Publications of Fleur Roland
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See detail5.1 The Meuse River basin
Descy, Jean-Pierre ULiege; Latli, A; Roland, Fleur ULiege et al

in Rivers of Europe (Second Edition) (2022)

The Meuse is an international river that has been used by man for centuries and it is still the main source of drinking water for large cities in Belgium and the Netherlands. In fact, water quantity and ... [more ▼]

The Meuse is an international river that has been used by man for centuries and it is still the main source of drinking water for large cities in Belgium and the Netherlands. In fact, water quantity and quality have been a major issue between the various riparian countries and political regions. Many kinds of data have been generated in the past decades on various aspects of the river: (a) hydrology for the need of predicting and controlling floods; (b) water chemistry in the context of water pollution assessment and control; and (c) biology and ecology for water quality assessment and studies on aquatic biodiversity community dynamics and ecosystem function. [less ▲]

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See detailNitrate-dependent anaerobic methane oxidation and chemolithotrophic denitrification in a temperate eutrophic lake
Roland, Fleur ULiege; Borges, Alberto ULiege; Bouillon, Steven et al

in FEMS Microbiology Ecology (2021), 97(10 (fiab124)), 1-10

While the emissions of methane (CH4) by the natural systems have been widely investigated, the aquatic sinks are still poorly constrained. Here, we investigated CH4 cycle and its interactions with ... [more ▼]

While the emissions of methane (CH4) by the natural systems have been widely investigated, the aquatic sinks are still poorly constrained. Here, we investigated CH4 cycle and its interactions with nitrogen (N), iron (Fe) and manganese (Mn) cycles in the oxic-anoxic interface and deep anoxic waters of a small, meromictic and eutrophic lake, during two summertime sampling campaigns. Anaerobic CH4 oxidation (AOM) was measured from the temporal decrease of CH4 concentrations, with addition of 3 potential electron acceptors (NO3–, iron oxides (Fe(OH)3) and manganese oxides (MnO2)). Experiments with addition of either 15N-labeled nitrate (15N-NO3–) or 15N-NO3– combined with sulfide (H2S), to measure denitrification, chemolithotrophic denitrification and anaerobic ammonium oxidation (anammox) rates were also performed. Measurements showed AOM rates up to 3.8 µmol CH4 L–1 d–1 that strongly increased with the addition of NO3– and moderately increased with the addition of Fe(OH)3. No stimulation was observed with MnO2 added. Potential denitrification and anammox rates up to 63 and 0.27 µmol N2 L–1 d–1, respectively, were measured when only 15N-NO3– was added. When H2S was added, both denitrification and anammox rates increased. Altogether, these results suggest that prokaryote communities in the redoxcline are able to efficiently use the most available substrates. [less ▲]

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See detailMethane paradox in tropical lakes? Sedimentary fluxes rather than pelagic production in oxic conditions sustain methanotrophy and emissions to the atmosphere
Morana, Cédric ULiege; Bouillon, S; Nolla-Ardèvol, V et al

Conference (2021, June 22)

Despite growing evidence that methane (CH4) formation could also occur in well-oxygenated surface freshwaters, its significance at the ecosystem scale is uncertain. Empirical models based on data gathered ... [more ▼]

Despite growing evidence that methane (CH4) formation could also occur in well-oxygenated surface freshwaters, its significance at the ecosystem scale is uncertain. Empirical models based on data gathered at high latitude predict that the contribution of oxic CH4 increases with lake size and should represent the majority of CH4 emissions in large lakes. However, such predictive models could not directly apply to tropical lakes which differ from their temperate counterparts in some fundamental characteristics, such as year-round elevated water temperature. We conducted stable isotope tracer experiments which revealed that oxic CH4 production is closely related to phytoplankton metabolism, and is a common feature in five contrasting African lakes. Nevertheless, methanotrophic activity in surface waters and CH4 emissions to the atmosphere were predominantly fuelled by CH4 generated in sediments and physically transported to the surface. Indeed, CH4 bubble dissolution flux and diffusive benthic CH4 flux were several orders of magnitude higher than CH4 production in surface waters. Microbial CH4 consumption dramatically decreased with increasing sunlight intensity, suggesting that the freshwater “CH4 paradox” might be also partly explained by photo-inhibition of CH4 oxidizers in the illuminated zone. Sunlight appeared as an overlooked but important factor determining the CH4 dynamics in surface waters, directly affecting its production by photoautotrophs and consumption by methanotrophs. [less ▲]

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See detailThe possible occurrence of iron-dependent anaerobic methane oxidation in an Ancient ocean analog
Roland, Fleur ULiege; Borges, Alberto ULiege; Darchambeau, François ULiege et al

in Scientific Reports (2021), 11(1597), 1-11

In the ferruginous and anoxic early Earth oceans, photoferrotrophy drove most of the biological production before the advent of oxygenic photosynthesis, but its association with ferric iron (Fe 3 ... [more ▼]

