Reactivity Of Dissolved Organic Carbon Research Articles

Controls on Terrestrial Carbon Fluxes in Simulated Networks of Connected Streams and Lakes

AbstractInland waters play a critical role in the carbon cycle by emitting significant amounts of land‐exported carbon to the atmosphere. While carbon gas emissions from individual aquatic systems have been extensively studied, how networks of connected streams and lakes regulate integrated fluxes of organic and inorganic forms remain poorly understood. Here, we develop a process‐based model to simulate the fate of terrestrial dissolved organic carbon (DOC) and carbon dioxide (CO2) in artificial inland water networks with variable topology, hydrology, and DOC reactivity. While the role of lakes is highly dependent on DOC reactivity, we find that the mineralization of terrestrial DOC is more efficient in lake‐rich networks. Regardless of typology and hydrology, terrestrial CO2 is emitted almost entirely within the network boundary. Consequently, the proportion of exported terrestrial carbon emitted from inland water networks increases with the CO2 versus DOC export ratio. Overall, our results suggest that CO2 emissions from inland waters are governed by interactions between the relative amount and reactivity of terrestrial DOC and CO2 inputs and the network configuration of recipient lakes and streams.

Unified understanding of intrinsic and extrinsic controls of dissolved organic carbon reactivity in aquatic ecosystems.

Despite our growing understanding of the global carbon cycle, scientific consensus on the drivers and mechanisms that control dissolved organic carbon (DOC) turnover in aquatic systems is lacking, hampered by the mismatch between research that approaches DOC reactivity from either intrinsic (inherent chemical properties) or extrinsic (environmental context) perspectives. Here we propose a conceptual view of DOC reactivity in which the combination of intrinsic and extrinsic factors controls turnover rates and determines which reactions will occur. We review three major types of reactions (biological, photochemical, and flocculation) from an intrinsic chemical perspective and further define the environmental features that modulate the expression of chemically inherent reactivity potential. Finally, we propose hypotheses of how extrinsic and intrinsic factors together shape patterns in DOC turnover across the land‐to‐ocean continuum, underscoring that there is no intrinsic DOC reactivity without environmental context. By acknowledging the intrinsic–extrinsic control duality, our framework intends to foster improved modeling of DOC reactivity and its impact on ecosystem services.

Spatiotemporal patterns and drivers of terrestrial dissolved organic carbon (DOC) leaching into the European river network

Abstract. Leaching of dissolved organic carbon (DOC) from soils into the river network is an important component of the land carbon (C) budget. At regional to global scales, its significance has been estimated through simple mass budgets, often using multi-year averages of observed fluvial DOC fluxes as a proxy of DOC leaching due to the limited availability of observations of the leaching flux itself. This procedure leads to a systematic underestimation of the leaching flux because of the decay of DOC during fluvial transport. Moreover, this procedure does not allow for revealing spatiotemporal variability in DOC leaching from soils, which is vital to better understand the drivers of DOC leaching and its impact on the local soil C budget. In this study, we use the land surface model (LSM) ORCHILEAK to simulate the terrestrial C budget, including leaching of DOC from the soil and its subsequent reactive transport through the river network of Europe. The model performance is evaluated not only against the sparse observations of the soil DOC leaching rate, but also against the more abundant observations of fluxes and reactivity of DOC in rivers, providing further evidence that our simulated DOC fluxes are realistic. The model is then used to simulate the spatiotemporal patterns of DOC leaching across Europe over the period 1972–2012, quantifying both the environmental drivers of these patterns and the impact of DOC leaching on the land C budget. Over the simulation period, we find that, on average, 14.3 Tg C yr−1 of DOC is leached from land into European rivers, which is about 0.6 % of the terrestrial net primary production (NPP), a fraction significantly lower than that reported for tropical river networks. On average, 12.3 Tg C yr−1 of the leached DOC is finally exported to the coast via the river network, and the rest is respired during transit. DOC leaching presents a large seasonal variability, with the maximum occurring in winter and the minimum in summer, except for most parts of northern Europe, where the maximum occurs in spring due to snowmelt. The DOC leaching rate is generally low in warm and dry regions, and high in the cold and wet regions of Europe. Furthermore, runoff and the ratio between runoff from shallower flow paths on one hand and deep drainage and groundwater flow on the other hand are the main drivers of the spatiotemporal variation of DOC leaching. Temperature, as a major control of DOC production and decomposition rates in the soils, plays only a secondary role.

