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See detailParticulate carbon export during a bloom of Emiliania huxleyi in the Northwest European continental margin (northern Bay of Biscay)
Schmidt, Sabine; Harlay, Jérôme ULg; Borges, Alberto ULg et al

Poster (2011, April 03)

Coccolithophores, the dominant pelagic calcifiers in the oceans, play a key role in the marine carbon cycle through calcification, primary production and carbon export, the main drivers of the biological ... [more ▼]

Coccolithophores, the dominant pelagic calcifiers in the oceans, play a key role in the marine carbon cycle through calcification, primary production and carbon export, the main drivers of the biological CO2 pump. Massive blooms of Emiliania huxleyi are observed each year at the continental margin of the Bay of Biscay. The BG02/11 cruise (RV Belgica), supported by near-real time remote sensing data, was conducted in early May 2002, along a transect on the outer shelf of the Northern Bay of Biscay between the La Chapelle Bank (southern region of the transect) and Goban Spur (northern region of the transect) (47.0°-50.5°N, 5.0°-11.0°W). Biogeochemical variables including primary production, calcification, partial pressure of CO2 (pCO2), chlorophyll-a (Chl-a), particle load, particulate organic and inorganic carbon (POC, PIC), Th-234 were measured in surface waters to assess particle dynamics, and carbon export in relation to the development of a coccolithophore bloom. We observed a marked northward decrease in water irradiance, Chl-a concentration and calcification rates: the bloom exhibited lower values and may have been less well developed in the Goban Spur area. There was also a large northward decrease in particulate 234Th settling fluxes along the transect from La Chapelle Bank to Goban Spur. The export fluxes of POC and PIC from the top 80 m, determined using the ratios of POC and PIC to Th-234 of particles, ranged from 81 to 323 mgC m-2 d-1 and from 30 to 128 mgC m-2 d-1, respectively. The highest fluxes were observed in waters presenting a well-developed coccolithophore bloom, as shown by high reflectance of surface waters. Despite the high calcification rates at the southernmost stations, surface waters were a net sink of atmospheric CO2 during this cruise. These results tend to demonstrate the enhancement of coccolithophore blooms on the efficiency of the surface community to export carbon to deep ocean. However, improvements in the estimation of POC and PIC export during coccolithophore blooms are needed to not only understand the present calcification to primary production (C:P) ratio, but also to help understand future sequestration of organic and inorganic carbon to the deep ocean. [less ▲]

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See detailDistribution and composition of organic carbon in the Tana River Basin, (Kenya)
Tamooh, F.; van den Meersche, K.; Borges, Alberto ULg et al

Conference (2011, February 13)

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See detailBiogeochemistry and carbon mass balance of a coccolithophore bloom in the northern Bay of Biscay (June 2006)
Harlay, Jérôme ULg; Chou, Lei; De Bodt, Caroline et al

in Deep-Sea Research Part I, Oceanographic Research Papers (2011), 58(2), 111-127

Primary production (PP), calcification (CAL), bacterial production (BP) and dark community respiration (DCR) were measured along with a set of various biogeochemical variables, in early June 2006, at ... [more ▼]

Primary production (PP), calcification (CAL), bacterial production (BP) and dark community respiration (DCR) were measured along with a set of various biogeochemical variables, in early June 2006, at several stations at the shelf break of the northern Bay of Biscay. The cruise was carried out after the main spring diatom bloom that, based on the analysis of a time-series of remotely sensed chlorophyll-a (Chl-a), peaked in mid-April. Remotely sensed sea surface temperature (SST) indicated the occurrence of enhanced vertical mixing (due to internal tides) at the continental slope, while adjacent waters on the continental shelf were stratified, as confirmed by vertical profiles of temperature acquired during the cruise. The surface layer of the stratified water masses (on the continental shelf) was depleted of inorganic nutrients. Dissolved silicate (DSi) levels probably did not allow significant diatom development. We hypothesize that mixing at the continental slope allowed the injection of inorganic nutrients that triggered the blooming of mixed phytoplanktonic communities dominated by coccolithophores (Emiliania huxleyi) that were favoured with regards to diatoms due to the low DSi levels. Based on this conceptual frame, we used an indicator of vertical stratification to classify the different sampled stations, and to reconstruct the possible evolution of the bloom from the onset at the continental slope (triggered by vertical mixing) through its development as the water mass was advected on-shelf and stratified. We also established a carbon mass balance at each station by integrating in the photic layer PP, CAL and DCR. This allowed computation at each station of the contribution of PP, CAL and DCR to CO2 fluxes in the photic layer, and how they changed from one station to another along the sequence of bloom development (as traced by the stratification indicator). This also showed a shift from net autotrophy to net heterotrophy as the water mass aged (stratified), and suggested the importance of extracellular production of carbon to sustain the bacterial demand in the photic and aphotic layers. [less ▲]

