Benthic Dissolved Organic Carbon Fluxes Research Articles

Dissolved organic matter distribution in the water column and sediment pore water in a highly anthropized coastal lagoon (Mar Menor, Spain): Characteristics, sources, and benthic fluxes

Coastal lagoons are among the most productive and biodiverse systems in the world and are important sentinels of climate change. The Mar Menor is one of the largest coastal lagoons in the Mediterranean, providing a variety of ecosystem services and resources to the community. However, in recent decades this lagoon has suffered drastic changes and degradation caused by human activities. We analyzed the concentration of dissolved organic carbon (DOC) and the optical properties of dissolved organic matter (DOM) in the water column and sediment pore water during the summer and winter of 2018 and during eighteen months from 2016 to 2018. Overall, we found that the composition of DOM is mainly related to and enhanced by anthropogenic activities and microbial metabolism. DOM enters the lagoon via urban and agricultural runoff, drainage systems, and wastewater treatment plants. Additionally, strong microbial metabolism in sediments leads to differences in DOM composition between water and sediments. In the water column, humic-like components accounted for 71 % of the total DOM, while protein-like compounds were most abundant in sediment pore water. We observed a strong seasonal variability associated with precipitation and the system collapse in 2016 (phytoplankton bloom), which resulted in the death of 80 % of macrophytes. The sediments act as a source of DOM to the overlying water, likely due to relatively high organic matter content and intense microbial activity, primarily through anaerobic pathways. Benthic fluxes of DOC ranged from 5.24 to 33.30 mmol m−2 d−1, being higher in winter than summer 2018 and decreasing from north to south, likely related to lower residence time in the northern basin, groundwater discharge and accumulation of organic matter from the dead meadows. We estimate a net flux of DOC from the Mar Menor toward the Mediterranean Sea of 1.57 × 107 mol yr−1.

Benthic fluxes of dissolved organic carbon from gas hydrate sediments in the northern South China Sea.

Hydrocarbon vents have recently been reported to contribute considerable amounts of dissolved organic carbon (DOC) to the oceans. Many such hydrocarbon vents widely exist in the northern South China Sea (NSCS). To investigate if these hydrocarbon vent sites release DOC, we used a real-time video multiple-corer to collect bottom seawater and surface sediments at vent sites. We analyzed concentrations of DOC in these samples and estimated DOC fluxes. Elevated DOC concentrations in the porewaters were found at some sites suggesting that DOC may come from these hydrocarbon vents. Benthic fluxes of DOC from these sediments were 28 to 1264 μmol m−2 d−1 (on average ~321 μmol m−2 d−1) which are several times higher than most DOC fluxes in coastal and continental margin sediments. The results demonstrate that the real-time video multiple-corer can precisely collect samples at vent sites. The estimated benthic DOC flux from the methane venting sites (8.6 × 106 mol y−1), is 24% of the DOC discharge from the Pearl River to the South China Sea, indicating that these sediments make an important contribution to the DOC in deep waters.

Benthic dissolved organic carbon fluxes in a drinking water reservoir

The trend of increasing dissolved organic carbon (DOC) in surface waters motivated us to gain a mechanistic understanding of DOC exchange at the sediment water interface in lakes. We quantified seasonal DOC and solute fluxes under different redox conditions, in a small drinking water reservoir using sediment core incubations. Processes governing benthic DOC exchange were microbial production of DOC and interaction with mineral surfaces. Mobilization of DOC in anoxic sediments seemed to be closely linked to reductive dissolution of ferric minerals as shown by the strong positive correlation (r2 = 0.99) between DOC and Fe fluxes. Oxidized surface sediments were an efficient DOC trap where DOC was bound to ferric minerals. Redox conditions appeared to be the primary regulator of the DOC exchange, resulting in sedimentary uptake of DOC (−1.8 mmol m−2 d−1) only under oxic conditions. DOC production was regulated by temperature, leading to higher DOC fluxes (up to 2.4 mmol m−2 d−1) in summer. The sediment was a net sink of DOC (−0.2 tonnes yr−1) but this was small compared with the annual DOC load (15 tonnes yr−1) of the reservoir. However, the benthic DOC flux was about 10–20% of the dissolved carbon flux at the sediment–water interface, making it a significant process in the lake internal carbon cycle. Climate change is supposed to promote reducing conditions at the bottom of lakes, which might increase benthic DOC production in the future.

