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See detailDynamic seasonal nitrogen cycling in response to anthropogenic N loading in a tropical catchment, Athi–Galana–Sabaki River, Kenya
Marwick, T. R.; Tamooh, F.; Ogwoka, B. et al

in Biogeosciences (2014), 11(2), 443--460

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See detailRates of consumption of atmospheric CO2 through the weathering of loess during the next 100 yr of climate change
Goddéris, Y.; Brantley, S. L.; François, Louis ULg et al

in Biogeosciences (2013), 10

Quantifying how C fluxes will change in the future is a complex task for models because of the coupling between climate, hydrology, and biogeochemical reactions. Here we investigate how pedogenesis of the ... [more ▼]

Quantifying how C fluxes will change in the future is a complex task for models because of the coupling between climate, hydrology, and biogeochemical reactions. Here we investigate how pedogenesis of the Peoria loess, which has been weathering for the last 13 kyr, will respond over the next 100 yr of climate change. Using a cascade of numerical models for climate (ARPEGE), vegetation (CARAIB) and weathering (WITCH), we explore the effect of an increase in CO2 of 315 ppmv (1950) to 700 ppmv (2100 projection). The increasing CO2 results in an increase in temperature along the entire transect. In contrast, drainage increases slightly for a focus pedon in the south but decreases strongly in the north. These two variables largely determine the behavior of weathering. In addition, although CO2 production rate increases in the soils in response to global warming, the rate of diffusion back to the atmosphere also increases, maintaining a roughly constant or even decreasing CO2 concentration in the soil gas phase. Our simulations predict that temperature increasing in the next 100 yr causes the weathering rates of the silicates to increase into the future. In contrast, the weathering rate of dolomite – which consumes most of the CO2 – decreases in both end members (south and north) of the transect due to its retrograde solubility. We thus infer slower rates of advance of the dolomite reaction front into the subsurface, and faster rates of advance of the silicate reaction front. However, additional simulations for 9 pedons located along the north–south transect show that the dolomite weathering advance rate will increase in the central part of the Mississippi Valley, owing to a maximum in the response of vertical drainage to the ongoing climate change. The carbonate reaction front can be likened to a terrestrial lysocline because it represents a depth interval over which carbonate dissolution rates increase drastically. However, in contrast to the lower pH and shallower lysocline expected in the oceans with increasing atmospheric CO2, we predict a deeper lysocline in future soils. Furthermore, in the central Mississippi Valley, soil lysocline deepening accelerates but in the south and north the deepening rate slows. This result illustrates the complex behavior of carbonate weathering facing short term global climate change. Predicting the global response of terrestrial weathering to increased atmospheric CO2 and temperature in the future will mostly depend upon our ability to make precise assessments of which areas of the globe increase or decrease in precipitation and soil drainage. [less ▲]

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See detailDynamics of dissolved inorganic carbon and aquatic metabolism in the Tana River basin, Kenya
Tamooh, F.; Borges, Alberto ULg; Meysman, F. J. R. et al

in Biogeosciences (2013), 10(11), 6911-6928

A basin-wide study was conducted in the Tana River basin (Kenya) in February 2008 (dry season), September–November 2009 (wet season) and June– July 2010 (end of the wet season) to assess the dynamics and ... [more ▼]

A basin-wide study was conducted in the Tana River basin (Kenya) in February 2008 (dry season), September–November 2009 (wet season) and June– July 2010 (end of the wet season) to assess the dynamics and sources of dissolved inorganic carbon (DIC) as well as to quantify CO2 fluxes, community respiration (R), and primary production (P). Samples were collected along the altitudinal gradient (from 3600 to 8 m) in several headwater streams, reservoirs (Kamburu and Masinga), and the Tana River mainstream. DIC concentrations ranged from 0.2 to 4.8 mmol L−1, with exceptionally high values (3.5±1.6 mmol L−1) in Nyambene Hills tributaries. The wide range of 13CDIC values (−15.0 to −2.4 ‰) indicate variable sources of DIC, with headwater streams recording more positive signatures compared to the Tana River mainstream. With with only a few exceptions, the entire riverine network was supersaturated in CO2, implying the system is a net source of CO2 to the atmosphere. pCO2 values were generally higher in the lower Tana River mainstream compared to headwater tributaries, opposite to the pattern typically observed in other river networks. This was attributed to high suspended sediment in the Tana River mainstream fuelling in-stream community respiration and net heterotrophy. This was particularly evident during the 2009 wet season campaign (median pCO2 of 1432 ppm) compared to the 2010 end of the wet season (1002 ppm) and 2008 dry season(579 ppm). First-order estimates show that in-stream community respiration was responsible for the bulk of total CO2 evasion (77 to 114 %) in the Tana River mainstream, while in the tributaries, this could only account for 5 to 68% of total CO2 evasion. This suggests that CO2 evasion in the tributaries was to a substantial degree sustained by benthic mineralisation and/or lateral inputs of CO2-oversaturated groundwater. While sediment loads increased downstream and thus light availability decreased in the water column, both chlorophyll a (0.2 to 9.6 μg L−1) and primary production (0.004 to 7.38 μmol CL−1 h−1) increased consistently downstream. Diurnal fluctuations of biogeochemical processes were examined at three different sites along the river continuum (headwater, reservoir and mainstream), and were found to be substantial only in the headwater stream, moderate in the reservoir and not detectable in the Tana River mainstream. The pronounced diurnal fluctuations observed in the headwater stream were largely regulated by periphyton as deduced from the low chlorophyll a in the water column. [less ▲]

