Net Dissolved Organic Carbon Production Research Articles

Partitioning the Export of Distinct Biogenic Carbon Pools in the Northeast Pacific Ocean Using a Biogeochemical Profiling Float

AbstractWe leverage observations from chemical and bio‐optical sensors mounted on a biogeochemical profiling float in the Northeast Pacific Ocean to quantify the cycling and export potential of distinct biogenic carbon pools, including particulate inorganic carbon (PIC), particulate organic carbon (POC), and dissolved organic carbon (DOC). Year‐round observations reveal complex carbon cycle dynamics among these carbon pools. Net DOC production peaked during bloom initiation, about 3 months prior to the summer peak in POC production. We validate the float estimates of DOC cycling with seasonal accumulation and removal rates derived from ship‐board DOC observations over the same period. By combining chemical and bio‐optical tracers of POC cycling, we estimate the instantaneous POC sinking flux (). The cooccurrence of DOC consumption and POC production and sinking during fall and winter resolves the regional conundrum of a persistent particle sinking flux observed by sediment traps during a season that is known to be heterotrophic. PIC production is small, and uncertainties are large. By combining float‐based estimates of instantaneous net primary production (NPP) and , we quantify a real‐time carbon export ratio ([/NPP] × 100%) for the euphotic zone. Elevated export ratios during summer are associated with an increase in the fraction of particles larger than 100 μm in size. Elevated export ratios during winter are associated with the physical redistribution of particles through seasonal deep mixing. Our study demonstrates how the combined use of multiple sensors on biogeochemical profiling floats can provide more nuanced information about upper ocean carbon cycle dynamics.

Controls on the net dissolved organic carbon production in tropical peat

<p>Soil factors such as pH and the presence of polyvalent cations can influence the net production of dissolved organic carbon (DOC). This study aimed to determine the main factors that control net DOC production. The study was conducted at Buatan Village, Siak Indrapura Regency, Riau Province, Indonesia. Soil and water sampling were done every month for a year observation, from July 2018 to June 2019. Soil sampling was carried out to determine the concentration of C-organic acids, pH, N, P, K, Cu, and soil water content (SWC). Peat water sampling was carried out using modified pore water sampling to measure DOC concentration. Groundwater level (GWL) and soil temperature were also observed. Multiple regression analysis was performed to find out the soil and environmental factors controlling the net DOC production. The results showed that the net DOC production fluctuated with seasonal changes and soil pH was a significant controlling factor (<em>P</em> = 0.035) and positively correlated (<em>P</em> = 0.040) to the net DOC production. In addition, N-mineral, PO<sub>4</sub>, and Cu were positively correlated with net DOC production (<em>P</em>-value: 0.026; 0.033; and 0.028; respectively) while C-organic acids and SWC were negatively correlated (<em>P</em>-value: 0.033; and 0.020; respectively). There was no correlation between net DOC production with GWL, soil temperature, and K concentration. This finding confirmed that pH was the main factor controlling the net DOC production and reflects DOC contribution to the solution acidity.</p>

Coupling carbon metabolism and dissolved organic carbon fluxes in benthic and pelagic coastal communities

The knowledge of the production and carbon transference dynamics between adjacent communities is essential to gain a better understanding on the role of coastal areas in the global carbon cycle. Here, we assess the carbon metabolism (through dissolved oxygen –DO– concentration) and the net dissolved organic carbon (DOC) fluxes for benthic (dominated by the seagrass Cymodocea nodosa, the rhizophytic green algae Caulerpa prolifera and unvegetated sediments) and pelagic plankton communities co–habiting the same shallow coastal area in Cadiz Bay, southern Spain. Both the seagrass and macroalgae meadows were highly autotrophic, but the carbon metabolism of C. prolifera community shifted seasonally from net autotrophic to net heterotrophic. Unvegetated benthic communities were slightly heterotrophic throughout the year, while plankton community was net autotrophic. This study reveals how the different components of the ecosystem (i.e. macrophyte, sediment or plankton community) may have a seasonal variability in the contribution to the NCP in shallow coastal areas. Moreover, our results suggest that pelagic communities can be essential to maintain autotrophy of coastal system during periods of low benthic productivity. Regarding DOC, benthic communities were net DOC producers, undergoing a marked seasonality with maximum net DOC production during summer. This indicated an extra carbon assimilation not usually contemplated in productivity studies, which entails an underestimation of gross production in these communities. A strong relationship between net DOC flux and net community production (NCP) was found for the three benthic communities. The plankton community was a net DOC consumer indicating a strong coupling between the benthic and pelagic compartments in the bay. When up–scaling our results to the whole bay, the NCP estimated for the benthos was 8800 ± 3770 Tons C y−1, and the net DOC flux was 2465 ± 830 Tons C y−1, which were mainly due to vegetated communities, indicating the importance of coastal vegetated benthic ones in the carbon metabolism and DOC fluxes.