In the ferruginous and anoxic early Earth oceans, photoferrotrophy drove most of the biological production before the advent of oxygenic photosynthesis, but its association with ferric iron (Fe 3+) dependent anaerobic methane (CH 4) oxidation (AOM) has been poorly investigated. We studied AOM in Kabuno Bay, a modern analogue to the Archean Ocean (anoxic bottom waters and dissolved Fe concentrations > 600 µmol L −1). Aerobic and anaerobic CH 4 oxidation rates up to 0.12 ± 0.03 and 51 ± 1 µmol L −1 d −1 , respectively, were put in evidence. In the Fe oxidation-reduction zone, we observed high concentration of Bacteriochlorophyll e (biomarker of the anoxygenic photoautotrophs), which co-occurred with the maximum CH 4 oxidation peaks, and a high abundance of Candidatus Methanoperedens, which can couple AOM to Fe 3+ reduction. In addition, comparison of measured CH 4 oxidation rates with electron acceptor fluxes suggest that AOM could mainly rely on Fe 3+ produced by photoferrotrophs. Further experiments specifically targeted to investigate the interactions between photoferrotrophs and AOM would be of considerable interest. Indeed, ferric Fe 3+-driven AOM has been poorly envisaged as a possible metabolic process in the Archean ocean, but this can potentially change the conceptualization and modelling of metabolic and geochemical processes controlling climate conditions in the Early Earth. [less ▲]

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See detail"Human impacts on ecosystem health and resources of Lake Edward" Final Report
Borges, Alberto ULiege; Lambert, Thibault ULiege; Descy, Jean-Pierre ULiege et al

Report (2021)

Lake Edward is one of the East African Rift lakes (surface 2325 km2, maximum depth 112 m), is located on the border between the Democratic Republic of Congo (DRC) and Uganda. It is connected to the ... [more ▼]