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See detailDrivers, mechanisms and long-term variability of seasonal hypoxia on the Black Sea northwestern shelf – is there any recovery after eutrophication?
Capet, Arthur ULg; Beckers, Jean-Marie ULg; Grégoire, Marilaure ULg

in Biogeosciences (2013), 10

The Black Sea northwestern shelf (NWS) is a shallow eutrophic area in which the seasonal stratification of the water column isolates the bottom waters from the atmosphere. This prevents ventilation from ... [more ▼]

The Black Sea northwestern shelf (NWS) is a shallow eutrophic area in which the seasonal stratification of the water column isolates the bottom waters from the atmosphere. This prevents ventilation from counterbalancing the large consumption of oxygen due to respiration in the bottom waters and in the sediments, and sets the stage for the development of seasonal hypoxia. A three-dimensional (3-D) coupled physical–biogeochemical model is used to investigate the dynamics of bottom hypoxia in the Black Sea NWS, first at seasonal and then at interannual scales (1981–2009), and to differentiate its driving factors (climatic versus eutrophication). Model skills are evaluated by a quantitative comparison of the model results to 14 123 in situ oxygen measurements available in the NOAA World Ocean and the Black Sea Commission databases, using different error metrics. This validation exercise shows that the model is able to represent the seasonal and interannual variability of the oxygen concentration and of the occurrence of hypoxia, as well as the spatial distribution of oxygen-depleted waters. During the period 1981–2009, each year exhibits seasonal bottom hypoxia at the end of summer. This phenomenon essentially covers the northern part of the NWS – which receives large inputs of nutrients from the Danube, Dniester and Dnieper rivers – and extends, during the years of severe hypoxia, towards the Romanian bay of Constanta. An index H which merges the aspects of the spatial and temporal extension of the hypoxic event is proposed to quantify, for each year, the intensity of hypoxia as an environmental stressor. In order to explain the interannual variability of H and to disentangle its drivers, we analyze the long time series of model results by means of a stepwise multiple linear regression. This statistical model gives a general relationship that links the intensity of hypoxia to eutrophication and climate-related variables. A total of 82% of the interannual variability of H is explained by the combination of four predictors: the annual riverine nitrate load (N), the sea surface temperature in the month preceding stratification (Ts), the amount of semi-labile organic matter accumulated in the sediments (C) and the sea surface temperature during late summer (Tf). Partial regression indicates that the climatic impact on hypoxia is almost as important as that of eutrophication. Accumulation of organic matter in the sediments introduces an important inertia in the recovery process after eutrophication, with a typical timescale of 9.3 yr. Seasonal fluctuations and the heterogeneous spatial distribution complicate the monitoring of bottom hypoxia, leading to contradictory conclusions when the interpretation is done from different sets of data. In particular, it appears that the recovery reported in the literature after 1995 was overestimated due to the use of observations concentrated in areas and months not typically affected by hypoxia. This stresses the urgent need for a dedicated monitoring effort in the Black Sea NWS focused on the areas and months concerned by recurrent hypoxic events. [less ▲]

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See detailDistribution and origin of suspended matter and organic carbon pools in the Tana River Basin, Kenya
Tamooh, F; Van den Meersche, K; Meysman, F et al

in Biogeosciences (2012), 9

We studied patterns in organic carbon pools and their origin in the Tana River Basin (Kenya), in February 2008 (dry season), September–November 2009 (wet season), and June–July 2010 (end of wet season ... [more ▼]