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See detailFirst assessment of the biogeochemistry of the Congo River and tributaries
Darchambeau, François ULg; Bouillon, S.; Wabakanghanzi, J. N. et al

Poster (2011)

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See detailOverview of CO2 dynamics within sea ice
Delille, Bruno ULg; Geilfus, Nicolas-Xavier ULg; Vancoppenolle, M. et al

Conference (2011)

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See detail5.04 - Carbon Dioxide and Methane Dynamics in Estuaries
Borges, Alberto ULg; Abril, Gwenaël

in Wolanski, Eric; McLusky, Donald (Eds.) Treatise on Estuarine and Coastal Science, Volume 5: Biogeochemistry (2011)

Estuaries profoundly transform the large amounts of carbon delivered from rivers before their transfer to the adjacent coastal zone. As a consequence of the complex biogeochemical reworking of ... [more ▼]

Estuaries profoundly transform the large amounts of carbon delivered from rivers before their transfer to the adjacent coastal zone. As a consequence of the complex biogeochemical reworking of allochthonous carbon in the sediments and the water column, CO2 and CH4 are emitted into the atmosphere. We attempt to synthesize available knowledge on biogeochemical cycling of CO2 and CH4 in estuarine environments, with a particular emphasis on the exchange with the atmosphere. Unlike CH4, the global emission of CO2 to the atmosphere from estuaries is significant compared to other components of the global carbon cycle [less ▲]

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See detailBiogeochemical response of Emiliania huxleyi (PML B92/11) to elevated CO2 and temperature under phosphorous limitation: A chemostat study
Borchard; Borges, Alberto ULg; Händel, Nicole et al

in Journal of Experimental Marine Biology & Ecology (2011), 410

The present study investigates the combined effect of phosphorous limitation, elevated partial pressure of CO2 (pCO2) and temperature on a calcifying strain of Emiliania huxleyi (PML B92/11) by means of a ... [more ▼]

The present study investigates the combined effect of phosphorous limitation, elevated partial pressure of CO2 (pCO2) and temperature on a calcifying strain of Emiliania huxleyi (PML B92/11) by means of a fully controlled continuous culture facility. Two levels of phosphorous limitation were consecutively applied by renewal of culture media (N:P = 26) at dilution rates (D) of 0.3 d− 1 and 0.1 d− 1. CO2 and temperature conditions were 300, 550 and 900 μatm pCO2 at 14 °C and 900 μatm pCO2 at 18 °C. In general, the steady state cell density and particulate organic carbon (POC) production increased with pCO2, yielding significantly higher concentrations in cultures grown at 900 μatm pCO2 compared to 300 and 550 μatm pCO2. At 900 μatm pCO2, elevation of temperature as expected for a greenhouse ocean, further increased cell densities and POC concentrations. In contrast to POC concentration, C-quotas (pmol C cell− 1) were similar at D = 0.3 d− 1 in all cultures. At D = 0.1 d− 1, a reduction of C-quotas by up to 15% was observed in the 900 μatm pCO2 at 18 °C culture. As a result of growth rate reduction, POC:PON:POP ratios deviated strongly from the Redfield ratio, primarily due to an increase in POC. Ratios of particulate inorganic and organic carbon (PIC:POC) ranged from 0.14 to 0.18 at D = 0.3 d− 1, and from 0.11 to 0.17 at D = 0.1 d− 1, with variations primarily induced by the changes in POC. At D = 0.1 d− 1, cell volume was reduced by up to 22% in cultures grown at 900 μatm pCO2. Our results indicate that changes in pCO2, temperature and phosphorus supply affect cell density, POC concentration and size of E. huxleyi (PML B92/11) to varying degrees, and will likely impact bloom development as well as biogeochemical cycling in a greenhouse ocean. [less ▲]