Dissolved organic carbon dynamics in anaerobic sediments of the Santa Monica Basin

Cycling of dissolved organic carbon (DOC) was investigated in anoxic sediments of the Santa Monica Basin, California Borderland, by analyzing the concentration and isotopic signatures (Δ14C and δ13C) of pore-water DOC and dissolved inorganic carbon (DIC), and organic compound classes extracted from the bulk sediments. DOC and DIC increased across the sediment–water interface, indicating net efflux of these solutes out of the sediments. Throughout the depth interval examined (0–30cm), the Δ14C value of DOC (Δ14CDOC) was similar to, or higher than, that of bulk sedimentary particulate organic carbon (POC), indicating degradation of relatively 14C-rich components of POC. There were prominent peaks in both Δ14CDOC and Δ14CDIC in the uppermost 2 cm of the sediment column, indicating degradation and remineralization of 14C-rich, labile organic matter in the near-surface sediments. However, below these sub-surface maxima, Δ14CDOC and Δ14CDIC decreased with depth by ∼200‰ and ∼50‰, respectively. Given the diffusive time scales, these decreases were too large to be explained by 14C loss due to radioactive decay. To help explain these observations, we constructed and implemented a selective degradation model that considers bulk pore-water DOC to be the sum of three kinetically- and isotopically-distinct sub-components. Based on this model, the most reactive DOC fraction, which supported ∼60% of the DIC production, had a Δ14C value indicating the presence of bomb-14C. The intermediate fraction had a Δ14C value of ∼−60‰ and accounted for most of the pore-water DOC standing stock. The least reactive fraction was virtually non-reactive in these sediments, and had a Δ14C value of ∼−500‰. The benthic DOC flux of this 14C-depleted, poorly-reactive DOC fraction may represent a source of pre-aged, refractory DOC to the oceans.

Insights into estuarine benthic dissolved organic carbon (DOC) dynamics using δ 13C-DOC values, phospholipid fatty acids and dissolved organic nutrient fluxes

Benthic dissolved organic carbon (DOC) flux rates and changes in DOC isotope ratios, along with nutrient fluxes, phospholipid fatty acids concentration and carbon isotope ratios were measured in productive estuarine sediments over a diel cycle to determine the mechanisms driving benthic–pelagic coupling of DOC. There was uptake of DOC during the dark and efflux during the light at all sites. DOC uptake rates were related to benthic respiration (dark O 2 uptake) and effluxes were coupled to the trophic status (ratio of production to respiration) of the sediments. Highest uptake and efflux rates were observed at two high nutrient concentration sites. The DOC:DON ratio of water column dissolved organic matter (DOM) decreased during the dark and increased during the light indicating preferential uptake and release of carbon rich dissolved organic matter. The calculated carbon isotope ratio of the DOC taken up by the benthos was significantly more depleted than the bulk water column DOC pool, suggesting preferential uptake of selected components of the water column DOC pool. Generally the isotope ratio of the DOC released during the light was more enriched than that taken up during the dark, which suggests that the benthos has the potential to significantly alter the estuarine DOC pool. Uptake and efflux were coupled to respiration and algal grazing/mineralization, therefore increased nutrient loading may shift the composition of the estuarine DOC pool through changes in the magnitude of benthic DOC fluxes. A combination of biological (diel shifts in DOC production and consumption) and abiotic processes (flocculation) appear to be driving the observed benthic DOC dynamics at the study sites. This study was the first to measure carbon isotopic changes in the water column DOC pool due to benthic processes, and shows that the benthos can alter the estuarine DOC pool through diel differences in DOC uptake and efflux.