Preferential loss of Yukon River delta colored dissolved organic matter under nutrient replete conditions

AbstractThe Yukon River exports a large amount of dissolved organic carbon (DOC), much of which is colored, absorbing visible and ultraviolet light. Yukon River water typically has low total dissolved nitrogen (TDN), with an average DOC : TDN ratio of 21.1 (mol C mol N−1) ~ 200 km upstream from the coast at Pilot Station, Alaska. TDN has been correlated to the bioavailability of DOC, but a direct estimate of the dependence of DOC reactivity on nutrient availability has not been conducted in the Yukon River delta. The biodegradability of DOC and colored dissolved organic matter (CDOM) was assessed along a gradient within the Yukon River delta and into the coastal ocean. Samples were dark incubated at 20°C and half of the incubations were amended with inorganic nutrients. At five time points through 27 d, CDOM absorption and DOC concentration were measured. Initial DOC concentration was 113–830 μmol C L−1 and CDOM absorption at 443 nm (a443) ranged from 0.16 to 7.8 m−1 from the ocean to a high CDOM lake. DOC was relatively unreactive, with 3–6% of DOC degrading in riverine samples and little difference between nutrient amended and control incubations. Control river delta a443 decreased 3–10%, while nutrient amended treatments saw a 6–22% decline over 27 d. In the nutrient amended riverine stations, CDOM absorption loss was greater at blue wavelengths and CDOM degradation was greater than total DOC loss. DOC was resistant to biodegradation in both nutrient amended and ambient conditions, while CDOM was preferentially degraded when nutrients were replete.

Soothsaying DOM: A Current Perspective on the Future of Oceanic Dissolved Organic Carbon

The vast majority of freshly produced oceanic dissolved organic carbon (DOC) is derived from marine phytoplankton, then rapidly recycled by heterotrophic microbes. A small fraction of this DOC survives long enough to be routed to the interior ocean, which houses the largest and oldest DOC reservoir. DOC reactivity depends upon its intrinsic chemical composition and extrinsic environmental conditions. Therefore, recalcitrance is an emergent property of DOC that is analytically difficult to constrain. New isotopic techniques that track the flow of carbon through individual organic molecules show promise in unveiling specific biosynthetic or degradation pathways that control the metabolic turnover of DOC and its accumulation in the deep ocean. However, a multivariate approach is required to constrain current carbon fluxes so that we may better predict how the cycling of oceanic DOC will be altered with continued climate change. Ocean warming, acidification, and oxygen depletion may upset the balance between the primary production and heterotrophic reworking of DOC, thus modifying the amount and/or composition of recalcitrant DOC. Climate change and anthropogenic activities may enhance mobilization of terrestrial DOC and/or stimulate DOC production in coastal waters, but it is unclear how this would affect the flux of DOC to the open ocean. Here, we assess current knowledge on the oceanic DOC cycle and identify research gaps that must be addressed to successfully implement its use in global scale carbon models.

