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See detailDissolved primary production and heterotrophic prokaryote reassimilation in a large oligotrophic tropical lake (Lake Kivu, Eastern Africa)
Morana, Cédric; Sarmento, Hugo; Bouillon, Steven et al

Conference (2012, July 11)

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See detailLake Kivu, Limnology and Biogeochemistry of a Tropical Great Lake
Descy, Jean-Pierre; Darchambeau, François ULg; Schmid, Martin

Book published by Springer (2012)

In the heart of Africa, a unique lake has attracted the attention of scientists since the beginning of the 20th century. At the foot of the Virunga volcano chain, Lake Kivu harbors a vast amount of ... [more ▼]

In the heart of Africa, a unique lake has attracted the attention of scientists since the beginning of the 20th century. At the foot of the Virunga volcano chain, Lake Kivu harbors a vast amount of dissolved carbon dioxide and methane, making it the most dangerous lake on Earth. But the lake also furnishes many goods and services for surrounding populations and may soon become the most important energy supplier in the area. At the beginning of gas exploitation, the time has come to gather the wealth of scientific information acquired during past and present research on Lake Kivu. The eleven chapters cover many aspects of the physics, geochemistry and biology of the lake, with a particular focus on the unique physical and geochemical features of the water column and on the ecological functioning of the surface waters. The impacts of the introduced fish species and the potential impacts of methane exploitation are also summarized. This multi-disciplinary book may also be used as an introduction to the limnology and biogeochemistry of large tropical lakes, as it covers various aspects of the physics, geochemistry, biology and ecology of the African Great Rift lakes. [less ▲]

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See detailLake Kivu Research: Conclusions and Perspectives
Descy, Jean-Pierre; Darchambeau, François ULg; Schmid, Martin

in Descy, Jean-Pierre; Darchambeau, François; Schmid, Martin (Eds.) Lake Kivu, Limnology and Biogeochemistry of a Tropical Great Lake (2012)

In this chapter the knowledge gained from the interdisciplinary research on Lake Kivu presented in the previous chapters is synthesized. The importance of the sublacustrine springs as a driving force for ... [more ▼]

In this chapter the knowledge gained from the interdisciplinary research on Lake Kivu presented in the previous chapters is synthesized. The importance of the sublacustrine springs as a driving force for physical and biogeochemical processes is highlighted, the special properties of the lake’s food web structure are discussed, and the consequences and impacts of both the introduction of a new fish species and methane extraction are evaluated. Finally, a list of open research questions illustrates that Lake Kivu has by far not yet revealed all of its secrets. [less ▲]

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See detailMicrobial Ecology of Lake Kivu
Llirós, Marc; Descy, Jean-Pierre; Libert, Xavier et al

in Descy, Jean-Pierre; Darchambeau, François; Schmid, Martin (Eds.) Lake Kivu, Limnology and Biogeochemistry of a Tropical Great Lake (2012)

We review available data on archaea, bacteria and small eukaryotes in an attempt to provide a general picture of microbial diversity, abundances and microbe-driven processes in Lake Kivu surface and ... [more ▼]