Benthic fluxes of dissolved organic carbon in three temperate Australian estuaries: Implications for global estimates of benthic DOC fluxes

Light and dark benthic dissolved organic carbon (DOC) fluxes and sediment characteristics (total organic carbon (TOC), total nitrogen (TN), TOC isotope ratio (δ13C), phospholipid fatty acid (PLFA) concentration, and compound‐specific δ13C) were measured seasonally in a range of habitats in three warm‐temperate Australian estuaries to determine what factors control benthic DOC fluxes. Benthic DOC fluxes were seasonal with the highest rates generally observed during summer. Most habitats displayed uptake of DOC during the dark and efflux during the light, and were a net source of DOC to the overlying waters over the diel cycle. Macrophyte habitats (seagrass and macroalgae) had the highest benthic fluxes of DOC (up to ∼50 mmol C m−2 d−1). Organic matter supply to the sediment was dominated by in situ production, and both quantity and source exerted control over benthic DOC fluxes. DOC flux was tightly coupled to bacteria and algal PLFA concentrations during summer, and algal PLFA concentrations during winter, indicating a strong influence of microbial interactions on benthic DOC flux and a temperature‐related decoupling of algae and bacteria under cooler conditions. Estimates of the global estuarine benthic DOC flux indicate that these areas are a significant source of DOC to the ocean (1–41 Tg C yr −1). Inclusion of macrophyte, salt marsh, mangrove, and intertidal benthic DOC flux estimates resulted in a higher global continental margin benthic DOC flux (106–416 Tg C yr −1) than the previous estimate (90 Tg C yr−1).

The effect of induced anoxia and reoxygenation on benthic fluxes of organic carbon, phosphate, iron, and manganese

Eutrophication causes seasonally anoxic bottom waters in coastal environments, but we lack information on effects of onset of anoxia and subsequent reoxygenation on benthic fluxes of redox-sensitive minerals and associated organic carbon (OC). As the first study, we determined the effect of inducing anoxia and subsequently restoring oxic conditions in mesocosms with surface sediment and water from a coastal environment. These concentration changes were compared with those in an oxygenated control. We determined water column concentrations of dissolved organic carbon (DOC), particulate organic carbon (POC), iron, manganese, and phosphate. Benthic fluxes of DOC, POC, and iron increased at the onset of anoxia in oxygen-depleted treatments. DOC and iron concentrations increased concomitantly towards maxima, which may have indicated reductive dissolution of FeOOH and release of associated OC. The subsequent concomitant concentration decreases may have been the result of coprecipitation of OC with iron-containing minerals. In contrast, the phosphate-concentration increase occurred several days after the onset of anoxia and the manganese concentration was not affected by the onset of anoxia. Restoring oxic conditions resulted in a decrease in DOC, POC, and phosphate concentrations, which may indicate coprecipitation of OC with phosphate-containing minerals. The high DOC fluxes at the onset of anoxia indicate that redox oscillations may be important in OC degradation. Further, our results indicate a close coupling between OC cycling and dissolution/precipitation of iron-containing minerals in intermittently anoxic sediments.

Respiration and sequestering of organic carbon in shelf sediments of the oligotrophic northern Aegean Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 269:33-48 (2004) - doi:10.3354/meps269033 Respiration and sequestering of organic carbon in shelf sediments of the oligotrophic northern Aegean Sea H. Ståhl1, P. O. J. Hall1,*, A. Tengberg1, A. B. Josefson2, N. Streftaris3, A. Zenetos3, A. P. Karageorgis3 1Department of Chemistry, Marine Chemistry, Göteborg University, Kemivägen 10, 412 96 Göteborg, Sweden 2National Environmental Research Institute, PO Box 359, 4000 Roskilde, Denmark 3National Centre for Marine Research, PO Box 712, Mavro Lithari, Anavyssos, 19013 Attiki, Greece *Corresponding author. Email: perhall@chem.gu.se ABSTRACT: Respiration and sequestering of organic carbon was investigated in northern Aegean Sea sediments (NE Mediterranean). Benthic total carbonate (CT, also called …CO2 or dissolved inorganic carbon, DIC) fluxes and O2 uptake rates were measured in situ using a benthic lander. Dissolved organic carbon (DOC) fluxes were calculated from pore water gradients, taking into account the influence of biodiffusion/bioirrigation. Macrofaunal biomass was determined in the sediment collected by the chambers of the benthic lander. Chl a distributions were used as a tracer of high-quality sedimentary organic carbon. The measured benthic CT fluxes were positively correlated with the O2 uptake rates. The obtained average apparent respiration ratio (CT flux:O2 flux) of 0.90 ± 0.36 suggests a clear dominance of aerobic respiration in these organic carbon-poor shelf sediments. The CT efflux, the O2 uptake rate, and the DOC flux were significantly higher in spring than in fall at 2 of the stations. The Black Sea water, which enters the Aegean Sea in the study area, did not influence benthic respiration rates or organic carbon sequestering rates. A strong positive correlation between both the CT and O2 fluxes and the mean chl a concentration in surficial sediment suggests that benthic respiration, to a large extent, was controlled by the availability of labile phytodetrital organic matter. There was no influence of macrofaunal biomass (dry weight) on CT fluxes or oxygen uptake rates. The calculated benthic DOC fluxes made up 7.2 to 27% (average 14 ± 8.1%) of the CT fluxes, indicating that their contribution to the overall recycling of organic carbon in these sediments was important. The organic carbon burial efficiency ranged from 0.1 to 5.3%, but at Stn KA1 it was considerably higher (average 4%) than at the other stations (average 0.3%). Except for this station, the obtained burial efficiencies were very low compared to other sediments with similar accumulation rates. The average burial flux of organic carbon corresponded to less than 1% of the annual mean primary production (PP) for the Aegean Sea at Stn KA1, and to less than 0.1% of PP at the other stations. KEY WORDS: Northern Aegean Sea · Sediment · Organic carbon · Dissolved inorganic carbon · Dissolved organic carbon · Oxygen · Chlorophyll · Benthic lander Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 269. Online publication date: March 25, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Fluxes of dissolved organic carbon from California continental margin sediments