A mechanistic microbial underpinning for the size-reactivity continuum of dissolved organic carbon degradation

The reservoir of dissolved organic carbon (DOC) in the ocean is modified by multiple input and removal processes. Incubation experiments as well as measurements of oceanic DOC have demonstrated that the high molecular weight (HMW) fraction of DOC typically has a younger radiocarbon age and is more reactive biologically than the low molecular weight (LMW) fraction of DOC. These observations have been summarized as a ‘size-reactivity continuum’ of DOC reactivity, but mechanistic explanations for these observations have been lacking. Here we describe how our recent discovery of ‘selfish’ HMW organic matter uptake among bacteria in surface ocean waters may help explain the rapid removal of HMW DOC. ‘Selfish’ substrate uptake by bacteria encompasses rapid binding and partial hydrolysis of intact polysaccharides on the outer membrane of bacteria, seamlessly followed by the transport of large oligosaccharide fragments into the periplasm with little to no loss of LMW hydrolysis products. ‘Selfish’ bacteria therefore process HMW substrates in a manner distinct from bacteria that carry out extracellular hydrolysis that yields LMW hydrolysis products in the environment. Recognition of the presence and prevalence of selfish bacteria in the ocean has profound implications for carbon flow – the source and quantity of LMW substrates made available to non-extracellular-enzyme producing bacteria – as well as for efforts to model and measure bacterial interactions during organic matter degradation. This discovery also highlights the importance of targeted substrate binding and uptake as key (often understudied) factors in geochemical investigations of microbially driven carbon cycling in the ocean. We conclude with some speculative thoughts about the factors that may determine the prevalence of selfish substrate uptake in the environment.

The influence of submarine groundwater discharge on nearshore marine dissolved organic carbon reactivity, concentration dynamics, and offshore export

Dissolved organic carbon (DOC) is the largest pool of reduced carbon in the oceans, with a reservoir equivalent to atmospheric CO2. In nearshore marine regions, DOC sources include primary production, terrestrial DOC delivered by river discharge, and/or terrestrial and marine DOC delivered via submarine groundwater discharge (SGD). While the importance of SGD to coastal carbon cycling has been implicated, the actual influence of this process on nearshore carbon dynamics and offshore export has not been explicitly identified. This study, conducted at a predominantly marine-influenced intertidal beach-nearshore ocean system along the Santa Barbara, California coastline, aimed to address this knowledge gap. Dark, temperature-controlled laboratory incubations, radioisotopic (radon-222) SGD estimates, and an SGD-DOC mixing-reaction box model were coupled to identify the influence of pore water mixing with seawater on nearshore DOC reactivity, concentration dynamics, and offshore export. Even with a relatively low volumetric contribution, SGD pore water mixing altered nearshore DOC reactivity, and elevated the nearshore DOC concentration by 0.4–6.6 µmol L−1 over nearshore seawater residence times spanning 1–6 days. These elevated DOC concentrations were equivalent to 0.5% to 9% of the mean offshore DOC concentration for the summer months in the Santa Barbara Channel, when the coastal water column is highly thermally stratified. Despite the challenge of assessing carbon dynamics in physically and biogeochemically complex nearshore marine regions, this study demonstrates the need for future investigations to assess and account for SGD as a non-trivial component of coastal marine carbon cycles.

Linked changes in marine dissolved organic carbon molecular size and radiocarbon age

AbstractMarine dissolved organic carbon (DOC) is a major global carbon reservoir, yet its cycling remains poorly understood. Previous work suggests that DOC molecular size and chemical composition can significantly affect its bioavailability. Thus, DOC size and composition may control DOC cycling and radiocarbon age (via Δ14C). Here we show that DOC molecular size is correlated to DOC Δ14C in the Pacific Ocean. Our results, based on a series of increasing molecular size fractions from three depths in the Pacific, show increasing DOC Δ14C with increasing molecular size. We use a size‐age distribution model to predict the DOC and Δ14C of ultrafiltered DOC. The model predicts both large and small surface DOC with high Δ14C and a narrow range (200–500 Da) of low Δ14C DOC. Deep model offsets suggest different size distributions and/or Δ14C sources at 670–915 m. Our results suggest that molecular size and composition are linked to DOC reactivity and storage in the ocean.