We studied patterns in organic carbon pools and their origin in the Tana River Basin (Kenya), in February 2008 (dry season), September–November 2009 (wet season), and June–July 2010 (end of wet season), covering the full continuum from headwater streams to lowland mainstream sites. A consistent downstream increase in total suspended matter (TSM, 0.6 to 7058 mg l−1) and particulate organic carbon (POC, 0.23 to 119.8 mg l−1) was observed during all three sampling campaigns, particularly pronounced below 1000m above sea level, indicating that most particulate matter exported towards the coastal zone originated from the mid and low altitude zones rather than from headwater regions. This indicates that the cascade of hydroelectrical reservoirs act as an extremely efficient particle trap. Although 7Be / 210Pbxs ratios/age of suspended sediment do not show clear seasonal variation, the gradual downstream increase of suspended matter during end of wet season suggests its origin is caused by inputs of older sediments from bank erosion and/or river sediment resuspension. During wet season, higher TSM concentrations correspond with relatively young suspended matter, suggesting a contribution from recently eroded material.With the exception of reservoir waters, POC was predominantly of terrestrial origin as indicated by generally high POC : chlorophyll a (POC : Chl a) ratios (up to 41 000). Stable isotope signatures of POC ( 13CPOC) ranged between −32 and −20‰and increased downstream, reflecting an increasing contribution of C4-derived carbon in combination with an expected shift in 13C for C3 vegetation towards the more semi-arid lowlands. 13C values in sediments from the main reservoir (−19.5 to −15.7 ‰) were higher than those found in any of the riverine samples, indicating selective retention of particles associated with C4 fraction. Dissolved organic carbon (DOC) concentrations were highest during the end of wet season (2.1 to 6.9 mg l−1), with stable isotope signatures generally between −28 and −22 ‰. A consistent downstream decrease in % organic carbon (%OC) was observed for soils, riverine sediments, and suspended matter. This was likely due to better preservation of the organic fraction in colder high altitude regions, with loss of carbon during downstream spiraling. 13C values for soil and sediment did not exhibit clear altitudinal patterns, but values reflect the full spectrum from C3-dominated to C4-dominated sites. Very low ratios of organic carbon to mineral surface area (OC : SA) were found in reservoir sediments and suspended matter in the lower Tana River, indicating that these are stable OC pools which have undergone extensive degradation. Overall, our study demonstrates that substantial differences occur in both the quantities and origin of suspended sediments and organic carbon along the river profile in this tropical river basin, as well as seasonal differences in the mechanisms causing such variations. [less ▲]

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See detailOrganic matter sources, fluxes and greenhouse gas exchange in the Oubangui River (Congo River basin)
Bouillon, S.; Yambélé, A.; Spencer, R.G.M. et al

in Biogeosciences (2012), 9

The Oubangui is a major tributary of the Congo River, draining an area of 500 000 km2 mainly consisting of wooded savannahs. Here, we report results of a one year long, 2-weekly sampling campaign in ... [more ▼]

The Oubangui is a major tributary of the Congo River, draining an area of 500 000 km2 mainly consisting of wooded savannahs. Here, we report results of a one year long, 2-weekly sampling campaign in Bangui (Central African Republic) since March 2010 for a suite of physicochemical and biogeochemical characteristics, including total suspended matter (TSM), bulk concentration and stable isotope composition of particulate organic carbon (POC and 13CPOC), particulate nitrogen (PN and 15NPN), dissolved organic carbon (DOC and 13CDOC), dissolved inorganic carbon (DIC and 13CDIC), dissolved greenhouse gases (CO2, CH4 and N2O), and dissolved ignin composition. 13C signatures of both POC and DOC showed strong seasonal variations −30.6 to −25.8 ‰, and −31.8 to −27.1 ‰, respectively), but their different timing indicates that the origins of POC and DOC may vary strongly over the hydrograph and are largely ncoupled, differing up to 6‰ in 13C signatures. Dissolved lignin characteristics (carbon- ormalised yields, cinnamyl:vanillyl phenol ratios, and vanillic acid to vanillin ratios) showed arked differences between high and low discharge conditions, consistent with major seasonal ariations in the sources of dissolved organic matter. We observed a strong seasonality in pCO2, ranging between 470 ± 203 ppm for Q<1000m3 s−1 (n = 10) to a maximum of 3750 pm during the first stage of the rising discharge. The low POC/PN ratios, high %POCand low and variable 13CPOC signatures during low flow conditions suggest that the majority of the POC pool during this period consists of in situ produced phytoplankton, consistent with oncurrent pCO2 (partial pressure of CO2) values only slightly above and, occasionally, below atmospheric equilibrium. Water-atmosphere CO2 fluxes estimated using two independent pproaches averaged 105 and 204 gCm−2 yr−1, i.e. more than an order of magnitude lower than current estimates for large tropical rivers globally. Although tropical rivers are often ssumed to show much higher CO2 effluxes compared to temperate systems, we show that in situ production may be high enough to dominate the particulate organic carbon pool, and lower CO2 values to near equilibrium values during low discharge conditions. The total annual flux of TSM, POC, PN, DOC and DIC are 2.33 Tg yr−1, 0.14 TgC yr−1, 0.014 TgNyr−1, 0.70 TgC yr−1, and 0.49 Tg Cyr−1, respectively. While our TSM and POC fluxes are similar to previous stimates for the Oubangui, DOC fluxes were 30% higher and bicarbonate fluxes were 35% ower than previous reports. DIC represented 58% of the total annual C flux, and under the ssumptions that carbonate weathering represents 25% of the DIC flux and that CO2 from espiration drives chemical weathering, this flux is equivalent to 50% of terrestrial-derived riverine C transport. [less ▲]