Fluxes of dissolved organic carbon (DOC) from marine sediments represent a poorly constrained component of the oceanic carbon cycle that may affect the concentration and composition of DOC in the ocean. Here we report the first in situ measurements of DOC fluxes from continental margin sediments (water depths ranging from 95 to 3,700 m), and compare these fluxes with measured benthic fluxes from 20 other coastal and continental margin sediments. With this combined data set data we have estimated that benthic DOC fluxes are less than ∼10% of sediment carbon oxidation rates, and that the integrated DOC flux from sediments in water depths less than 2,000 m is ∼180 Tg C/yr. These fluxes are roughly equivalent to the riverine DOC flux, and the organic carbon burial rate in marine sediments. Benthic DOC fluxes therefore represent an important net source of DOC to the oceans. We also note that: (1) benthic DOC fluxes represent a loss of organic carbon from sediments; (2) in many sediments these fluxes appear to be controlled by molecular diffusion (i.e., by pore water concentration gradients); (3) pore water DOC may be an important intermediate in sediment carbon burial and preservation. These observations therefore suggest a linkage between benthic DOC fluxes and sediment carbon preservation that may be mediated by pore water DOC concentrations and cycling. The magnitude and fate of DOC effluxing from marine sediments is thus important to understanding carbon cycles and budgets in the marine environment.

Dissolved organic carbon cycling in a subtropical seagrass-dominated lagoon

The cycling of dissolved organic matter (DOM) and its significance to ecosystem metabolism was studied over a 16 mo period in a Thalassia testudinum dominated meadow. The benthos was usually net autotrophic (annual gross primary production to respiration ratio [P:R] = 1.3) while water column respiration (R) exceeded gross primary production (annual P:R = 0.3). Net fluxes of dissolved organic carbon (DOC) from the benthos primarily occurred in the light (0 to 18 mmol C m -2 d -1 ) and from seagrass-dominated areas, suggesting that release of DOC was mainly due to seagrass exudation. Net benthic DOC fluxes measured in the light were significantly correlated (p 50 %) of the DOC released from the benthos on a daily basis. Annual estimates of bacterioplankton C demand, based on water column R (∼8 mol C m -2 yr -1 ), represented >50% of the benthic NPP (∼14 mol C m -2 yr -1 ) These measurements indicate a stronger linkage between benthic and water column processes than previously believed, and it appears that water column heterotrophic processes are largely dependent upon seagrass exudation.