Lake Edward is one of the East African Rift lakes (surface 2325 km2, maximum depth 112 m), is located on the border between the Democratic Republic of Congo (DRC) and Uganda. It is connected to the smaller lake George (surface 250 km2, maximum depth 3m) through Kazinga Channel, and is part of the Nile watershed. On both sides of the border, Lake Edward is bordered by two major natural reserves, the Virunga National Park, Queen Elisabeth National Park. A collapse of the fisheries of Lake Edward was observed from the 1960’s to 1980’s that matched the reduction of the numbers of hippos observed from the 1960s and 1970s. The reason of this collapse is unknown, nor if fisheries continued to decline until present. The aim of HIPE was to test the causal relationship between the recent environmental changes and the drastic reduction of fisheries productivity. Our working hypothesis was that several environmental pressures in the watershed of Lake Edward have disrupted the biogeochemical, structural and functional links between the terrestrial and aquatic ecosystems, leading to a collapse of the main ecosystem service provided by Lake Edward. The fishing effort has dramatically increased since the 1960s. This in combination with a relaxed enforcement, resulted in an overexploitation of the fish stock and a spread of the use of damaging fishing techniques around the lake. Other potential causes are changes in the hydrological cycle, which could have also modified the transfer of nutrients from the watershed to the lake, and climate variability affecting nutrient cycling and availability in the Great Lakes. Shallow sediment cores from Lake Edward show a shift in stable carbon isotope composition in the upper 10 cm of the core (approximately last 40 yrs) that could be explained by (i) an increase in the delivery of C3-derived organic matter to the lake, and/or (ii) a decrease in phytoplankton productivity, which would increase isotope fractionation in phytoplankton due to alleviation of diffusive CO2 limitation. Museum-archived fish specimens confirm a shift in the baseline δ13C values in the lake, corresponding to a lower productivity or a change in the composition of terrestrial organic matter input to L. Edward over the past century. A range of bivalve shells analyzed for C and O isotope ratios along their growth lines indicate these are excellent tools to differentiate habitats, but this compromises their use as archives for paleo-environmental reconstruction in the area since exact provenance of museum collection specimens is typically not sufficiently well documented. The data from tooth enamel from Hippopotamus canines show a long-term trend towards decreasing δ13C values of the Hippopotamus diet in the region, although showing a high inter-individual variability in δ13C values, most pronounced in more recent specimens, indicating that dietary differences exist on small spatial scales. As Hippopotamus are thought to be rather indiscriminate grazers, this should reflect local differences in relative C3-C4 cover. The oxygen stable isotope data of Hippopotamus were shown to be likely good proxies for their lacustrine versus riverine habitat use. The phytoplankton community in Lake Edward was largely dominated cyanobacteria (60% of the phytoplankton biomass), followed by diatoms (25% of the phytoplankton biomass), and by green algae, chrysophytes and cryptophytes. 248 taxa phytoplankton were identified with clear prevalence of cyanobacteria (104 taxa), from the morphological groups of coccal and filamentous species (non-heterocytous and heterocytous). Compared to historical data from the 1930’s it seems a shift in algal diversity (number of species) from diatoms to cyanobacteria. Lack of historical data does not allow to determine if a change in total phytoplankton biomass occurred. The primary production measured in Lake Edward during the three cruises was approximately 4 times lower than the single historical value (June 1960), but values seemed comparable in Lake George compared to those from the 1960’s. This is consistent with an overall decrease of present phosphorus loading compared to historical data (1990’s). Primary production rates were high, and in excess of community respiration, indicating a net autotrophic status leading to low values of dissolved CO2. Consequently, Lake George and the part of Lake Edward influenced by the outflow from Lake George acted as sinks for atmospheric CO2. This challenges the paradigm that inland waters are systematically sources of atmospheric CO2 that was derived from data mostly collected in boreal and temperate lakes. The sampled sites were in some cases very strong CH4 emitters to the atmosphere, although a very strong spatial variability was observed due to a combination of several factors such as the occurrence of anoxia in bottom waters and productivity, both related to depth (the shallowest systems being the most productive and devoid of bottom anoxia). Most systems were sinks of atmospheric N2O due to denitrification. Data on stable C, N and H isotope composition indicate that aquatic primary production is the dominant energy source at the basis of the foodweb, but the wide variability in consumer stable isotope signatures also indicates that a wider variety of sources is used, with contributions of terrestrial and possibly CH4-derived carbon. The basin of Lake Edward has a fish fauna that is typical for East Africa and consists of two components: a species-rich assemblage of Haplochromis species and a relatively species-poor assemblage of non-Haplochromis species. A total of 34 non-Haplochromis species belonging to 10 families and 21 genera are recorded from the system. These include six species of major importance to the fisheries. Three non-Haplochromis species are endemic to the system and two others have been introduced in the region. Six species are new records for the Lake Edward system. The Haplochromis species diversity was seriously underreported and went from 27 species known to 56, of which 13 new ones have been or are being described. We have currently no indication that species have disappeared from the system in the last few decades, though one catfish species known from historical records, H. longifillis, could not be retrieved during the recent expeditions. Substantial dietary niche overlaps were found between several commercially important fish species, while niche overlaps between Haplochromis species were small, which suggests that they display a high trophic differentiation in accordance with their specialised morphologies. We did not find indications that different stocks exist for the commercial species. However, we found that the populations in the lakes were separated from those inhabiting the rivers. We did find intraspecific differences in morphology and ecology between populations from Lake Edward and Lake George (including the Kazinga Channel) of some Haplochromis species. No signs of large-scale hybridization were found between the two important tilapias. Recent reports on the state of the fisheries of the Lake Edward system remain scarce. Lakes Edward and George are almost completely within protected areas. However, occupied land area, the number of inhabitants and the number of boats has risen substantially during the last decades. Total annual yields have been increasing in the last ten years, but Catch Per Unit Effort (CPUE) appears to be continuously decreasing. Tilapias, which were traditionally dominant in the catches, seem to have crashed in the eighties and their share in the catches keeps decreasing. Recent estimates showed poor stock status for most commercially important species with most stocks defined as either collapsed, recruitment impaired or overfished. Higher catches could be obtained under sustainable management. The immediate target of management should be rebuilding biomass to the biomass at maximum sustainable yield (Bmsy). A survey was conducted at the different landing sites along lakes Edward and George. This revealed that fishers were aware of the changes in the state of the fishery. Fishers attributed declining catches to excessive effort, fishing malpractices, destruction of breeding grounds, and pollution. Communities reported large-scale non-compliance with fisheries regulation, especially at Lake George. [less ▲]

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See detailMethane paradox in tropical lakes? Sedimentary fluxes rather than pelagic production in oxic conditions sustain methanotrophy and emissions to the atmosphere
Morana, Cédric ULiege; Bouillon, Steven; Nolla-Ardèvol, V et al

in Biogeosciences (2020), 17(0), 5209-5221

Despite growing evidence that methane (CH4) formation could also occur in well-oxygenated surface freshwaters, its significance at the ecosystem scale is uncertain. Empirical models based on data gathered ... [more ▼]