Production of tyrosine-like fluorescence and labile chromophoric dissolved organic matter (DOM) and low surface accumulation of low molecular weight-dominated DOM in a productive Antarctic sea

The Antarctic seas play critical roles in global carbon cycling. Yet, little is known about the dissolved organic matter (DOM) characteristics and the dynamics there. Here, we conducted an extensive study on the seawater DOM in the Amundsen Sea in the Pacific sector of the Southern Ocean. We found that low molecular weight fractions quantitatively dominated the DOM composition at the surface of the highly productive Amundsen Sea Polynya with the relative abundance reaching up to ~89%. Moreover, CDOM and tyrosine-like fluorescence generation were observed, with the average values of~2.4–2.6 m−1 (a254) and ~0.3 RU, respectively. While there is a net positive accumulation of dissolved organic carbon (DOC) at the ocean's surface, the net accumulation was negative for the chromophoric DOM (CDOM), which suggests a labile nature for the freshly produced CDOM. The estimated net DOC production ratio was only ~9 ± 6%, which was less than the global level (~17%). This finding signified a low surface accumulation of DOM in the austral summer, which is potentially explained by its nonlimiting nutrients, photo- and/or bio-labile nature of produced DOM, and long water residence time.

Soil freeze–thaw with root litter alters N transformations during the dormant season in soils under two temperate forests in northern Japan

Extreme soil freeze–thaw regimes caused by winter climate change can alter nitrogen (N) transformations through plant root mortality. In this study, we conducted a snow removal experiment to simulate extreme soil freeze–thaw to understand the effect of soil freeze–thaw on N transformations in soils under two northern temperate forests (oak and larch) in northern Japan. We also investigated the effect of root input on microbial biomass and transformations of dissolved organic carbon (DOC) and N by experimentally adding root litter to obtain mechanistic insight. Snow removal significantly reduced net nitrification and N mineralization, while it increased net ammonification in both soils. Root litter addition provided contrasting effects of vegetation type on net DOC change; it significantly decreased net DOC production in the oak soil, while it was increased in the larch soil. The contrasting responses of DOC change with vegetation were possibly related to specific microbial composition and physiology of the vegetation. Further, root addition eliminated the differences in net N mineralization and microbial biomass between the two vegetation types in the snow removal plots. Our results indicated that soil freeze–thaw could reduce the differences in vegetation type on soil N dynamics through root litter input.

Dynamics and production of dissolved organic carbon in a large continental shelf system under the influence of both river plume and coastal upwelling

AbstractWe examined the dynamics and production of dissolved organic carbon (DOC) on a large continental shelf in the northern South China Sea, which is largely shaped by a river plume and coastal upwelling, based on a cruise in summer 2008. The plume water extended from the mouth of the Pearl River estuary to the middle shelf and was characterized by high DOC concentrations, while the upwelled water occupying the nearshore area featured low DOC concentrations. Biological production of DOC was observed in both the river plume and the coastal upwelling zones with different behavior between regions. The system appeared to be autotrophic in terms of DOC throughout the plume, while in the upwelling circulation, the metabolism of DOC was mixed trophic. Nevertheless, the integrated net DOC production rate of 11.5 ± 6.9 mmol C m−2 d−1 in the upwelling zone was comparable to that in the plume (7.1 ± 7.0 mmol C m−2 d−1). The net DOC production correlated strongly with net consumption of dissolved inorganic carbon (DIC) and inorganic nutrients, suggesting that the net DOC production was highly coupled to net community production (NCP) in both the plume and upwelling zones. Both regimes had similar DOC/NCP partitioning, with 19–27% of NCP in the plume and 24–26% of NCP in the upwelling zones converted to DOC. A positive correlation was also found between particulate organic carbon (POC) and net DIC consumption, with higher POC production in the upwelling zones where large phytoplankton prevailed. Most NCP removal occurred through POC sinking and/or the diffusion and horizontal transport of DOC.