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See detailSpatial and temporal CO2 exchanges measured by Eddy Covariance over a temperate intertidal flat and their relationships to net ecosystem production
Polsenaere, P.; Lamaud, E.; Lafon, V. et al

in Biogeosciences (2012), 9(1), 249--268

Measurements of carbon dioxide fluxes were performed over a temperate intertidal mudflat in southwestern France using the micrometeorological Eddy Covariance (EC) technique. EC measurements were carried ... [more ▼]

Measurements of carbon dioxide fluxes were performed over a temperate intertidal mudflat in southwestern France using the micrometeorological Eddy Covariance (EC) technique. EC measurements were carried out in two contrasting sites of the Arcachon flat during four periods and in three different seasons (autumn 2007, summer 2008, autumn 2008 and spring 2009). In addition, satellite images of the tidal flat at low tide were used to link the net ecosystem CO2 exchange (NEE) with the occupation of the mudflat by primary producers, particularly by Zostera noltii meadows. CO2 fluxes during the four deployments showed important spatial and temporal variations, with the flat rapidly shifting from sink to source with the tide. Absolute CO2 fluxes showed generally small negative (influx) and positive (efflux) values, with larger values up to −13 μmol m−2 s−1 for influxes and 19 μmol m−2 s−1 for effluxes. Low tide during the day was mostly associated with a net uptake of atmospheric CO2. In contrast, during immersion and during low tide at night, CO2 fluxes where positive, negative or close to zero, depending on the season and the site. During the autumn of 2007, at the innermost station with a patchy Zostera noltii bed (cover of 22 ± 14% in the wind direction of measurements), CO2 influx was −1.7 ± 1.7 μmol m−2 s−1 at low tide during the day, and the efflux was 2.7 ± 3.7 μmol m−2 s−1 at low tide during the night. A gross primary production (GPP) of 4.4 ± 4.1 μmol m−2 s−1 during emersion could be attributed to microphytobenthic communities. During the summer and autumn of 2008, at the central station with a dense eelgrass bed (92 ± 10%), CO2 uptakes at low tide during the day were −1.5 ± 1.2 and −0.9 ± 1.7 μmol m−2 s−1, respectively. Night time effluxes of CO2 were 1.0 ± 0.9 and 0.2 ± 1.1 μmol m−2 s−1 in summer and autumn, respectively, resulting in a GPP during emersion of 2.5 ± 1.5 and 1.1 ± 2.0 μmol m−2 s−1, respectively, attributed primarily to the seagrass community. At the same station in April 2009, before Zostera noltii started to grow, the CO2 uptake at low tide during the day was the highest (−2.7 ± 2.0 μmol m−2 s−1). Influxes of CO2 were also observed during immersion at the central station in spring and early autumn and were apparently related to phytoplankton blooms occurring at the mouth of the flat, followed by the advection of CO2-depleted water with the flooding tide. Although winter data as well as water carbon measurements would be necessary to determine a precise CO2 budget for the flat, our results suggest that tidal flat ecosystems are a modest contributor to the CO2 budget of the coastal ocean. [less ▲]

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See detailCarbonate system in the water masses of the Southeast Atlantic sector of the Southern Ocean during February and March 2008
Gonzalez-Davila, M.; Santana-Casiano, J. M.; Fine, R. A. et al

in Biogeosciences (2011), 8

Carbonate system variables were measured in the South Atlantic sector of the Southern Ocean along a transect from South Africa to the southern limit of the Antarctic Circumpolar Current (ACC) from ... [more ▼]

Carbonate system variables were measured in the South Atlantic sector of the Southern Ocean along a transect from South Africa to the southern limit of the Antarctic Circumpolar Current (ACC) from February to March 2008. Eddies detached from the retroflection of the Agulhas Current increased the gradients observed along the fronts. Minima in the fugacity of CO2, fCO2, and maxima in pH on either side of the frontal zone were observed, noting that within the frontal zone fCO2 reached maximum values and pH was at a minimum. Vertical distributions of water masses were described by their carbonate system properties and their relationship to CFC concentrations. Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW) offered pHT,25 values of 7.56 and 7.61, respectively. The UCDW also had higher concentrations of CFC-12 (>0.2 pmol kg−1) as compared to deeper waters, revealing that UCDW was mixed with recently ventilated waters. Calcite and aragonite saturation states (Ω) were also affected by the presence of these two water masses with high carbonate concentrations. The aragonite saturation horizon was observed at 1000 m in the subtropical area and north of the Subantarctic Front. At the position of the Polar Front, and under the influence of UCDW and LCDW, the aragonite saturation horizon deepened from 800 m to 1500 m at 50.37° S, and reached 700 m south of 57.5° S. High latitudes proved to be the most sensitive areas to predicted anthropogenic carbon increase. Buffer coefficients related to changes in [CO2], [H+] and Ω with changes in dissolved inorganic carbon (CT) and total alkalinity (AT) offered minima values in the Antarctic Intermediate Water and UCDW layers. These coefficients suggest that a small increase in CT will sharply decrease the status of pH and carbonate saturation. Here we present data that suggest that south of 55° S, surface water will be under-saturated with respect to aragonite within the next few decades. [less ▲]