Despite growing evidence that methane (CH4) formation could also occur in well-oxygenated surface freshwaters, its significance at the ecosystem scale is uncertain. Empirical models based on data gathered at high latitude predict that the contribution of oxic CH4 increases with lake size and should represent the majority of CH4 emissions in large lakes. However, such predictive models could not directly apply to tropical lakes which differ from their temperate counterparts in some fundamental characteristics, such as year-round elevated water temperature. We conducted stable isotope tracer experiments which revealed that oxic CH4 production is closely related to phytoplankton metabolism, and is a common feature in five contrasting African lakes. Nevertheless, methanotrophic activity in surface waters and CH4 emissions to the atmosphere were predominantly fuelled by CH4 generated in sediments and physically transported to the surface. Indeed, CH4 bubble dissolution flux and diffusive benthic CH4 flux were several orders of magnitude higher than CH4 production in surface waters. Microbial 20 CH4 consumption dramatically decreased with increasing sunlight intensity, suggesting that the freshwater “CH4 paradox”might be also partly explained by photo-inhibition of CH4 oxidizers in the illuminated zone. Sunlight appeared as an overlooked but important factor determining the CH4 dynamics in surface waters, directly affecting its production by photoautotrophs and consumption by methanotrophs. [less ▲]

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See detailDiversity and ecology of phytoplankton in Lake Edward (East Africa): Present status and long-term changes
Stoyneva-Gärtner, Maya P.; Morana, Cédric ULiege; Borges, Alberto ULiege et al

in Journal of Great Lakes Research (2020), 46(4), 741-751

Lake Edward is one of the African Rift Valley lakes draining into the Nile River basin. We conducted three sampling series in Lake Edward in October-November 2016, March-April 2017 and January 2018, in ... [more ▼]

Lake Edward is one of the African Rift Valley lakes draining into the Nile River basin. We conducted three sampling series in Lake Edward in October-November 2016, March-April 2017 and January 2018, in distinct seasonal conditions and in several sites varying by depth and proximity to river outlets, including the Kazinga Channel, which connects the hypertrophic Lake George to Lake Edward. The phytoplankton was examined using microscopy and marker pigment analysis by high performance liquid chromatography (HPLC) and subsequent CHEMTAX processing for estimating abundance of phytoplankton groups. Chlorophyll a concentration in the pelagic and littoral open lake sites barely exceeded 10 µg L−1 whereas, in contrast, in the semi-enclosed Bay of Katwe influenced by the Kazinga Channel chlorophyll a was up to 100 µg L−1. Despite substantial seasonal variations of limnological conditions such as photic and mixed layer depths, cyanoprokaryotes/cyanobacteria represented on average 60% of the phytoplankton biomass, followed by diatoms, which contributed ~25% of chlorophyll a, and by green algae, chrysophytes and cryptophytes. 248 taxa were identified with clear prevalence of cyanobacteria (104 taxa), from the morphological groups of coccal and filamentous species (non-heterocytous and heterocytous). The high proportion of heterocytous cyanobacteria, along with a relatively high particulate organic carbon to nitrogen (C:N) ratio, suggest N limitation as well as light limitation, most pronounced in the pelagic sites. During the rainy season, the most abundant diatoms in the plankton were needle-like Nitzschia. Comparison with previous studies found differences in water transparency, total phosphorus, and phytoplankton composition. [less ▲]

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See detailMethane production in oxic surface waters is widespread but insufficient to support methanotrophy in tropical lakes
Morana, Cédric ULiege; Bouillon, S; Nolla-Ardèvol, V et al

Scientific conference (2019, May 19)

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See detailAnaerobic methane oxidation and aerobic methane production in an east African great lake (Lake Kivu)
Roland, Fleur ULiege; Morana, Cédric ULiege; Darchambeau, François ULiege et al

in Journal of Great Lakes Research (2018), 44(6), 1183-1193

We investigated CH4 oxidation in the water column of Lake Kivu, a deep meromictic tropical lake with CH4-rich anoxic deep waters. Depth profiles of dissolved gases (CH4 and N2O) and a diversity of ... [more ▼]

We investigated CH4 oxidation in the water column of Lake Kivu, a deep meromictic tropical lake with CH4-rich anoxic deep waters. Depth profiles of dissolved gases (CH4 and N2O) and a diversity of potential electron acceptors for anaerobic CH4 oxidation (NO3−, SO4 2−, Fe and Mn oxides) were determined during six field campaigns between June 2011 and August 2014. Denitrification measurements based on stable isotope labelling experiments were performed twice. In addition, we quantified aerobic and anaerobic CH4 oxidation, NO3− and SO4 2− consumption rates,with and without the presence of an inhibitor of SO4 2−-reducing bacteria activity. Aerobic CH4 production was also measured in parallel incubations with the addition of an inhibitor of aerobic CH4 oxidation. Themaximu m aerobic and anaerobic CH4 oxidation rates were estimated to be 27±2 and 16±8 μmol/L/d, respectively. We observed a difference in the relative importance of aerobic and anaerobic CH4 oxidation during the rainy and the dry season, with a greater role for aerobic oxidation during the dry season. Lower anaerobic CH4 oxidation rates were measured in presence of molybdate in half of the measurements, suggesting the occurrence of linkage between SO4 2− reduction and anaerobic CH4 oxidation. NO3− consumption and dissolved Mn production rates were never high enough to sustain themeasured anaerobic CH4 oxidation, reinforcing the idea of a coupling between SO4 2− reduction and CH4 oxidation in the anoxic waters of Lake Kivu. Finally, significant rates (up to 0.37 μmol/L/d) of pelagic CH4 production were also measured in oxygenated waters. [less ▲]