Production of dissolved organic carbon by Arctic plankton communities: Responses to elevated carbon dioxide and the availability of light and nutrients

The extracellular release of dissolved organic carbon (DOC) by phytoplankton is a potentially important source of labile organic carbon for bacterioplankton in pelagic ecosystems. In the context of increasing seawater partial pressure of CO2 (pCO2), via the oceanic absorption of elevated atmospheric CO2 (ocean acidification), several previous studies have reported increases to the relative amount of carbon fixed into particulates, via primary production (PP), and dissolved phases (DOC). During the summer of 2012 we measured DOC production by phytoplankton communities in the Nordic seas of the Arctic Ocean (Greenland, Norwegian and Barents Sea) from both in situ sampling and during three bioassay experiments where pCO2 levels (targets ~550µatm, ~750µatm, ~1000µatm) were elevated relative to ambient conditions. Measurements of DOC production and PP came from 24h incubations and therefore represent net DOC production rates, where an unknown portion of the DOC released has potentially been utilised by heterotrophic organisms. Production of DOC (net pDOC) by in situ communities varied from 0.09 to 0.64mmolCm−3d−1 (average 0.25mmolCm−3d−1), with comparative rates in two of the experimental bioassays (0.04–1.23mmolCm−3d−1) and increasing dramatically in the third (up to 5.88mmolCm−3d−1). When expressed as a fraction of total carbon fixation (i.e., PP plus pDOC), percentage extracellular release (PER) was 14% on average (range 2–46%) for in situ measurements, with PER in the three bioassays having a very similar range (2–50%). A marked increase in pDOC (and PER) was only observed in one of the bioassays where nutrient levels (nitrate, silicic acid) dropped dramatically relative to starting (ambient) concentrations; no pCO2 treatment effect on pDOC (or PER) was evident across the three experiments. Examination of in situ net pDOC (and PER) found significant correlations with decreasing silicic acid and increasing euphotic zone depth, indicating that nutrient and light availability were strong drivers of the partitioning of primary production between particulate and dissolved phases. Furthermore, the third bioassay experiment had relatively high levels of diatom biomass as well as a strong response to nitrate and silicic acid depletion, and we suggest that nutrient starved or light limited diatom communities may be strong producers of DOC in Arctic ecosystems.

Organic matter cycling across the sulfate-methane transition zone of the Santa Barbara Basin, California Borderland

Consumption of sulfate (SO42−) in the sulfate-methane transition zone (SMTZ) has often been considered to be due solely to anaerobic oxidation of methane (AOM). However, recent studies show SO42− fluxes into the SMTZ that exceed methane (CH4) fluxes, thereby challenging this conceptual model. Co-occurrence of organoclastic SO42− reduction (oSR) with AOM in the SMTZ has been hypothesized to be the cause for this flux imbalance, but conclusive evidence is lacking. To address this knowledge gap, we investigated organic matter cycling in the SMTZ of the organic-rich sediments of the Santa Barbara Basin, California Borderland, and examined the occurrence of oSR within this zone using bulk solute profiles and Δ14C and δ13C values of selected carbon pools. We also tested the hypothesis that the SMTZ acts as an oxidation front not just for CH4, but also for dissolved organic carbon (DOC) that is produced below the SMTZ and migrates upward. Mass balance calculations for the SMTZ based on reaction stoichiometry and Δ14C and δ13C values of associated carbon fluxes indicate that ∼35–45% of total SO42− reduction in the SMTZ occurs via oSR, with the remainder attributable to AOM. The δ13C value of net DOC production is distinct from that of the fraction of bulk POC undergoing degradation, suggesting that pore-water DOC represents a compositionally unique slice of the metabolizable POC pool. DOC diffusing upward at 450cm is virtually free of 14C and contain low levels of short-chain organic acids. Radiocarbon mass balance shows that >30% of this pre-aged, and presumably refractory, DOC is removed from the pore waters within or immediately below the SMTZ. Although the SMTZ does not appear to be a major net DOC oxidation front, these results show that DOC dynamics provide unique insights into organic matter processing in these subsurface sediments.