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See detailDiffusive methane emissions to the atmosphere from Lake Kivu (Eastern Africa)
Borges, Alberto ULg; Abril, Gwenaël; Delille, Bruno ULg et al

in Journal of Geophysical Research. Biogeosciences (2011), 116(G03032),

We report a data-set of methane (CH4) concentrations in the surface waters of Lake Kivu obtained during four cruises (March 2007, September 2007, June 2008, April 2009) covering the two main seasons ... [more ▼]

We report a data-set of methane (CH4) concentrations in the surface waters of Lake Kivu obtained during four cruises (March 2007, September 2007, June 2008, April 2009) covering the two main seasons, rainy (October to May) and dry (June to September). Spatial gradients of CH4 concentrations were modest in the surface waters of the main basin. In Kabuno Bay (a small sub-basin), CH4 concentrations in surface waters were significantly higher than in the main basin. Seasonal variations of CH4 in the main basin were strongly driven by deepening of the mixolimnion and mixing of surface waters with deeper waters rich in CH4. On an annual basis, both Kabuno Bay and the main basin of Lake Kivu were over-saturated in CH4 with respect to atmospheric equilibrium (7330% and 2510%, respectively), and emitted CH4 to the atmosphere (39 mmol m-2 yr-1 and 13 mmol m-2 yr-1, respectively). The source of CH4 to atmosphere was two orders of magnitude lower than the CH4 upward flux. The source of CH4 to the atmosphere from Lake Kivu corresponded to ~60% of the terrestrial sink of atmospheric CH4 over the lake’s catchment. A global cross-system comparison of CH4 in surface waters of lakes shows that both Kabuno Bay and the main basin are at the lower end of values in lakes globally, despite the huge amounts of CH4 in the deeper layers of the lake. This is related to the strongly meromictic nature of the lake that promotes an intense removal of CH4 by bacterial oxidation. [less ▲]

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See detailMicrobial Diversity and Processes in Lake Kivu (East Africa)
Llirós, M.; Darchambeau, François ULg; Garcia-Armisen, T. et al

Conference (2011)

Lake Kivu is a deep meromictic and oligotrophic tropical African lake with a permanent thermal- and haline stratification with huge accumulations of dissolved CO2 and CH4 (ca. 300 km3 and 60 km3 ... [more ▼]

Lake Kivu is a deep meromictic and oligotrophic tropical African lake with a permanent thermal- and haline stratification with huge accumulations of dissolved CO2 and CH4 (ca. 300 km3 and 60 km3, respectively) in the deep anoxic monimolimnion (from 60 o 480 m depth). Although there are a wealth of information on the ecology of small eukaryotes and their trophic role on Kivu, available information on prokaryotic planktonic assemblages is scarce. Molecular analysis of archaeal and bacterial communities showed a vertical segregation imposed by the permanent redoxcline. In relation to Bacteria, Actinobacteria, Betaproteobacteria, Green Sulfur Bacteria and Bacteroidetes were the most commonly retrieved groups. For Archaea, a marked dominance of Thaumarchaeota and Crenarchaeota (75% of all archaeal OTUs) over Euryarchaeota was observed. In the anoxic hypolimnion, Euryarchaoeta (Methanosarcinales and Methanocellales) lineages together with Miscellaneous Crenarchaeotic Group phylotypes were mainly recovered. In turn, Thaumarchaeota phylotypes were recovered in oxic and suboxic waters. CARDFISH analyses over the first 100 m revealed the dominance of Bacteria (51.4% – 95.7% of DAPI-stained cells), especially Actinobacteria (epilimnion), Betaproteobacteria (oxic-anoxic interface) and Bacteroidetes (upper hypolimnion), over Archaea (1.0% – 4.5%; maximum abundances at the oxic-anoxic interface). In turn, flow cytometry evidenced the dominance of HNA cells in the euphotic layer, whereas the proportion of LNA cells increased with depth. HNA and LNA populations were still observed in the anoxic hypolimnion suggesting facultative or strict anaerobic metabolisms. The detection of distinct depth maxima of nitrate, nitrite, archaeal amoA and Marine Thaumarchaeota 16S gene copy numbers together with regularly detection of deep maxima of 3H-Thymidine uptake, and the presence of low-light adapted GSB species point towards a strong link of N, C, and S cycles in the redoxcline of Lake Kivu. [less ▲]

