing conditions, with humic binding being insignificant, and that precipitation as pure sulfides or coprecipitation with FeS were likely major removal processes for Zn and Cu under anoxic conditions.

Temporal variability of organic C and nitrate in a shallow aquifer

The loading of organic substrates into shallow aquifers may follow seasonal cycles, which will impact the transport and fate of agrichemicals. The objective of this research was to measure temporal changes in the groundwater dissolved organic C (DOC) and nitrate concentrations. Groundwater monitoring wells were installed and sediment samples from the aquifer were collected in 1991. Sediment samples were used to evaluate denitrification potentials, while water samples were collected at periodic intervals in 1992 and 1993 from the surface of the aquifer. Water samples were analyzed for nitrate-N and DOC-C. Denitrification was observed in sediment amended with nitrate and incubated under anaerobic conditions at 10°C. Addition of algae lazed biomass increased denitrification, establishing that denitrification was substrate limited. In the aquifer, DOC concentrations followed seasonal patterns. DOC concentrations were highest following spring recharge and then decreased. Peak timing indicates that freezing and thawing were responsible for seasonal DOC patterns. These findings show that seasonally driven physical processes, such as freezing and thawing, influence organic substrate transport from surface to subsurface environments, and that this process should be taken into account when assessing agrichemical detoxification rates in shallow aquifers.

The attenuation of solar UV radiation in lakes and the role of dissolved organic carbon

Diffuse attenuation coefficients (Kd) for solar UV radiation (UVR) (305, 320, 340, 380 nm, and PAR) were measured in the mixed layer of 65 lake sites in Alaska, Colorado, and Pennsylvania and the Bariloche region of Argentina. Integrated mixed layer samples of lake water were concurrently collected, and a multivariate approach was used to model Kd with a number of optical and chemical variables.Substantial variation in transparency was observed among lakes. Attenuation depths (z1%) for UV‐B radiation ranged from several centimeters to &gt; 10 m. In some shallow, low DOC (dissolved organic C) lakes typical of high elevation ecosystems, substantial fluxes of UVR penetrated the entire water column. In deeper lakes with low DOC concentrations, high fluxes of UVR were found in a significant proportion of the mixed layer. Much of the among‐lake variation in Kd (87–96%) was explained by differences in DOC concentration, which strongly influenced dissolved absorbance. On average, dissolved absorbance accounted for between 33% (for PAR) and 68% (for 305 nm) of Kd measured in situ. Throughout the solar UV‐A and UV‐B range, Kd was best estimated with a univariate power model based solely on DOC concentration. Models are also presented that relate absorption coefficients to Kd. These models can be used with archival DOC or color data to provide approximate estimates of UV transparency of lakes.

Dynamics of dissolved organic carbon in a coastal ecosystem

In the Bothnian Sea, there was a marked seasonal variation of dissolved organic C (DOC) in 1990-1992, with a large increase in DOC concentrations in summer at two stations. The accumulation of DOC ...

The use of in situ and airborne fluorescence measurements to determine UV absorption coefficients and DOC concentrations in surface waters

We compared the fluorescence (λex = 355 nm, λem = 450 nm) of chromophore‐containing dissolved organic matter (CDOM) determined in the laboratory to that determined with a shipboard fluorometer and an airborne laser fluorosensor off the U.S. mid‐Atlantic coast. Fluorescence was highly correlated with CDOM absorption at the excitation wavelength despite the presence of multiple sources of CDOM and a changing contribution of CDOM to the total dissolved organic C (DOC) pool, both in surface waters and down the water column. The mean quantum yield (0.8±0.1%) fell within the narrow range of yields previously determined for U.S. east coast and south Florida waters. These results show that CDOM absorption can be reliably determined from in situ or airborne fluorescence over wide areas of the ocean and that it may be possible to estimate DOC concentration from fluorescence or absorption measurements.

Dissolved organic carbon concentrations in marine pore waters determined by high‐temperature oxidation

We have developed sampling methods and an analytical system to determine the concentration of dissolved organic C (DOC) in marine pore waters. Our analytical approach is a modification of recently developed high‐temperature, Pt‐catalyzed oxidation methods; it uses chromatographic trapping of the DOC‐derived CO2, followed by reduction to CH4 and flame ionization detection. Sampling experiments with nearshore sediments indicate that pore‐water separation by whole‐core squeezing causes artificially elevated DOC concentrations, while pore‐water recovery by sectioning and centrifugation does not appear to introduce DOC artifacts. Results from a set of northwestern Atlantic continental slope cores suggest that net DOC production accounts for &gt; 50% of the organic C that is recycled at the sediment‐water interface.