Transformations in DOC along a source to sea continuum; impacts of photo-degradation, biological processes and mixing

Peatlands export significant amounts of dissolved organic carbon (DOC) to freshwaters, but the quantity of DOC reaching marine environments is typically less than the input to the fluvial system due to processing within the water column. Key removal processes include photo-chemical degradation, and heterotrophic bacterial respiration. In this study we examined these processes using 14C-labelled DOC to quantify the extent of DOC breakdown and to determine its fate following irradiation under controlled laboratory conditions. We examined the influence of microbial processes occurring within the water column, the potential role of stream-bed biofilms, and the possible modifying effects of downstream mixing, as DOC in water from the peatland encounters runoff from upland mineral soils (“Mountain”), nutrient-rich runoff from agricultural soils, and seawater in an estuary. Our results demonstrated conservative mixing of DOC from Peatland and Mountain waters but interactive effects when Peatland water was mixed with Agricultural and Estuary waters and exposed to solar radiation. The mixing of Peatland and Agricultural waters led to net DOC production, suggesting that DOC was only partially degraded by solar radiation and that the products of this might have fuelled autotrophic microbial growth in the samples. The mixing of Peatland water with saline estuary water resulted in net DOC loss following irradiation, suggesting a role for sunlight in enhancing the flocculation of DOC to particulate organic carbon (POC) in saline environments.

Effect of environmental factors on production and bioavailability of dissolved organic carbon from substrates available in a constructed and reference fens in the Athabasca oil sands development region

Fen peatlands cover a large proportion of the landscape in boreal Alberta and thus these ecosystems are likely to be constructed as part of mine closure following oil sands mining. Peatlands are large sources of dissolved organic carbon (DOC); however, little is known about DOC dynamics in constructed peatlands. We investigated DOC production and its bioavailability from substrates collected from a constructed fen and natural fens (reference ecosystems) in the Athabasca oil sands region near Fort McMurray, Alberta. Dominant vegetation and other substrates used in fen construction, like peat, petroleum coke, tailings sand and upland soil, were collected for the study. The effect of substrate type, salinity and temperature on DOC production and bioavailability were analyzed in a laboratory incubation study. Sedges and trees produced the largest amount of DOC, followed by peat and moss in all the fens, whereas the substrates from the constructed fen such as petroleum coke, tailings sand and upland soil produced very little DOC per unit dry mass. In addition, temperature and salinity were also significant explanatory variables for both initial (30 day) and final (60 day) net DOC production rates. It was also observed that greater production of DOC resulted in lower pH and higher electrical conductivity (EC) in incubation jar water. In addition, the quality of DOC depended upon the type of substrate. Specific ultraviolet absorbance at 254nm wavelength (SUVA254) indicated that sedges produced DOC having less aromatic humic substances than peat, tailings sand, upland soil and petroleum coke. Substrate, salinity and temperature at which DOC was produced, and interactions among these factors were all found to significantly explain variation in DOC bioavailability. Aromatic content of DOC produced, based on SUVA254, had an effect on its bioavailability. As various substrates produced different amounts and quality of DOC, and since construction material like petroleum coke and tailings sand produce very little DOC, it is likely that establishment of the vegetation community and its composition will play an important role in DOC dynamics at the constructed fen in the future.

Organic and inorganic carbon dynamics in a karst aquifer: Santa Fe River Sink‐Rise system, north Florida, USA