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See detailDistribution and composition of organic carbon in the Tana River Basin, (Kenya)
Tamooh, F.; van den Meersche, K.; Borges, Alberto ULg et al

Poster (2011)

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See detailIsotopic Composition and sources of Organic Carbon Pools within the Tana River Basin, (Kenya)
Tamooh, F.; Van Den Meersche, K.; Borges, Alberto ULg et al

Poster (2011)

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See detailDry Season Carbon Dynamics in Savannah Grassland and Rainforest Dominated River Basins of Madagascar
Marwick, T.; van Acker, K.; Darchambeau, François ULg et al

Poster (2011)

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See detailCarbon and nitrogen flows during a bloom of the coccolithophore Emiliania huxleyi: Modelling a mesocosm experiment
Joassin, Pascal ULg; Delille, Bruno ULg; Soetaert, Karline et al

in Journal of Marine Systems (2011), 85

A dynamic model has been developed to represent biogeochemical variables and processes observed during experimental blooms of the coccolithophore Emiliania huxleyi induced inmesocosms over a period of 23 ... [more ▼]

A dynamic model has been developed to represent biogeochemical variables and processes observed during experimental blooms of the coccolithophore Emiliania huxleyi induced inmesocosms over a period of 23 days. The model describes carbon (C), nitrogen (N), and phosphorus (P) cycling through E. huxleyi and the microbial loop, and computes pH and the partial pressure of carbon dioxide (pCO2) from dissolved inorganic carbon (DIC) and total alkalinity (TA). The main innovations are: 1) the representation of E. huxleyi dynamics using an unbalanced growthmodel in carbon and nitrogen, 2) the gathering of formulations describing typical processes involved in the export of carbon such as primary production, calcification, cellular dissolved organic carbon (DOC) excretion, transparent exopolymer (TEP) formation and viral lyses, and 3) an original and validated representation of the calcification process as a function of the net primary production with a modulation by the intra-cellular N:C ratio mimicking the effect of nutrients limitation on the onset of calcification. It is shown that this new mathematical formulation of calcification provides a better representation of the dynamics of TA, DIC and calcification rates derived from experimental data compared to classicaly used formulations (e.g. function of biomass or of net primary production without anymodulation term). In a first step, the model has been applied to the simulations of present pCO2 conditions. It adequately reproduces the observations for chemical and biological variables and provides an overall view of carbon and nitrogen dynamics. Carbon and nitrogen budgets are derived from the model for the different phases of the bloom, highlighting three distinct phases, reflecting the evolution of the cellular C:N ratio and the interaction between hosts and viruses. During the first phase, inorganic nutrients are massively consumed by E. huxleyi increasing its biomass. Uptakes of carbon and nitrogen are maintained at a constant ratio. The second phase is triggered by the exhaustion of phosphate (PO4 3−). Uptake of carbon and nitrogen being uncoupled, the cellular C:N ratio of E. huxleyi increases. This stimulates the active release of DOC, acting as precursors for TEP. The third phase is characterised by an enhancement of the phytoplankton mortality due to viral lysis. A huge amount of DOC has been accumulated in the mesocosm. [less ▲]

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See detailPresent day carbon dioxide fluxes in the coastal ocean and possible feedbacks under global change
Borges, Alberto ULg

in da Silva Duarte, P. M.; Santana Casiano, J. M. (Eds.) Oceans and the atmospheric carbon content (2011)

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See detailBenthic remineralization in the northwest European continental margin (northern Bay of Biscay)
Suykens, Kim; Schmidt, Sabine; Delille, Bruno ULg et al

in Continental Shelf Research (2011), 31

We report a dataset of sediment characteristics and biogeochemical fluxes at the watersediment interface at the northwest European continental margin (northern Bay of Biscay). Cores were obtained in June ... [more ▼]