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See detailAnaerobic methane oxidation and aerobic methane production in Lake Kivu
Roland, Fleur ULiege; Morana, Cédric ULiege; Darchambeau, François ULiege et al

Conference (2018, September)

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See detailDenitrification, anaerobic ammonium oxidation, and dissimilatory nitrate reduction to ammonium in an East African Great Lake (Lake Kivu)
Roland, Fleur ULiege; Darchambeau, François ULiege; Borges, Alberto ULiege et al

in Limnology and Oceanography (2018), 63(2), 687-701

We investigated anaerobic nitrogen (N) cycling in the water column of Lake Kivu, a deep meromictic tropical lake in East Africa. Data were collected at one station in the Northern Basin and one in the ... [more ▼]

We investigated anaerobic nitrogen (N) cycling in the water column of Lake Kivu, a deep meromictic tropical lake in East Africa. Data were collected at one station in the Northern Basin and one in the Southern Basin, during two sampling campaigns (June 2011—dry season, and February 2012—rainy season). Short-term incubations of sulfide-free water with 15N-labeled substrates revealed high potential denitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates (up to 350 and 36 nmol N produced L−1 h−1, respectively), while anaerobic ammonium oxidation (anammox) was lower (up to 3.3 nmol N produced L−1 h−1). However, anammox rates were 15 nmol N produced L−1 h−1 when 15 NH4+ was added at depths where NH4+ concentrations were very low (< 1 μmol L−1). With the addition of 5 μmol L−1 of 15 NO3− and 10 μmol L−1 of H2S, denitrification and anammox were stimulated in the Northern Basin, while the increase of DNRA rates was less notable. In the Southern Basin, the addition of H2S decreased denitrification rates, probably because of competition with DNRA, which increased, while no effect was observed on anammox. This study puts into evidence the co-occurrence of denitrification, anammox and DNRA, for the first time in a great tropical lake, and underlines the spatial heterogeneity of these processes. Contrary to numerous reports in literature, we show that anammox can significantly occur in presence of H2S, suggesting that the contribution of anammox in the N cycle may be underestimated. [less ▲]

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See detailEffects of agricultural land use on fluvial carbon dioxide, methane and nitrous oxide concentrations in a large European river, the Meuse (Belgium)
Borges, Alberto ULiege; Darchambeau, François ULiege; Lambert, Thibault ULiege et al

in Science of the Total Environment (2018), 610–611(-), 342-355

We report a data-set of CO2, CH4, and N2O concentrations in the surface waters of the Meuse river network in Belgium, obtained during four surveys covering 50 stations (summer 2013 and late winter 2013 ... [more ▼]

We report a data-set of CO2, CH4, and N2O concentrations in the surface waters of the Meuse river network in Belgium, obtained during four surveys covering 50 stations (summer 2013 and late winter 2013, 2014 and 2015), from yearly cycles in four rivers of variable size and catchment land cover, and from 111 groundwater samples. Surface waters of the Meuse river network were over-saturated in CO2, CH4, N2O with respect to atmospheric equilibrium, acting as sources of these greenhouse gases to the atmosphere, although the dissolved gases also showed marked seasonal and spatial variations. Seasonal variations were related to changes in freshwater discharge following the hydrological cycle, with highest concentrations of CO2, CH4, N2O during low water owing to a longer water residence time and lower currents (i.e. lower gas transfer velocities), both contributing to the accumulation of gases in the water column, combined with higher temperatures favourable to microbial processes. Inter-annual differences of discharge also led to differences in CH4 and N2O that were higher in years with prolonged low water periods. Spatial variations were mostly due to differences in land cover over the catchments, with systems dominated by agriculture (croplands and pastures) having higher CO2, CH4, N2O levels than forested systems. This seemed to be related to higher levels of dissolved and particulate organic matter, as well as dissolved inorganic nitrogen in agriculture dominated systems compared to forested ones. Groundwater had very low CH4 concentrations in the shallow and unconfined aquifers (mostly fractured limestones) of the Meuse basin, hence, should not contribute significantly to the high CH4 levels in surface riverine waters. Owing to high dissolved concentrations, groundwater could potentially transfer important quantities of CO2 and N2O to surface waters of the Meuse basin, although this hypothesis remains to be tested. [less ▲]