AbstractSpatiotemporal variations in dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), major ions concentrations and other geochemical parameters including stable carbon isotopes of DIC (δ13CDIC), were measured in surface water and deep and shallow well water samples of the Santa Fe River Sink‐Rise eogenetic karst system, north Florida, USA. Three end‐member water sources were identified: one DOC‐rich/DIC‐poor/δ13CDIC‐depleted, one DOC‐poor/DIC‐rich/δ13CDIC‐enriched, and one enriched in major ions. Given their spatiotemporal distributions, they were presumed to represent soil water, upper aquifer groundwater, and deep aquifer water sources, respectively. Using assumed ratios of Na+, Cl, and SO42− for each end‐member, a mixing model calculated the contribution of each water source to each sample. Then, chemical effects of biogeochemical reactions were calculated as the difference between those predicted by the mixing model and measured species concentrations. In general, carbonate mineral dissolution occurred throughout the Sink‐Rise system, surface waters were net autotrophic and the subsurface was in metabolic balance, i.e., no net DOC or DIC production or consumption. However, there was evidence for chemolithoautotrophy, perhaps by hydrogen oxidizing microbes, at some deep aquifer sites. Mineralization of this autochthonous natural dissolved organic matter (NDOM) led to localized carbonate dissolution as did surface water‐derived NDOM supplied to shallow well sites during the highest flow periods. This study demonstrates linkages between hydrology, abiotic and microbial processes and carbon dynamics and has important implications for groundwater quality, karst morphologic evolution, and hydrogeologic projects such as aquifer storage and recovery in karst systems.

Seasonal dynamics and net production of dissolved organic carbon in an oligotrophic coastal environment

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 456:7-19 (2012) - DOI: https://doi.org/10.3354/meps09677 Seasonal dynamics and net production of dissolved organic carbon in an oligotrophic coastal environment Gemma Vila-Reixach1,*, Josep M. Gasol2, Clara Cardelús2, Montserrat Vidal1 1Departament d’Ecologia, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Catalunya, Spain 2Departament de Biologia Marina i Oceanografia, Institut de Ciències del Mar-CSIC, 08003 Barcelona, Catalunya, Spain *Email: gemmavila@ub.edu ABSTRACT: To understand dissolved organic carbon (DOC) seasonal dynamics in a coastal oligotrophic site in the north-western Mediterranean Sea, we monitored DOC concentrations monthly over 3 yr, together with the meteorological data and the food-web-related biological processes involved in DOC dynamics. Additional DOC samples were taken in several inshore−offshore transects along the Catalan coast. We found DOC concentrations of ~60 µmol C l−1 in winter, with increasing values through the summer and autumn and reaching 100 to 120 µmol C l−1 in November. There was high inter-annual variability in this summer DOC accumulation, with values of 36, 69 and 13 µmol C l−1 for 2006, 2007 and 2008, respectively. The analysis of the microbial food-web processes involved in the DOC balance did not reveal the causes of this accumulation, since the only occasion on which we observed net DOC production (0.3 ± 1 µmol C l−1 d−1 on average) was in 2007, and the negative DOC balance of 2006 and 2008 did not prevent DOC accumulating. The DOC accumulation episodes coincided with low rates of water renewal (average 0.037 ± 0.021 d−1 from May to October) compared with those of winter to early spring (average 0.11 ± 0.048 d−1 from November to April). Indeed, the amount of DOC accumulated each year was inversely correlated with the average summer rainfall. We hypothesize that decreased DOC turn-over due to photochemical or biological processes—mostly active during the summer—and low water renewal rate combine to determine seasonal DOC accumulation and influence its inter-annual variability. KEY WORDS: Coastal DOC · Seasonality · Primary production · Bacterial metabolism · Physical transport Full text in pdf format PreviousNextCite this article as: Vila-Reixach G, Gasol JM, Cardelús C, Vidal M (2012) Seasonal dynamics and net production of dissolved organic carbon in an oligotrophic coastal environment. Mar Ecol Prog Ser 456:7-19. https://doi.org/10.3354/meps09677 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 456. Online publication date: June 07, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Dissolved organic carbon production and runoff quality following peatland extraction and restoration