Characterisation of algogenic organic matter during an algal bloom and its implications for trihalomethane formation

Abstract It is predicated that algal blooms will become an increasing problem under changing climatic conditions. This is particularly concerning for the potable water treatment industry since algogenic organic matter (AOM) in surface waters supplying water treatment works (WTWs) can cause a number of treatment issues. However, whilst previous studies have shown that AOM is distinct from terrigenous, humic-dominated organic matter, limited information exists relating to changes in the character of AOM during different algal growth phases. In this study, reservoir water containing dissolved organic carbon (DOC) dominated by humic material was enriched with nutrient medium to create an algal bloom. Over the course of the algal bloom, DOC was characterised using XAD-fractionation and UV absorbance measurements. In addition, the reactivity of DOC with chlorine both before and after XAD-fractionation was assessed using trihalomethane formation potential (THMFP) and bromine incorporation measurements to monitor whether THM yield and speciation varied between different growth phases. Characterisation of DOC during the algal bloom indicated a shift towards more hydrophilic, aliphatic (low specific UV absorbance; SUVA) DOC with the release of extracellular organic matter (EOM) and later intracellular organic matter (IOM) during cell lysis. XAD-fractionation results suggest that algae produce predominantly hydrophilic neutral (HPIN) DOC. In contrast to some existing research, our study shows a marked change in DOC reactivity over time with a reduction in standardised THMFP (STHMFP) and the initial rate of THM formation observed as the algal bloom progressed. However, bromine incorporation increased with culture age.

Surface production fuels deep heterotrophic respiration in northern peatlands

Multiple analyses of dissolved organic carbon (DOC) from pore waters were conducted to define the processes that govern carbon balance in peatlands: (1) source, reactivity, and transport of DOC with respect to vegetation, peat, and age of carbon substrate, (2) reactivity of DOC with respect to molecular size, and (3) lability to photoxidation of surficial DOC. We found that surface organic production fuels heterotrophic respiration at depth in advection‐dominated peatlands, especially in fens. Fen DOC was Δ14C enriched relative to the surrounding fen peat, and fen respiration products were similar to this enriched DOC indicating that DOC was the main microbial substrate. Bog DOC was more variable showing either enrichment in ∆14C at depth or ∆14C values that follow peat values. This variability in bogs is probably controlled by the relative importance of vertical transport of labile carbon substrates within the peat profile versus DOC production from bog peat. These results extended our set of observations to 10 years at one bog‐fen pair and add two additional bog‐fen pairs to our series of observations. Anaerobic incubations of peat, rinsed free of residual DOC, produced DOC and respiration products that were strikingly similar to the peat values in a bog and two fens. This result demonstrated conclusively that downward advection is the process responsible for the presence of modern DOC found at depth in the peat column. Fen DOC has lower C/N values and up to twice as much LMW (<1 kDa) DOC as bogs due to differences in organic inputs and greater microbial processing. Fluorescence irradiation experiments showed that fen DOC is more photolabile than bog DOC.

Mercury and Methylmercury Dynamics in the Hyporheic Zone of an Oregon Stream

The role of the hyporheic zone in mercury (Hg) cycling has received limited attention despite the biogeochemically active nature of this zone and, thus, its potential to influence Hg behavior in streams. An assessment of Hg geochemistry in the hyporheic zone of a coarse-grained island in the Coast Fork Willamette River in Oregon, USA, illustrates the spatially dynamic nature of this region of the stream channel for Hg mobilization and attenuation. Hyporheic flow through the island was evident from the water-table geometry and supported by hyporheic-zone chemistry distinct from that of the bounding groundwater system. Redox-indicator species changed abruptly along a transect through the hyporheic zone, indicating a biogeochemically reactive stream/hyporheic-zone continuum. Dissolved organic carbon (DOC), total Hg, and methylmercury (MeHg) concentrations increased in the upgradient portion of the hyporheic zone and decreased in the downgradient region. Total Hg (collected in 2002 and 2003) and MeHg (collected in 2003) were correlated with DOC in hyporheic-zone samples: r2 = 0.63 (total Hg-DOC, 2002), 0.73 (total Hg-DOC, 2003), and 0.94 (MeHg-DOC, 2003). Weaker Hg/DOC association in late summer 2002 than in early summer 2003 may reflect seasonal differences in DOC reactivity. Observed correlations between DOC and both total Hg and MeHg reflect the importance of DOC for Hg mobilization, transport, and fate in this hyporheic zone. Correlations with DOC provide a framework for conceptualizing and quantifying Hg and MeHg dynamics in this region of the stream channel, and provide a refined conceptual model of the role hyporheic zones may play in aquatic ecosystems.