Mineralization and burial of organic carbon in sediments of the southern Weddell Sea (Antarctica)

Benthic fluxes of oxygen, alkalinity (AT), total carbonate (CT or ΣCO 2) and dissolved organic carbon (DOC) were measured during sediment-water incubations at 16 stations in the southern Weddell Sea (Antarctica) with water depths between 280 and 2514 m. The total sediment oxygen consumption rates (TSOC) were in general low (1.74-3.61 mmol m −2 day −1) and more comparable to measurements in slope and deep-sea sediments at a few thousand meters water depth. The decrease of TSOC with water depth was lower than that observed in many other seas. The mean carbon to nitrogen ratio (C/N) in the solid phase of surficial sediment was 8.3. Measured benthic fluxes of alkalinity, corrected for contributions from nitrification and denitrification, were quantitatively used to correct the fluxes of total carbonate for dissolution of solid phase carbonates. The ΣCO 2 fluxes, originating from carbonate dissolution (0.1661–1.77 mmol m −2 day −1 were 2.6–71 % of the ΣC0 2 fluxes (0.984–3.73 mmol m −2 day −1) resulting from organic carbon oxidation. Measured benthic fluxes of oxygen, ΣC0 2 and nitrate were, together with estimated denitrification rates and sediment C/N ratios, used to model respiration quotients (RQ) for organic carbon oxidation and estimate composition of the organic matter undergoing degradation. Modelled RQ varied roughly between 2/3 and 1 (mean 0.87). Measured fluxes of ΣC0 2 were 1.6–3.2 times higher than integrated organic C mineralization rates (measured during closed incubations of sieved, homogenized sediment), indicating macrofaunal (plus possibly meiofaunal) respiration to be important. However, low abundances of bioirrigating benthic macrofauna and small differences in benthic fluxes of oxygen, ΣCO 2 and alkalinity found between replicate sediment cores, suggested that macrofaunal respiration was quantitatively unimportant in these sediments. The higher measured fluxes of ΣC0 2 compared to the integrated mineralization rates, were therefore most likely caused by a large fraction of the respiration occurring directly on the sediment surface. This degradation of newly deposited organic matter was not reflected in the integrated organic C mineralization rates. Also, there was no obvious effect of this surficial degradation process on the pore water distributions of ΣCO 2. Benthic mass balances of carbon revealed that benthic fluxes of DOC were 3–147% of the corrected fluxes of ΣCO 2, and the recycling efficiencies (E) were up to 35% higher if the DOC fluxes were included in the calculations of E, rather than the inorganic ΣCO 2 flux alone. The recycling efficiencies, including the benthic flux of DOC, ranged between 57 and 88% (mean 78%). Measured rates of inorganic C accumulation (for most stations <0.3 mmol C m −2 day −1 were a factor of 6–7 lower than organic C accumulation rates (0.457–1.94 mmol C m −2 day −1).

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.

The relationship between dissolved organic carbon (DOC) and fluorescence in anoxic marine porewaters: Implications for estimating benthic DOC fluxes

Fluorescence and dissolved organic carbon (DOC) measurements of porewaters from the Santa Barbara Basin, the Guaymas Basin, and the upper sections of the Nankai Trough suggest that ultraviolet fluorescence ( λ ex = 325 nm, λ em = 450 nm) may be used as a first order estimate of DOC in anoxic marine porewaters. The majority of porewater organic carbon appears to be fluorescent, while a constant ~1 mM DOC, probably the low molecular weight compounds, is not fluorescent. These data are consistent with a model in which low molecular weight compounds dissolved in porewater act as the common intermediate between labile sedimentary organic matter and remineralization or polymerization products. Fluorescence may also be used to sensitively estimate benthic DOC fluxes to the overlying water column. Results from the Santa Barbara Basin, if representative of global anoxic oceanic regions, indicate that DOC release from anoxic sediments is not a major source of oceanic DOC when compared to internal recycling rates, but may be comparable to external input or permanent removal processes.

The Role of Benthic Fluxes of Dissolved Organic Carbon in Oceanic and Sedimentary Carbon Cycling

Benthic fluxes (sediment‐water exchange) of dissolved organic carbon (DOC) represent a poorly quantified component of sedimentary and oceanic carbon cycling. In this paper we use pore water DOC data and direct DOC benthic flux measurements to begin to quantitatively examine this problem. These results suggest that marine sediments represent a significant source of DOC to the oceans, as a lower limit of the globally‐integrated benthic DOC flux is comparable in magnitude to riverine inputs of organic carbon to the oceans. Benthic fluxes of DOC also appear to be similar in magnitude to other sedimentary processes such as organic carbon oxidation (remineralization) in surface sediments and organic carbon burial with depth.