We review available data on archaea, bacteria and small eukaryotes in an attempt to provide a general picture of microbial diversity, abundances and microbe-driven processes in Lake Kivu surface and intermediate waters (ca. 0–100 m). The various water layers present contrasting physical and chemical properties and harbour very different microbial communities supported by the vertical redox structure. For instance, we found a clear vertical segregation of archaeal and bacterial assemblages between the oxic and the anoxic zone of the surface waters. The presence of specific bacterial (e.g. Green Sulfur Bacteria) and archaeal (e.g. ammonia-oxidising archaea) communities and the prevailing physico-chemical conditions point towards the redoxcline as the most active and metabolically diverse water layer. The archaeal assemblage in the surface and intermediate water column layers was mainly composed by the phylum Crenarchaeota , by the recently defined phylum Thaumarchaeota and by the phylum Euryarchaeota . In turn, the bacterial assemblage comprised mainly ubiquitous members of planktonic assemblages of freshwater environments (Actinobacteria, Bacteroidetes and Betaproteobacteria among others) and other less commonly retrieved phyla (e.g. Chlorobi, Clostridium and Deltaproteobacteria). The community of small eukaryotes (<5 µm) mainly comprised Stramenopiles , Alveolata , Cryptophyta , Chytridiomycota , Kinetoplastea and Choanoflagellida, by decreasing order of richness. The total prokaryotic abundance ranged between 0.5 × 10^6 and 2.0 × 10^6 cells mL−1 , with maxima located in the 0–20 m layer, while phycoerythrin-rich Synechococcus-like picocyanobacteria populations were comprised between 0.5 × 10^5 and 2.0 × 10^5 cells mL−1 in the same surface layer. Brown-coloured species of Green Sulfur Bacteria permanently developed at 11m depth in Kabuno Bay and sporadically in the anoxic waters of the lower mixolimnion of the main basin. The mean bacterial production was estimated to 336 mg C m−2 day−1 . First estimates of the re-assimilation by bacterioplankton of dissolved organic matter excreted by phytoplankton showed high values of dissolved primary production (ca. 50% of total production). The bacterial carbon demand can totally be fuelled by phytoplankton production. Overall, recent studies have revealed a high microbial diversity in Lake Kivu, and point towards a central role of microbes in the biogeochemical and ecological functioning of the surface layers, comprising the mixolimnion and the upper chemocline. [less ▲]

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See detailPhytoplankton of Lake Kivu
Sarmento, Hugo; Darchambeau, François ULg; Descy, Jean-Pierre

in Descy, Jean-Pierre; Darchambeau, François; Schmid, Martin (Eds.) Lake Kivu, Limnology and Biogeochemistry of a Tropical Great Lake (2012)

This chapter reviews taxonomic composition, biomass, production and nutrient limitation of the phytoplankton of Lake Kivu. Present Lake Kivu phytoplankton is dominated by cyanobacteria – mainly ... [more ▼]

This chapter reviews taxonomic composition, biomass, production and nutrient limitation of the phytoplankton of Lake Kivu. Present Lake Kivu phytoplankton is dominated by cyanobacteria – mainly Synechococcus spp. and thin filaments of Planktolyngbya limnetica – and by pennate diatoms, among which Nitzschia bacata and Fragilaria danica are dominant. Seasonal shifts occur, with cyanobacteria developing more in the rainy season, and the diatoms in the dry season. Other groups present are cryptophytes, chrysophytes, chlorophytes and dinoflagellates. According to a survey conducted in the period 2002–2008, the composition of the phytoplankton assemblage was quasi homogeneous among lake basins. The mean euphotic depth varied between 17 and 20 m, and the increase in the ratio between mixed layer depth and euphotic depth to about 2 in the dry season may have selected for diatoms and cryptophytes, which tended to present their maximal development in this season, when cyanobacteria slightly decreased. Mean chlorophyll a concentration was 2.16 mg m−3, and the mean daily primary production was 0.62 g C m−2 day−1 (range, 0.14–1.92), i.e. in the same range as in other large oligotrophic East African Rift lakes. Seston elemental ratios indicated a moderate P deficiency during the dry, mixed season and a severe P limitation during part of the rainy, stratified season; the C:N ratio indicated a moderate N limitation throughout the year. Nutrient addition assays pointed to a direct N limitation and co-limitation by P during rainy seasons and P or N limitation during dry seasons depending on the year. Thus, phytoplankton ecology in Lake Kivu does not differ from that of other Rift lakes, where seasonal variations result in a trade-off between low light with high nutrient supply and high light with low nutrient supply. Phytoplankton production in Lake Kivu is also similar to that of other Rift lakes, and nutrient limitation of phytoplankton growth may occur as a result of variable availability of N and P, as in Lakes Tanganyika and Malawi, even though the extent of P limitation seems greater in Lake Kivu. [less ▲]