We examined dissolved organic carbon (DOC) production and quality in a cutover (extracted) and restored peatland in eastern Québec through a combined laboratory DOC production study and an accompanying field DOC runoff quality study. Both temperature and substrate were significant for explaining variability in net DOC production rate in both short (5 days) and longer term (25 day) laboratory incubations. Moss ( Sphagnum spp. and Polytricum spp.) produced the least amount of DOC, with initial release rates of 0.02–0.35 mg DOC g −1 d −1. Shrubs, and to a lesser extent herbaceous vegetation, showed an initial high release of DOC followed by a decrease. Peat from the restored site had significantly higher longer-term DOC production rates than the cutover site. Humic acid (HA) was more likely to be produced by shrub and herbaceous plant material than by peat, mosses and straw. The hydrophilic (HPI) fraction of the DOC increased at higher temperatures. Despite differences in the surface cover of available substrate (e.g. vascular vegetation, moss, and straw), there was no difference in the quality of DOC exported from the cutover and restored sites. However, hydrological and biogeochemical controls were apparent in temporal variability of DOC quality in discharge. DOC exported during snowmelt and most storm events was high in HPI with little contribution from HA. Since contact time between water and soil is limited during these high flow periods, larger more hydrophobic HA molecules are less likely to be mobilized. Higher export of HA in summer compared to spring and autumn suggests that polycondensation and increased vascular plant productivity are important for controlling seasonal patterns of DOC export quality. As various substrates produced different amounts and quality of DOC, it is likely that the DOC quality exported from the restored site will continue to change as the new vegetation community develops. DOC production rates and DOC quality should be considered when developing a plan to control DOC runoff/export from managed peatlands.

In situ dissolved organic carbon (DOC) release by submerged macrophyte–epiphyte communities in southern Quebec lakes

We studied the in situ release of dissolved organic carbon (DOC) by growing a submerged freshwater macrophyte–epiphyte complex. Incubations with benthic chambers in five southeastern Quebec lakes show a net DOC production for different communities of Myriophyllum spicatum and Potamogeton spp. Daytime DOC release rates range from undetectable to 9.7 mg C·m–2·h–1. Although DOC release was restricted to daylight hours and thus suggestive of a photosynthesis-related process, we found no strong link between DOC release rates and concurrent illumination or temperature. We found no difference in DOC release rates between the three main colonizing species of the studied region. The overall mean DOC release rate was 4.57 mg C·m–2·h–1(standard deviation (SD), ±0.65) or 56 µg C·g dry weight–1·h–1(SD, ±8), which we suggest can be used for extrapolations at the lake scale.

Increased temperature sensitivity of net DOC production from ombrotrophic peat due to water table draw‐down

AbstractThe production and release of dissolved organic carbon (DOC) from peat soils is thought to be sensitive to changes in climate, specifically changes in temperature and rainfall. However, little is known about the actual rates of net DOC production in response to temperature and water table draw‐down, particularly in comparison to carbon dioxide (CO2) fluxes. To explore these relationships, we carried out a laboratory experiment on intact peat soil cores under controlled temperature and water table conditions to determine the impact and interaction of each of these climatic factors on net DOC production. We found a significant interaction (P< 0.001) between temperature, water table draw‐down and net DOC production across the whole soil core (0 to −55 cm depth). This corresponded to an increase in theQ10(i.e. rise in the rate of net DOC production over a 10 °C range) from 1.84 under high water tables and anaerobic conditions to 3.53 under water table draw‐down and aerobic conditions between −10 and − 40 cm depth. However, increases in net DOC production were only seen after water tables recovered to the surface as secondary changes in soil water chemistry driven by sulphur redox reactions decreased DOC solubility, and therefore DOC concentrations, during periods of water table draw‐down. Furthermore, net microbial consumption of DOC was also apparent at − 1 cm depth and was an additional cause of declining DOC concentrations during dry periods. Therefore, although increased temperature and decreased rainfall could have a significant effect on net DOC release from peatlands, these climatic effects could be masked by other factors controlling the biological consumption of DOC in addition to soil water chemistry and DOC solubility. These findings highlight both the sensitivity of DOC release from ombrotrophic peat to episodic changes in water table draw‐down, and the need to disentangle complex and interacting controls on DOC dynamics to fully understand the impact of environmental change on this system.