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See detailBiogeochemical processing of greenhouse gases (methane and nitrous oxide) in meromictic lakes
Roland, Fleur ULiege

Doctoral thesis (2017)

During this study, we focused on the biogeochemical cycles of carbon and nitrogen in two tropical lakes, Lake Kivu and one of its bays, Kabuno Bay, located on the border between the Democratic Republic of ... [more ▼]

During this study, we focused on the biogeochemical cycles of carbon and nitrogen in two tropical lakes, Lake Kivu and one of its bays, Kabuno Bay, located on the border between the Democratic Republic of the Congo and Rwanda, and a small temperate lake located in a limestone quarry in Belgium (Lake Dendre). Seasonal and inter-annual variations in methane (CH4) and nitrous oxide (N2O) fluxes to the atmosphere in Lake Kivu were calculated based on the concentrations of these two elements measured monthly for almost two consecutive years. These data show that Lake Kivu is a low CH4 emitter throughout the year (mean of 86 µmol m-2 d-1), in proportion to the high levels of CH4 present in its deep waters, and alternates between a source and a sink of N2O. The oxidation of CH4 has been proposed to explain these low emissions to the atmosphere. This study concentrated particularly on the detection of the anaerobic oxidation of CH4 (AOM) within the water column and on the identification of the potential electron acceptors: nitrate (NO3-), nitrite (NO2-), sulfate (SO42-), iron (Fe) and manganese (Mn). Significant levels of AOM, up to 16 and 75 μmol m-2 d-1, were found in the water column of Lake Kivu and Kabuno Bay, respectively, but the identification of the potential electron acceptors is not so obvious. At Kabuno Bay, which is considered as a ferruginous basin, Fe seems to be the most likely electron acceptor, since NO3-, NO2- and Mn concentrations are very low, and the sulfur cycle seems to be not really developed. Despite the high concentrations of SO42- measured in oxic waters (up to 600 μmol L-1), concentrations of sulfide (HS-) remained very low in anoxic waters (<1 μmol L- 1), suggesting a poor occurrence of SO42- reduction. In Lake Kivu, the main electron acceptor is most likely SO42- in view of the high concentrations recorded (up to 225 μmol L-1) compared to the concentrations of the other elements (NO3-: <10 μmol L-1, NO2- <1.5 μmol L-1, Mn and Fe total <15 μmol L-1), and high HS- concentrations (up to 120 µmol L-1 at 80 m depth) suggesting the occurrence of significant SO42- reduction. However, some vertical profiles observed in the rainy season, which showed that AOM levels were higher in the NO3- accumulation zone, suggest that NO3- could be an electron acceptor for AOM, but at a low extent. NO3- concentrations were mostly too low to explain the AOM rates observed, and competition with other processes is most likely too high. Indeed, during this study, we also highlighted the occurrence of denitrification, the dissimilatory reduction of NO3- to ammonium (NH4+) (DNRA) and the anaerobic oxidation of NH4+ (Anammox), which compete for substrates. Finally, since Lake Dendre shares some characteristics with Lake Kivu (ie mainly they are both meromictic and have high CH4 concentrations in their anoxic waters), we also measured CH4 oxidation within the water column and put in evidence AOM rates up to 14 μmol L-1 d-1. Despite these high oxidation rates, Lake Dendre was a large emitter of CH4 for the atmosphere. SO42- was likely the primary electron acceptor, but high concentrations of NO3- (up to 80 μmol L-1) suggest that they could also be used for AOM, since AOM coupled to denitrification is thermodynamically much more favorable than the AOM coupled to the reduction of SO42-. [less ▲]

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See detailNitrous oxide and methane seasonal variability in the epilimnion of a large tropical meromictic lake (Lake Kivu, East-Africa)
Roland, Fleur ULiege; Darchambeau, François ULiege; Morana, Cédric ULiege et al

in Aquatic Sciences (2017), 79(0), 209-218

We report a data-set of monthly vertical profiles obtained from January 2012 to October 2013, from the surface to 70 m depth of nitrous oxide (N2O) and dissolved methane (CH4) in Lake Kivu, a large and ... [more ▼]