Radiocarbon evidence for the importance of surface vegetation on fermentation and methanogenesis in contrasting types of boreal peatlands

We found a consistent distribution pattern for radiocarbon in dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and methane replicated across spatial and temporal scales in northern peatlands from Minnesota to Alaska. The 14C content of DOC is relatively modern throughout the peat column, to depths of 3 m. In sedge‐dominated peatlands, the 14C contents of the products of respiration, CH4 and DIC, are essentially the same and are similar to that of DOC. In Sphagnum‐ and woody plant–dominated peatlands with few sedges, however, the respiration products are similar but intermediate between the 14C contents of the solid phase peat and the DOC. Preliminary data indicates qualitative differences in the pore water DOC, depending on the extent of sedge cover, consistent with the hypothesis that the DOC in sedge‐dominated peatlands is more reactive than DOC in peatlands where Sphagnum or other vascular plants dominate. These data are supported by molecular level analysis of DOC by ultrahigh‐resolution mass spectrometry that suggests more dramatic changes with depth in the composition of DOC in the sedge‐dominated peatland pore waters relative to changes observed in DOC where Sphagnum dominates. The higher reactivity of DOC from sedge‐dominated peatlands may be a function of either different source materials or environmental factors that are related to the abundance of sedges in peatlands.

Relationships between specific ultraviolet absorbance and trihalomethane precursors of different carbon sources

A rapid and effective detection method is essential for water utilities to monitor variability of dissolved organic carbon (DOC) in source waters in order to apply strategies to minimize formation of disinfection by-products in treated waters. Ultraviolet absorbance at 254 nm (UVA254) and specific UVA254 (SUVA254) have been widely used as surrogates of concentration and reactivity of DOC, respectively. However, poor correlations between SUVA254 and specific trihalomethane formation potential (STHMFP) have been occasionally reported and the reliability of using SUVA254 to predict trihalomethane (THM) formation has been questioned. In this study, the correlations of SUVA254 and THM reactivity of three different DOC sources commonly found in water treatment facilities (aquatic carbon, soil carbon, and fecal matter) were evaluated. A 0.22 μm filter, instead of 0.45 μm filter, was used for water filtration to minimize the effects of colloidal materials on UVA254. UVA254 and DOC after chlorination were also examined and differential UVA254 and SUVA254 (ΔSUVA254) were compared to THM reactivity. Results showed correlations between UVA254 and DOC were source dependent suggesting natural humification and degradation processes did not alter DOC characteristics from its original sources. The STHMFP of river and soil DOC samples were comparable, whereas their UVA254 normalized THMFP were different ( p <0.05), suggesting that UVA254 is a better indicator in predicting THM formation potential than DOC concentrations. ΔSUVA254 showed a stronger correlation with STHMFP than the conventional surrogate—SUVA254.

Evaluation of specific ultraviolet absorbance as an indicator of the chemical composition and reactivity of dissolved organic carbon.