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See detailIndividual and interacting effects of pCO2 and temperature on Emiliania huxleyi calcification: Study of the calcite production, the coccolith morphology and the coccosphere size
De Bodt, Caroline; Van Oostende, Nicolas; Harlay, Jérôme ULg et al

in Biogeosciences (2010), 7

The impact of ocean acidification and increased water temperature on marine ecosystems, in particular those involving calcifying organisms, has been gradually recognised. We examined the individual and ... [more ▼]

The impact of ocean acidification and increased water temperature on marine ecosystems, in particular those involving calcifying organisms, has been gradually recognised. We examined the individual and combined effects of increased pCO2 (180 ppm V CO2, 380 ppm V CO2 and 750 ppm V CO2 corresponding to past, present and future CO2 conditions, respectively) and temperature (13°C and 18°C) during the calcification phase of the coccolithophore E. huxleyi using batch culture experiments. We showed that the cell abundance-normalized particulate organic carbon concentration (POC) increased from the present to the future CO2 treatments. A significant effect of pCO2 and of temperature on calcification was found, manifesting itself in a lower cell abundance-normalized particulate inorganic carbon (PIC) content as well as a lower PIC:POC ratio at future CO2 levels and at 18°C. Coccosphere-sized particles showed a size reduction trend with both increasing temperature and CO2 concentration. The influence of the different treatments on coccolith morphology was studied by categorizing SEM coccolith micrographs. The number of well-formed coccoliths decreased with increasing pCO2 while temperature did not have a significant impact on coccolith morphology. No interacting effect of pCO2 and temperature was observed on calcite production, coccolith morphology or on coccosphere size. Finally, our results suggest that ocean acidification might have a larger adverse impact on coccolithophorid calcification than surface water warming. [less ▲]

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See detailThe role of ocean transport in the uptake of anthropogenic CO2
Cao, L.; Eby, M.; Ridgwell, A. et al

in Biogeosciences (2009), 6(3), 375-390

We compare modeled oceanic carbon uptake in response to pulse CO2 emissions using a suite of global ocean models and Earth system models. In response to a CO2 pulse emission of 590 Pg C (corresponding to ... [more ▼]

We compare modeled oceanic carbon uptake in response to pulse CO2 emissions using a suite of global ocean models and Earth system models. In response to a CO2 pulse emission of 590 Pg C (corresponding to an instantaneous doubling of atmospheric CO2 from 278 to 556 ppm), the fraction of CO2 emitted that is absorbed by the ocean is: 37 +/- 8\%, 56 +/- 10\%, and 81 +/- 4 (model mean +/- 2 sigma) in year 30, 100, and 1000 after the emission pulse, respectively. Modeled oceanic uptake of pulse CO2 on timescales from decades to about a century is strongly correlated with simulated present-day uptake of chlorofluorocarbons (CFCs) and CO2 across all models, while the amount of pulse CO2 absorbed by the ocean from a century to a millennium is strongly correlated with modeled radiocarbon in the deep Southern and Pacific Ocean. However, restricting the analysis to models that are capable of reproducing observations within uncertainty, the correlation is generally much weaker. The rates of surface-to-deep ocean transport are determined for individual models from the instantaneous doubling CO2 simulations, and they are used to calculate oceanic CO2 uptake in response to pulse CO2 emissions of different sizes pulses of 1000 and 5000 Pg C. These results are compared with simulated oceanic uptake of CO2 by a number of models simulations with the coupling of climate-ocean carbon cycle and without it. This comparison demonstrates that the impact of different ocean transport rates across models on oceanic uptake of anthropogenic CO2 is of similar magnitude as that of climate-carbon cycle feed-backs in a single model, emphasizing the important role of ocean transport in the uptake of anthropogenic CO2. [less ▲]

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See detailDistribution, origin and cycling of carbon in the Tana River (Kenya): a dry season basin-scale survey from headwaters to the delta
Bouillon, S.; Abril, G.; Borges, Alberto ULg et al

in Biogeosciences (2009), 6

The Tana River basin (TRB) is the largest in Kenya (_120 000 km2). We conducted a survey during the dry season throughout the TRB, analyzing a broad suite of biogeochemical parameters. Biogeochemical ... [more ▼]