Effect of water table drawdown on peatland dissolved organic carbon export and dynamics

AbstractPeatlands play an important role in the global carbon cycle, and loss of dissolved organic carbon (DOC) has been shown to be important for peatland carbon budgets. The objective of this study was to determine how net production and export of DOC from a northern peatland may be affected by disturbance such as drainage and climate change.The study was conducted at a poor fen containing several pool–ridge complexes: (1) control site–no water table manipulation; (2) experimental site–monitored for one season in a natural state and then subjected to a water table drawdown for 3 years; (3) drained site–subjected to a water table drawdown 9 years prior to monitoring. The DOC concentration was measured in pore water along a microtopographic gradient at each site (hummock, lawn and hollow), in standing water in pools, and in discharge from the experimental and drained sites. The initial water table drawdown released ∼3 g of carbon per square metre in the form of DOC, providing a large pulse of DOC to downstream ecosystems. This value, however, represents only 1–9% of ecosystem respiration at this site. Seasonal losses of DOC following drainage were 8–11 g of carbon per square metre, representing ∼17% of the total carbon exchange at the experimental study site. Immediately following water table drawdown, DOC concentrations were elevated in pore water and open water pools. In subsequent seasons, DOC concentration in the pool declined, but remained higher than the control site even 11 years after water‐table drawdown. This suggests continued elevated net DOC production under lower water table conditions likely related to an increase in vegetation biomass and larger water table fluctuations at the experimental and drained sites. However, the increase in concentration was limited to initially wet microforms (lawns and hollows) reflecting differences in vegetation community changes, water table and soil subsidence along the microtopographic gradient. Copyright © 2008 John Wiley & Sons, Ltd and Her Majesty the Queen in right of Canada.

Determining net dissolved organic carbon production in the hydrographically complex western Arctic Ocean

Each spring and summer, the Chukchi Sea in the western Arctic Ocean experiences a short but intense phytoplankton bloom that is controlled by sea ice cover, nutrient availability, and river runoff. During the bloom, dissolved inorganic carbon (DIC) concentrations are drawn down in the mixed layer (shelf, ~Δ100 µmol kg−1; shelf‐break, ~Δ50 µmol kg−1; basin, <Δ5 µmol kg−1) and converted to particulate organic carbon (POC) and dissolved organic carbon (DOC). We use δ18O and salinity to estimate DOC inputs by river and sea ice melt, allowing the marine portion of the DOC pool to be compared between seasons and to facilitate estimates of net DOC production in the region. We estimate that ~10% of the DIC consumed during net community production is converted to DOC. The remaining carbon deficit (90% of DIC drawdown) is most likely exported from the mixed layer as POC. In 2002 a carbon (C) mass balance was developed for the Chukchi Sea that showed that rates of vertical export were highest over the Chukchi Shelf near Point Barrow (64 mmol C m− d−1), but more commonly ranged from 10 to 35 mmol C m− d−1 over the remainder of the shelf. At the shelf‐break and in the Canada Basin export production was <10 mmol C m− d−1. DOC plays a relatively small role as a carbon sink in the system, but quantifying its contributions provides more robust estimates of export production in the Arctic.

Factors influencing organic carbon recycling and burial in Skagerrak sediments

Different factors influencing recycling and burial rates of organic carbon (OC) were investigated in the continental margin sediments of the Skagerrak (NE North Sea). Two different areas, one in the southern and one in the northeastern part of the Skagerrak were visited shortly after a spring bloom (March 1999) and in late summer (August 2000). Results suggested that: (1) Organic carbon oxidation rates (C ox ) (2.2-18 mmol C m -2 d -1 ) were generally larger than the O 2 uptake rates (1.9-25 mmol m -2 d -1 ). Both rates were measured in situ using a benthic lander. A mean apparent respiration ratio (C ox :O 2coor ) of 1.3 ± 0.5 was found, indicating some long-term burial of reduced inorganic substances in these sediments. Measured O 2 fluxes increased linearly with increasing C ox rates during the late summer cruise but not on the early spring cruise, indicating a temporal uncoupling of anaerobic mineralization and reoxidation of reduced substances. (2) Dissolved organic carbon (DOC) fluxes (0.2-1.0 mmol C m -2 d -1 ) constituted 3-10% of the C ox rates and were positively correlated with the latter, implying that net DOC production rates were proportional to the overall sediment OC remineralization rates. (3) Chlorophyll a (Chl-a) concentrations in the sediment were significantly higher in early spring compared to late summer. The measured C ox rates, but not O 2 fluxes, showed a strong positive correlation with the Chl-a inventories in the top 3 cm of the sediment. (4) Although no relationship was found between the benthic fluxes and the macrofaunal biomass in the chambers, total in situ measured dissolved inorganic carbon (C T ) fluxes were 1-5.4 times higher than diffusive mediated C T fluxes, indicating that macrofauna have a significant impact on benthic exchange rates of OC remineralization products in Skagerrak sediments. (5) OC burial fluxes were generally higher in northeastern Skagerrak than in the southern part. The same pattern was observed for burial efficiencies, with annual means of ∼62% and ∼43% for the two areas respectively. (6) On a basin-wide scale, there was a significant positive linear correlation between the burial efficiencies and sediment accumulation rates. (7) The calculated particulate organic carbon (POC) deposition, from benthic flux and burial measurements, was only 24-78% of the sediment trap measured POC deposition, indicating a strong near-bottom lateral transport and resuspension of POC. (8) A larger fraction of the laterally advected material of lower quality seemed to seetle in the northeastern Skagerrak rather than in the southern Skagerrak. (9) Skagerrak sediments, especially in the northeastern part, act as an efficient net sink for organic carbon, even in a global continental margin context.