Specific UV absorbance (SUVA) is defined as the UV absorbance of a water sample at a given wavelength normalized for dissolved organic carbon (DOC) concentration. Our data indicate that SUVA, determined at 254 nm, is strongly correlated with percent aromaticity as determined by 13C NMR for 13 organic matter isolates obtained from a variety of aquatic environments. SUVA, therefore, is shown to be a useful parameter for estimating the dissolved aromatic carbon content in aquatic systems. Experiments involving the reactivity of DOC with chlorine and tetramethylammonium hydroxide (TMAH), however, show a wide range of reactivity for samples with similar SUVA values. These results indicate that, while SUVA measurements are good predictors of general chemical characteristics of DOC, they do not provide information about reactivity of DOC derived from different types of source materials. Sample pH, nitrate, and iron were found to influence SUVA measurements.

DOC removal by multi-stage ozonation-biological treatment

Multi-stage ozonation-biological treatment process for dissolved organic carbon (DOC) removal was evaluated to apply for drinking water treatment. Waters with different types of DOC were used, i.e. a reservoir water for drinking water supply, a secondary effluent from a municipal wastewater treatment plant and a solution of humic substances extracted from leaf mold. The multi-stage ozonation-biological treatment process was compared with conventional single-stage ozonation-biological treatment process. Amount of DOC removed in biological treatment was defined as amount of biodegradable dissolved organic carbon (BDOC) in influent of biological treatment. DOC removal in the multi-stage ozonation-biological treatment was higher than that in the conventional single-stage ozonation-biological treatment with the same total ozonation time for the reservoir water and humic substances solution. Moreover, three- or four-stage ozonation for 5 min followed by biological treatment (total ozonation time 15 or 20 min) showed higher removal of DOC than the single-stage ozonation (60 min) and biological treatment. The higher DOC removal in the multi-stage treatment was due to the production of BDOC by ozonation. The long-term ozonation was not effective to produce BDOC because most of ozone was utilized to oxidize BDOC produced in the early stage of ozonation. In the multi-stage treatment, ozonation was effective to decompose refractory DOC and to produce BDOC because BDOC was removed by biological treatment. However, multi-stage ozonation-biological treatment was not effective for the secondary effluent. The reason seems to be high concentration of ozone scavengers in that water and low reactivity of DOC for ozone.

Composition and reactivity of DOC in forest floor soil solutions in relation to tree species and soil type

Metal coordinating propertiesof DOC (dissolved organic carbon), and henceits influence on heavy metal release andmineral weathering, depend on the compositionand properties of DOC. Tree species producelitter with different chemical composition anddegradability, and these differences mightinfluence the composition and reactivity of DOCin soil solutions. Accordingly, analysis ofcomposition and reactivity of DOC in soilsolution samples collected by centrifugationfrom 16 forest soil O horizons from the fourtree species beech (Fagus sylvatica L.),oak (Quercus robur L.), grand fir (Abies grandis Lindl.), and Norway spruce (Picea abies (L.) Karst.) on two clayey and twosandy soils were carried out. The compositionand properties of DOC were determined bycapillary zone electrophoresis, acid-basetitration, Cu ion titration, total elementalanalysis, IR and UV spectroscopy, and metalrelease assays. Concentrations of DOC rangedfrom 20 to 163 mM with pH ranging from 4.6 to7.3. Norway spruce produced the highest DOCconcentration, and the lowest pH. Carbon inlow-molecular-weight aliphatic carboxylic acids(LACA) accounted for less than 6% of DOC withformic and acetic acids as the most abundantLACAs. The DOC was cation exchanged and protonsaturated to obtain comparable forms of DOC.Titratable carboxylic acid and phenolic groupswere in the range 51 to 82 and 20 to64 mmol·mol−1 C, respectively, with fewerphenolic groups in grand fir DOC as the onlysignificant difference. Infrared spectra offreeze-dried DOC samples suggest low contentsof aromatic C in the DOC especially from grandfir stands. Stability constants, log K of Cu-DOC complexes, determined by Cu ion titrationof DOC samples with fitting of the data to atwo-site binding model, were in the range 5.63to 6.21 for the strong binding sites and 3.58to 4.10 for the weak sites, but with nosignificant effects of tree species or site.Freeze-dried DOC samples were found to consistof 41 to 45% C, 38 to 49% O, 4.4 to 5.4% Hand 1.2 to 2.0% N and C/N ratios in the range26 to 42. Reactivity of DOC in terms of releaserates of Cd, Cu and Fe cations from a soilsample (flow cell experiments) showed nosignificant differences among DOC samples fromdifferent tree species and soil types.Apparently, only minor differences occur inchemical composition and reactivity ofequivalent concentrations of DOC in forestfloor soil solutions irrespective of origin,i.e. four tree species and two soil types. Soilsolution pH and the concentration of DOCproduced by various tree species are thecritical parameters when distinguishing amongtree species in relation to heavy metal releaseand mineral weathering.