The Tana River basin (TRB) is the largest in Kenya (_120 000 km2). We conducted a survey during the dry season throughout the TRB, analyzing a broad suite of biogeochemical parameters. Biogeochemical signatures in headwater streams were highly variable. Along the middle and lower river course, total suspended matter (TSM) concentrations increased more than 30-fold despite the absence of tributary inputs, indicating important resuspension events of internally stored sediment. These resuspended sediment inputs were characterized by a lower and 14C-depleted OC content, suggesting selective degradation of more recent material during sediment retention. Masinga Dam (a large reservoir on the upper river) induced a strong nutrient retention (_50% for inorganic N, _72% for inorganic phosphate, and _40% for dissolved silicate). Moreover, while DOC pools and _13C signatures were similar above, in and below the reservoir, the POC pool in Masinga surface waters was dominated by 13C-depleted phytoplankton, which contributed to the riverine POC pool immediately below the dam, but rapidly disappeared further downstream, suggesting rapid remineralization of this labile C pool in the river system. Despite the generally high turbidity, the combination of relatively high oxygen saturation levels, low _18O signatures of dissolved O2 (all <+24.2‰), and the relatively low pCO2 values suggest that in-stream primary production was significant, even though pigment data suggest that phytoplankton makes only a minor contribution to the total POC pool in the Tana River. [less ▲]

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See detailEnhanced ocean carbon storage from anaerobic alkalinity generation in coastal sediments
Thomas, H.; Schiettecatte, L.-S.; Suykens, Kim ULg et al

in Biogeosciences (2009), 6

The coastal ocean is a crucial link between land, the open ocean and the atmosphere. The shallowness of the water column permits close interactions between the sedimentary, aquatic and atmospheric ... [more ▼]

The coastal ocean is a crucial link between land, the open ocean and the atmosphere. The shallowness of the water column permits close interactions between the sedimentary, aquatic and atmospheric compartments, which otherwise are decoupled at long time scales ( =1000 yr) in the open oceans. Despite the prominent role of the coastal oceans in absorbing atmospheric CO2 and transferring it into the deep oceans via the continental shelf pump, the underlying mechanisms remain only partly understood. Evaluating observations from the North Sea, a NW European shelf sea, we provide evidence that anaerobic degradation of organic matter, fuelled from land and ocean, generates total alkalinity (AT) and increases the CO2 buffer capacity of seawater. At both the basin wide and annual scales anaerobic AT generation in the North Sea’s tidal mud flat area irreversibly facilitates 7–10%, or taking into consideration benthic denitrification in the North Sea, 20–25% of the North Sea’s overall CO2 uptake. At the global scale, anaerobic AT generation could be accountable for as much as 60% of the uptake of CO2 in shelf and marginal seas, making this process, the anaerobic pump, a key player in the biological carbon pump. Under future high CO2 conditions oceanic CO2 storage via the anaerobic pump may even gain further relevance because of stimulated ocean productivity. [less ▲]

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See detailIron oxide deposits associated with the ectosymbiotic bacteria in the hydrothermal vent shrimp Rimicaris exoculata
Corbari, Laure; Cambon-Bonavita, Marie-Anne; Long, Gary et al

in Biogeosciences (2008), 5

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See detailEffects of CO2 on particle size distribution and phytoplankton abundance during a mesocosm bloom experiment (PeECE II)
Engel, Anja; Schulz, K. G.; Riebesell, U. et al

in Biogeosciences (2008), 5(2), 509-521

The influence of seawater carbon dioxide (CO2) concentration on the size distribution of suspended particles (2-60 mu m) and on phytoplankton abundance was investigated during a mesocosm experiment at the ... [more ▼]

The influence of seawater carbon dioxide (CO2) concentration on the size distribution of suspended particles (2-60 mu m) and on phytoplankton abundance was investigated during a mesocosm experiment at the large scale facility (LFS) in Bergen, Norway, in the frame of the Pelagic Ecosystem CO2 Enrichment study (PeECE II). In nine outdoor enclosures the partial pressure of CO2 in seawater was modified by an aeration system to simulate past (similar to 190 mu atm CO2), present (similar to 370 mu atm CO2) and future (similar to 700 mu atm CO2) CO2 conditions in triplicates. Due to the initial addition of inorganic nutrients, phytoplankton blooms developed in all mesocosms and were monitored over a period of 19 days. Seawater samples were collected daily for analysing the abundance of suspended particles and phytoplankton with the Coulter Counter and with Flow Cytometry, respectively. During the bloom period, the abundance of small particles (< 4 mu m) significantly increased at past, and decreased at future CO2 levels. At that time, a direct relationship between the total-surface-to-total-volume ratio of suspended particles and DIC concentration was determined for all mesocosms. Significant changes with respect to the CO2 treatment were also observed in the phytoplankton community structure. While some populations such as diatoms seemed to be insensitive to the CO2 treatment, others like Micromonas spp. increased with CO2, or showed maximum abundance at present day CO2 (i.e. Emiliania huxleyi). The strongest response to CO2 was observed in the abundance of small autotrophic nano-plankton that strongly increased during the bloom in the past CO2 mesocosms. Together, changes in particle size distribution and phytoplankton community indicate a complex interplay between the ability of the cells to physiologically respond to changes in CO2 and size selection. Size of cells is of general importance for a variety of processes in marine systems such as diffusion-limited uptake of substrates, resource allocation, predator-prey interaction, and gravitational settling. The observed changes in particle size distribution are therefore discussed with respect to biogeochemical cycling and ecosystem functioning. [less ▲]