We report a data-set of monthly vertical profiles obtained from January 2012 to October 2013, from the surface to 70 m depth of nitrous oxide (N2O) and dissolved methane (CH4) in Lake Kivu, a large and deep meromictic tropical lake (East Africa). Vertical variations of N2O were modest, with ranges of 6--9 and 0--16 nmol L−1 in surface and bottom waters, respectively, and occasionally peaks of N2O (up to 58 nmol L−1) were observed at the oxic-anoxic interface. On the contrary, steep vertical gradients of CH4 were observed with values changing several orders of magnitude from surface (19--103 nmol L−1) to 70 m (113,000--520,000 nmol L−1). Seasonal variations of CH4 were caused by annual cycles of mixing and stratification, during the dry and rainy seasons, respectively. This mixing allowed the establishment of a thick oxic layer (maximum 65 m deep), leading to decreased CH4 concentrations (minimum of 8 nmol L−1), presumably due to bacterial CH4 oxidation. During the stratification period, the oxic mixed layer was thinner (minimum 25 m deep), and an increase of CH4 concentrations in surface waters was observed (maximum of 103 nmol L−1), probably due to a lower integrated CH4 oxidation on the water column. Lake Kivu seasonally alternated between a source and a sink for atmospheric N2O, but on an annual scale was a small source of N2O to the atmosphere (on average 0.43 \textmumol m−2 day−1), while it was a small source of CH4 to the atmosphere throughout the year (on average 86 \textmumol m−2 day−1). Vertical and seasonal variations of N2O are discussed in terms of nitrification and denitrification, although from the present data-set it is not possible to unambiguously identify the main drivers of N2O production. [less ▲]

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See detailEmission and oxidation of methane in a meromictic, eutrophic and temperate lake (Dendre, Belgium)
Roland, Fleur ULiege; Darchambeau, François ULiege; Morana, Cédric ULiege et al

in Chemosphere (2017), 168(-), 756-764

We sampled the water column of the Dendre stone pit lake (Belgium) in spring, summer, autumn and winter. Depth profiles of several physico-chemical variables, nutrients, dissolved gases (CO2, CH4, N2O ... [more ▼]

We sampled the water column of the Dendre stone pit lake (Belgium) in spring, summer, autumn and winter. Depth profiles of several physico-chemical variables, nutrients, dissolved gases (CO2, CH4, N2O), sulfate, sulfide, iron and manganese concentrations and d13C-CH4 were determined. We performed incubation experiments to quantify CH4 oxidation rates, with a focus on anaerobic CH4 oxidation (AOM), without and with an inhibitor of sulfate reduction (molybdate). The evolution of nitrate and sulfate concentrations during the incubations was monitored. The water column was anoxic below 20 m throughout the year, and was thermally stratified in summer and autumn. High partial pressure of CO2 and CH4 and high concentrations of ammonium and phosphate were observed in anoxic waters. Important nitrous oxide and nitrate concentration maxima were also observed (up to 440 nmol L- 1 and 80 mmol L -1, respectively). Vertical profiles of d13C-CH4 unambiguously showed the occurrence of AOM. Important AOM rates (up to 14 mmol L -1 d- 1) were observed and often co-occurred with nitrate consumption peaks, suggesting the occurrence of AOM coupled with nitrate reduction. AOM coupled with sulfate reduction also occurred, since AOM rates tended to be lower when molybdate was added. CH4 oxidation was mostly aerobic (~80% of total oxidation) in spring and winter, and almost exclusively anaerobic in summer and autumn. Despite important CH4 oxidation rates, the estimated CH4 fluxes from the water surface to the atmosphere were high (mean of 732 mmol m- 2 d- 1 in spring, summer and autumn, and up to 12,482 mmol m- 2 d- 1 in winter). [less ▲]

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See detailEffects of human land use on the terrestrial and aquatic sources of fluvial organic matter in a temperate river basin (The Meuse River, Belgium)
Lambert, Thibault ULiege; Bouillon, Steven; Darchambeau, François ULiege et al

in Biogeochemistry (2017), 136(-), 191-211

The impact of human activities on the concentrations and composition of dissolved organic matter (DOM) and particulate organic matter (POM) was investigated in the Walloon Region of the Meuse River basin ... [more ▼]

The impact of human activities on the concentrations and composition of dissolved organic matter (DOM) and particulate organic matter (POM) was investigated in the Walloon Region of the Meuse River basin (Belgium). Water samples were collected at different hydrological periods along a gradient of human disturbance (50 sampling sites ranging from 8.0 to 20,407 km2) and during a 1.5 year monitoring of the Meuse River at the city of Liège. This dataset was completed by the characterization of the DOM pool in groundwaters. The composition of DOM and POM was investigated through elemental (C:N ratios), isotopic ($\delta$13C) and optical measurements including excitation emission matrix fluorescence with parallel factor analysis (EEM--PARAFAC). Land use was a major driver on fluvial OM composition at the regional scale of the Meuse Basin, the composition of both fluvial DOM and POM pools showing a shift toward a more microbial/algal and less plant/soil-derived character as human disturbance increased. The comparison of DOM composition between surface and groundwaters demonstrated that this pattern can be attributed in part to the transformation of terrestrial sources by agricultural practices that promote the decomposition of soil organic matter in agricultural lands and subsequent microbial inputs in terrestrial sources. In parallel, human land had contrasting effects on the autochthonous production of DOM and POM. While the in-stream generation of fresh DOM through biological activity was promoted in urban areas, summer autochthonous POM production was not influenced by land use. Finally, soil erosion by agricultural management practices favored the transfer of terrestrial organic matter via the particulate phase. Stable isotope data suggest that the hydrological transfer of terrestrial DOM and POM in human-impacted catchment are not subject to the same controls, and that physical exchange between these two pools of organic matter is limited. [less ▲]