Production and bioavailability of autochthonous dissolved organic carbon: effects of mesozooplankton

A phytoplankton bloom and decay sequence was created in 2 laboratory containers and mesozooplankton was added to one container before the peak of algal biomass. Each day for 22 d, the net production of autochthonous dissolved organic carbon (DOC) was measured and on 5 occasions the degradation kinetics and the total pool of biodegradable DOC (BDOC) were assayed in experi- ments lasting 230 d. Net accumulation of new DOC was 235 and 280 µM in the containers with and without zooplankton, respectively. The best description of microbial DOC degradation was a 2-pool model and 1st order exponential decay. Without mesozooplankton present, the degradation experi- ments showed accumulation of a large pool of labile BDOC characterised by decay coefficients >0.2 d -1 . The least labile pools in the 2 containers had similar coefficients (average 0.02 d -1 ). The amount of newly produced recalcitrant DOC (RDOC) accounted for about 12% of new DOC. The differences observed with respect to degradation kinetics and net DOC production are explained by food web interactions and nutrient limitation. The presence of mesozooplankton resulted in high bacterial production keeping labile BDOC at low concentrations. In the container without meso- zooplankton, the bacterial uptake capacity was reduced, probably by a combination of protist grazing and nutrient limitation. Consequently, about 75 µM BDOC with a half-life of less than 3 d accumu- lated during the experiment. Mineralisation of the accumulated dissolved organic matter (DOM) during microbial degradation in a nutrient replete environment was measured as the decrease in DOC and net mineralisation/immobilisation of inorganic N and P. The mineralisation of DOC was accompanied by low mineralisation of N and P and even immobilisation of phosphate during degra- dation of DOM produced in the container with mesozooplankton present. Bacterial production of DON and DOP is believed to result in a recalcitrant DOM pool enriched in N and P, and the activity of mesozooplankton seems to enhance this scenario.

Plankton metabolism and dissolved organic carbon use in the Bay of Palma, NW Mediterranean Sea

A study was conducted to assess the annual variability in planktonic metabolism and dissolved organic carbon (DOC) utilization in an oligotrophic Mediterranean Bay (Bay of Palma, Spain) and to test the role of elevated DOC concentrations in driving planktonic metabolism off balance. We examined, at monthly intervals over 17 mo, gross primary production (GPP), community respiration (R), net community production (NCP), DOC concentration, total chlorophyll a (chl a) concentration, and, for a smaller subset of 11 to 14 mo, net DOC fluxes, bacterial abundance (BA) and bacterial respiration (BR). The community was net heterotrophic in autumn, winter and the first summer studied, and shifted to net autotrophic towards the end of the study period. This period of sustained autotrophy was an anomalous period characterized by frequent storms that stimulated autotrophic processes in the bay, leading to the development of a bloom of the cyanobacteria Synechoccocus. Use of DOC was consistent with the trophic state of the system, as DOC consumption was observed during periods when the system was net heterotrophic and there was a net DOC production when the system shifted to autotrophic. Bacterial respiration accounted for, on average, 51.76% of R and increased as the percent of cells with high DNA content increased. The planktonic community was net heterotrophic on an annual basis, suggesting that the system imports DOC. In particular, the organic carbon import may derive from the excess production of the underlying Posidonia oceanica meadow.