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See detailInter-annual variability of the carbon dioxide oceanic sink south of Tasmania
Borges, Alberto ULg; Tilbrook, B.; Metzl, N. et al

in Biogeosciences (2008), 5(1), 141-155

We compiled a large data-set from 22 cruises spanning from 1991 to 2003, of the partial pressure of CO2 (pCO(2)) in surface waters over the continental shelf (CS) and adjacent open ocean (43 degrees to 46 ... [more ▼]

We compiled a large data-set from 22 cruises spanning from 1991 to 2003, of the partial pressure of CO2 (pCO(2)) in surface waters over the continental shelf (CS) and adjacent open ocean (43 degrees to 46 degrees S; 145 degrees to 150 degrees E), south of Tasmania. Climatological seasonal cycles of pCO(2) in the CS, the subtropical zone (STZ) and the subAntarctic zone (SAZ) are described and used to determine monthly pCO(2) anomalies. These are used in combination with monthly anomalies of sea surface temperature (SST) to investigate inter-annual variations of SST and pCO(2). Monthly anomalies of SST (as intense as 2 degrees C) are apparent in the CS, STZ and SAZ, and are indicative of strong inter-annual variability that seems to be related to large-scale coupled atmosphere-ocean oscillations. Anomalies of pCO(2) normalized to a constant temperature are negatively related to SST anomalies. A reduced winter-time vertical input of dissolved inorganic carbon (DIC) during phases of positive SST anomalies, related to a poleward shift of westerly winds, and a concomitant local decrease in wind stress is the likely cause of the negative relationship between pCO(2) and SST anomalies. The observed pattern is an increase of the sink for atmospheric CO2 associated with positive SST anomalies, although strongly modulated by inter-annual variability of wind speed. Assuming that phases of positive SST anomalies are indicative of the future evolution of regional ocean biogeochemistry under global warming, we show using a purely observational based approach that some provinces of the Southern Ocean could provide a potential negative feedback on increasing atmospheric CO2. [less ▲]

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See detailBiomass production in experimental grasslands of different species richness during three years of climate warming
De Boeck, H. J.; Lemmens, CMHM; Zavalloni, C. et al

in Biogeosciences (2008), 5

Here we report on the single and combined impacts of climate warming and species richness on the biomass production in experimental grassland communities. Projections of a future warmer climate have ... [more ▼]

Here we report on the single and combined impacts of climate warming and species richness on the biomass production in experimental grassland communities. Projections of a future warmer climate have stimulated studies on the response of terrestrial ecosystems to this global change. Experiments have likewise addressed the importance of species numbers for ecosystem functioning. There is, however, little knowledge on the interplay between warming and species richness. During three years, we grew experimental plant communities containing one, three or nine grassland species in 12 sunlit, climate-controlled chambers in Wilrijk, Belgium. Half of these chambers were exposed to ambient air temperatures (unheated), while the other half were warmed by 3 degrees C (heated). Equal amounts of water were added to heated and unheated communities, so that warming would imply drier soils if evapotranspiration was higher. Biomass production was decreased due to warming, both aboveground (-29%) and belowground (-25%), as negative impacts of increased heat and drought stress in summer prevailed. Complementarity effects, likely mostly through both increased aboveground spatial complementarity and facilitative effects of legumes, led to higher shoot and root biomass in multi-species communities, regardless of the induced warming. Surprisingly, warming suppressed productivity the most in 9-species communities, which may be attributed to negative impacts of intense interspecific competition for resources under conditions of high abiotic stress. Our results suggest that warming and the associated soil drying could reduce primary production in many temperate grasslands, and that this will not necessarily be mitigated by efforts to maintain or increase species richness. [less ▲]

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See detailDependence of CO2 advection patterns on wind direction on a gentle forested slope
Heinesch, Bernard ULg; Yernaux, Michel ULg; Aubinet, Marc ULg

in Biogeosciences (2008), 5(3), 657-668

Gravitational flows generated on a gentle slope in stable conditions were analysed at a forested site at Vielsalm in Belgium. There were two distinct situations at the site, one corresponding to vertical ... [more ▼]