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See detailIron-dependent nitrogen cycling in a ferruginous lake and the nutrient status of Proterozoic oceans
Michiels, Celine C.; Darchambeau, François ULiege; Roland, Fleur ULiege et al

in Nature Geoscience (2017), 10(0), 217221

Nitrogen limitation during the Proterozoic has been inferred from the great expanse of ocean anoxia under low-O2 atmospheres, which could have promoted NO3- reduction to N2 and fixed N loss from the ocean ... [more ▼]

Nitrogen limitation during the Proterozoic has been inferred from the great expanse of ocean anoxia under low-O2 atmospheres, which could have promoted NO3- reduction to N2 and fixed N loss from the ocean. The deep oceans were Fe rich (ferruginous) during much of this time, yet the dynamics of N cycling under such conditions remain entirely conceptual, as analogue environments are rare today. Here we use incubation experiments to show that a modern ferruginous basin, Kabuno Bay in East Africa, supports high rates of NO3- reduction. Although 60 of this NO3- is reduced to N2 through canonical denitrification, a large fraction (40\%) is reduced to NH4+, leading to N retention rather than loss. We also find that NO3- reduction is Fe dependent, demonstrating that such reactions occur in natural ferruginous water columns. Numerical modelling of ferruginous upwelling systems, informed by our results from Kabuno Bay, demonstrates that NO3- reduction to NH4+ could have enhanced biological production, fuelling sulfate reduction and the development of mid-water euxinia overlying ferruginous deep oceans. This NO3- reduction to NH4+ could also have partly offset a negative feedback on biological production that accompanies oxygenation of the surface ocean. Our results indicate that N loss in ferruginous upwelling systems may not have kept pace with global N fixation at marine phosphorous concentrations (0.04-0.13[thinsp][mu]M) indicated by the rock record. We therefore suggest that global marine biological production under ferruginous ocean conditions in the Proterozoic eon may thus have been P not N limited. [less ▲]

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See detailChemoautotrophy and anoxygenic photosynthesis within the water column of a large meromictic tropical lake (Lake Kivu, East Africa)
Morana, Cédric ULiege; Roland, Fleur ULiege; Crowe, Sean A. et al

in Limnology and Oceanography (2016), 61(4), 1424-1437

We quantified chemoautotrophic and anoxygenic photosynthetic microbial production in the water column of Lake Kivu, a permanently stratified tropical lake situated amidst volcanic activity, and aimed to ... [more ▼]

We quantified chemoautotrophic and anoxygenic photosynthetic microbial production in the water column of Lake Kivu, a permanently stratified tropical lake situated amidst volcanic activity, and aimed to identify the microorganisms involved in these processes through the analysis of their phospholipid fatty acid (PLFA) content and stable isotope (13C) labelling of PLFA in a set of incubation experiments. Data demonstrate the existence of a biogeochemically active chemoautotrophic bacterial community in the redoxcline of Lake Kivu (50–70 m). PLFA data indicate that the bacterial communities are structured vertically in the water column, with a large dissimilarity between the oxic and anoxic waters. Maximum volumetric dark CO2 fixation rates measured in Lake Kivu were in the same range as values reported from H2S-rich marine redoxclines, such as the Black and Baltic Seas, and the Cariaco Basin. Similarly, maximal chemoautotrophic activities in Lake Kivu were observed in sulfidic waters, just below the oxycline. Anoxygenic photosynthetic production was never observed in the main basin of Lake Kivu. However, anoxygenic phototrophs largely dominated CO2 fixation in the illuminated redoxcline of Kabuno Bay, a shallower ferruginous sub-basin. Overall, this study supports the idea that chemoautotrophs and/or anoxygenic photoautotrophs might play an important role in the flow of carbon and energy in permanently stratified tropical ecosystems. In Lake Kivu, these processes significantly contribute to organic matter biosynthesis and exert an indirect control on oxygenic photoautotrophs by shortcircuiting the vertical transport of nutrients to the illuminated and oxygenated surface waters. [less ▲]

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