We report a dataset of sediment characteristics and biogeochemical fluxes at the watersediment interface at the northwest European continental margin (northern Bay of Biscay). Cores were obtained in June 2006, May 2007 and 2008, at 18 stations on the shelf break (120 to 180 m), and at 2 stations on the continental slope (520 m and 680 m). Water-sediment fluxes of dissolved oxygen (O2), total alkalinity (TA), nitrate (NO3-), and dissolved silicate (DSi) were measured at a total of 20 stations. Sediment characteristics include: grain size, chlorophyll-a (Chl-a) and phaeopigment (Phaeo) content, particulate organic (POC) and inorganic (PIC) carbon content, and lead-210 (210Pb) and thorium-234 (234Th) activities. Sediments were sandy (fine to coarse) with organic matter (OM) (1.0 - 4.0 %) and Chl-a (0.01 - 0.95 μg g-1) contents comparable to previous investigations in the same region, and a relatively high PIC fraction (0.8 - 10.2 %). Water-sediment O2 fluxes (-2.4 to -8.4 mmol O2 m-2 d-1) were low compared to other coastal environments and correlated well with OM and Chl-a content. 234Th activity profiles indicated that Chl-a sediment content was mainly controlled by physical mixing processes related to local hydrodynamics. The correlation between water-sediment fluxes of O2 and NO3- indicated a close coupling of nitrification/denitrification and total benthic organic carbon degradation. Dissolution of biogenic silica (0.05 to 0.95 mmol m-2 d-1) seemed uncoupled from organic carbon degradation, as characterized by water-sediment O2 fluxes. The link between water-sediment fluxes of TA and O2 indicated the occurrence of metabolic driven dissolution of calcium carbonates (CaCO3) in the sediments (~ 0.33 ± 0.47 mmol m-2 d-1) which represented ~ 1 % of the pelagic calcification rates due to coccolithophores measured during the cruises. These CaCO3 dissolution rates were below those reported in sediments of continental slopes and of the deep ocean, probably due to the high over-saturation with respect to CaCO3 of the water column overlying the continental shelf sediments of the northern Bay of Biscay. Rates of total benthic organic carbon degradation were low compared to water column rates of primary production and aphotic community respiration obtained during the cruises. [less ▲]

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See detailInorganic carbon in the Tana River Basin (Kenya): Distribution, composition and process rates
Van den Meersche, K.; Tamooh, F.; Meysman, F. et al

Conference (2011)

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See detailSeasonal and inter-annual variability of air-sea CO2 fluxes and seawater carbonate chemistry in the Southern North Sea
Gypens, N.; Lacroix, G.; Lancelot, C. et al

in Progress in Oceanography (2011), 88

A 3D coupled biogeochemical–hydrodynamic model (MIRO-CO2&CO) is implemented in the English Channel (ECH) and the Southern Bight of the North Sea (SBNS) to estimate the present-day spatio-temporal ... [more ▼]

A 3D coupled biogeochemical–hydrodynamic model (MIRO-CO2&CO) is implemented in the English Channel (ECH) and the Southern Bight of the North Sea (SBNS) to estimate the present-day spatio-temporal distribution of air–sea CO2 fluxes, surface water partial pressure of CO2 (pCO2) and other components of the carbonate system (pH, saturation state of calcite (Xca) and of aragonite (Xar)), and the main drivers of their variability. Over the 1994–2004 period, air–sea CO2 fluxes show significant interannual variability, with oscillations between net annual CO2 sinks and sources. The inter annual variability of air–sea CO2 fluxes simulated in the SBNS is controlled primarily by river loads and changes of biological activities (net autotrophy in spring and early summer, and net heterotrophy in winter and autumn), while in areas less influenced by river inputs such as the ECH, the inter annual variations of air–sea CO2 fluxes are mainly due to changes in sea surface temperature and in near-surface wind strength and direction. In the ECH, the decrease of pH, of Xca and of Xar follows the one expected from the increase of atmospheric CO2 (ocean acidification), but the decrease of these quantities in the SBNS during the considered time period is faster than the one expected from ocean acidification alone. This seems to be related to a general pattern of decreasing nutrient river loads and net ecosystem production (NEP) in the SBNS. Annually, the combined effect of carbon and nutrient loads leads to an increase of the sink of CO2 in the ECH and the SBNS, but the impact of the river loads varies spatially and is stronger in river plumes and nearshore waters than in offshore waters. The impact of organic and inorganic carbon (C) inputs is mainly confined to the coast and generates a source of CO2 to the atmosphere and low pH, of Xca and of Xar values in estuarine plumes, while the impact of nutrient loads, highest than the effect of C inputs in coastal nearshore waters, also propagates offshore and, by stimulating primary production, drives a sink of atmospheric CO2 and higher values of pH, of Xca and of Xar. [less ▲]

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