Gravitational flows generated on a gentle slope in stable conditions were analysed at a forested site at Vielsalm in Belgium. There were two distinct situations at the site, one corresponding to vertical convergence, characterised by a negative vertical velocity at the canopy top and horizontal velocity divergence below the canopy, the other corresponding to an equilibrium situation without any vertical movement. The causes of these two distinct flow patterns were analysed. These measurements combined with those of the horizontal CO2 concentration gradient below the canopy supported the dilution hypothesis suggested by Aubinet et al. (2003): the horizontal CO2 concentration gradient is negative in convergence situations but slightly positive in equilibrium conditions. The existence of such patterns allows us to confirm the coherence of advection observations made at the site. However, the sum of turbulent CO2 flux, changes in CO2 storage and advective terms were shown to greatly overestimate the expected net ecosystem exchange in the convergence conditions. The most probable cause was identified as being a poor estimate of the vertical profile of the vertical velocity component. [less ▲]

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See detailThe impact of lateral carbon fluxes on the European carbon balance
Ciais, P.; Borges, Alberto ULg; Abril, Gwenaël et al

in Biogeosciences (2008), 5

To date, little is known about the impact of processes which cause lateral carbon fluxes over continents, and from continents to oceans on the CO2 – and carbon budgets at local, regional and continental ... [more ▼]

To date, little is known about the impact of processes which cause lateral carbon fluxes over continents, and from continents to oceans on the CO2 – and carbon budgets at local, regional and continental scales. Lateral carbon fluxes contribute to regional carbon budgets as follows: Ecosystem CO2 sink=Ecosystem carbon accumulation+Lateral carbon fluxes. We estimated the contribution of wood and food product trade, of emission and oxidation of reduced carbon species, and of river erosion and transport as lateral carbon fluxes to the carbon balance of Europe (EU-25). The analysis is completed by new estimates of the carbon fluxes of coastal seas. We estimated that lateral transport (all processes combined) is a flux of 165 Tg C yr−1 at the scale of EU-25. The magnitude of lateral transport is thus comparable to current estimates of carbon accumulation in European forests. The main process contributing to the total lateral flux out of Europe is the flux of reduced carbon compounds, corresponding to the sum of non-CO2 gaseous species (CH4, CO, hydrocarbons, . . . ) emitted by ecosystems and exported out of the European boundary layer by the large scale atmospheric circulation. [less ▲]

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See detailImportance of intertidal sediment processes and porewater exchange on the water column biogeochemistry in a pristine mangrove creek (Ras Dege, Tanzania)
Bouillon, Steven; Middelburg, Jack J.; Dehairs, Frank et al

in Biogeosciences (2007), 4

We sampled a tidal creek (Ras Dege, Tanzania) during a 24-h cycle to document the variations in a suite of creek water column characteristics and to determine the relative influence of tidal and ... [more ▼]

We sampled a tidal creek (Ras Dege, Tanzania) during a 24-h cycle to document the variations in a suite of creek water column characteristics and to determine the relative influence of tidal and biological driving forces. Since the creek has no upstream freshwater inputs, highest salinity was observed at low tide, due to evaporation effects and porewater seepage. Total suspended matter (TSM) and particulate organic carbon (POC) showed distinct maxima at periods of highest water flow, indicating that erosion of surface sediments and/or resuspension of bottom sediments were an important source of particulate material. Dissolved organic carbon (DOC), in contrast, varied in phase with water height and was highest at low tide. Stable isotope data of POC and DOC displayed large variations in both pools, and similarly followed the variations in water height. Although the variation of 13CDOC (−23.8 to −13.8‰) was higher than that of 13CPOC (−26.2 to −20.5‰), due to the different endmember pool sizes, the 13C signatures of both pools differed only slightly at low tide, but up to 9‰ at high tide. Thus, at low tide both DOC and POC originated from mangrove production. At high tide, however, the DOC pool had signatures consistent with a high contribution of seagrass-derived material, whereas the POC pool was dominated by marine phytoplankton. Daily variations in CH4, and partial pressure of CO2 (pCO2) were similarly governed by tidal influence and were up to 7- and 10-fold higher at low tide, which stresses the importance of exchange of porewater and diffusive fluxes to the water column. When assuming that the high dissolved inorganic carbon (DIC) levels in the upper parts of the creek (i.e. at low tide) are due to inputs from mineralization, 13C data on DIC indicate that the organic matter source for mineralization had a signature of −22.4‰. Hence, imported POC and DOC from the marine environment contributes strongly to overall mineralization within the mangrove system. Our data demonstrate how biogeochemical processes in the intertidal zone appear to be prominent drivers of element concentrations and isotope signatures in the water column, and how pathways of dissolved and particulate matter transport are fundamentally different. [less ▲]

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