Transformations in dissolved organic carbon through constructed wetlands

Constructed wetlands have emerged as a viable option for addressing a wide range of water quality problems, especially in treating wastewater effluent. This paper presents longitudinal profiles in dissolved organic carbon (DOC) concentrations and structural characteristics across a full-scale wastewater treatment wetland receiving lagoon-treated wastewater (DOC=15–25 mg/L). DOC removal through the wetland varied seasonally, achieving a maximum net removal of 47% in February and minimum net removal of 9% in June. During summer months, when the wetland plants were actively growing, DOC decreased across the first half of the wetland and then increased through the second half of the wetland. Specific ultraviolet absorbance at 254 nm always increased across the wetland, with the largest increases (>130%) occurring during summer months. DOC lability decreased across the wetland. DOC reactivity to form trihalomethanes was also reduced on both an absolute and per carbon mass basis. Laboratory experiments employing a series of wetland microcosms with HRTs ranging from 1.6 to 7.4 days were employed to determine the amount of DOC leached from Typha wetland plant material. During fifty-six day steady-state experiments, roughly 5–8% of the total Typha biomass added was leached as DOC, 45–60% remained in the reactor as accumulated biomass, the remainder of the carbon (30–50%) exited as particulate organic carbon or was microbially respired. We hypothesized that DOC in the wastewater effluent biodegraded over the first-half of the wetland, and that DOC leaching from plant material occurred throughout the wetland. A DOC-wetland model was developed, and the results suggested that the percentage of plant-derived DOC increases with longer HRTs, and while the overall DOC concentration exiting a wetland may only be slightly lower than influent levels that a majority of the DOC, which contains a large percentage of refractory DOC, could be plant-derived. Wetlands with short HRTs would reduce the amount of DOC leached from plant material.

Fluxes of dissolved organic carbon from Chesapeake Bay sediments

Benthic fluxes of dissolved organic carbon (DOC) were measured over an annual cycle at two contrasting sites in Chesapeake Bay. At an organic-rich, sulfidic site in the mesohaline portion of the Bay (site M) DOC fluxes from the sediments ranged from 1.4 to 2.9 mmol/m 2/d. Measured benthic DOC fluxes at site M corresponded to ~3–13% of the depth-integrated benthic C remineralization rates (ΣOCR), and agreed well with calculated diffusive DOC fluxes based on porewater DOC profiles. This agreement suggests that DOC fluxes from site M sediments were likely controlled by molecular diffusion. The second site that was studied is a heavily bioturbated site in the southern Bay (site S). The activity of macrobenthos did not appear to enhance DOC fluxes from these sediments, since measured benthic DOC fluxes (< 0.5 mmol/m 2/d) were lower than those at site M. The ratios of benthic DOC fluxes to ΣOCR values at site S were also slightly smaller than those observed at site M. Benthic DOC fluxes from Chesapeake Bay sediments do not appear to significantly affect the transport of DOC through this estuary, although uncertainties in the reactivity of DOC in estuaries makes this conclusion somewhat tentative at this time. However, when these results are used to make a lower limit estimate of the globally integrated benthic DOC flux from marine sediments, a value similar to that previously calculated by Burdige et al. (1992) is obtained. This observation further supports suggestions in this paper about the importance of benthic DOC fluxes in the oceanic C cycle.