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See detailZooplankton of Lake Kivu
Darchambeau, François ULg; Isumbisho, Mwapu; Descy, Jean-Pierre

in Descy, Jean-Pierre; Darchambeau, François; Schmid, Martin (Eds.) Lake Kivu, Limnology and Biogeochemistry of a Tropical Great Lake (2012)

The dominant species of the crustacean plankton in Lake Kivu are the cyclopoid copepods Thermocyclops consimilis and Mesocyclops aequatorialis and the cladoceran Diaphanosoma excisum. Mean crustacean ... [more ▼]

The dominant species of the crustacean plankton in Lake Kivu are the cyclopoid copepods Thermocyclops consimilis and Mesocyclops aequatorialis and the cladoceran Diaphanosoma excisum. Mean crustacean biomass over the period 2003–2004 was 0.99 g C m−2. The seasonal dynamics closely followed variations of chlorophyll a concentration and responded well to the dry season phytoplankton peak. The mean annual crustacean production rate was 23 g C m−2 year−1. The mean trophic transfer efficiency between phytoplankton and herbivorous zooplankton was equal to 6.8 %, indicating a coupling between both trophic levels similar to that in other East African Great lakes. These observations suggest a predominant bottom-up control of plankton dynamics and biomass in Lake Kivu. Whereas the present biomass of crustacean plankton in Lake Kivu is comparable to that of other African Rift lakes, the zooplankton biomass before Limnothrissa introduction was 2.6 g C m−2, based on estimation from available historical data. So, if the sardine introduction in the middle of the last century led to a threefold decrease of zooplankton biomass, it did not affect zooplankton production to a level which would lead to the collapse of the food web and of the fishery. [less ▲]

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See detailFirst assessment of the biogeochemistry of the upper Congo River
Darchambeau, François ULg; Bouillon, S.; Borges, Alberto ULg

Poster (2012, April 22)

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See detailVariability of methane in the epilimnion of Lake Kivu
Borges, Alberto ULg; Abril, G.; Morana, C. et al

Poster (2012, April 22)

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See detailEvaluating COSMO’s lake module (FLake) for an East-African lake using a comprehensive set of lake temperature profiles
Thiery, Wim; Martynov, Andrey; Darchambeau, François ULg et al

Conference (2012, April)

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See detailIs the fishery of the introduced Tanganyika sardine (Limnothrissa miodon) in Lake Kivu (East Africa) sustainable?
Guillard, J.; Darchambeau, François ULg; Mulungula, P. M. et al

in Journal of Great Lakes Research (2012), 38(3), 524-533

Limnothrissa miodon, a small pelagic clupeid fish introduced at the end of the 1950s into Lake Kivu, became an important resource for the human populations of this area. The total stock of pelagic fish ... [more ▼]

Limnothrissa miodon, a small pelagic clupeid fish introduced at the end of the 1950s into Lake Kivu, became an important resource for the human populations of this area. The total stock of pelagic fish populations of this lake was estimated in 2008 by two hydroacoustic surveys, using an EK60 split-beam sounder (frequency 70. kHz). The total fish stocks were estimated to be approximately 5000. t in the rainy season and 6000 tons in the dry season. These values are similar to previous estimations performed in the 1980s. During 2008, the stock did not fluctuate throughout the seasons; however, the spatial distributions were different in the two hydrological seasons. Interestingly, the L. miodon stock has appeared to remain stable over the last two decades, which suggests that the pelagic fishery in Lake Kivu has not been overexploited and that it is sustainable. © 2012 Elsevier B.V. [less ▲]

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See detailA descriptive study of physico-chemical characteristics of Posidonia oceanica litter accumulation
Lepoint, Gilles ULg; Borges, Alberto ULg; Darchambeau, François ULg et al

Poster (2012)

The Mediterranean seagrass Posidonia oceanica meadow losses every fall the major part of its leaf biomass after senescing. These phytodetritus may decay within the meadow, be buried or be exported to ... [more ▼]

The Mediterranean seagrass Posidonia oceanica meadow losses every fall the major part of its leaf biomass after senescing. These phytodetritus may decay within the meadow, be buried or be exported to other habitats. They form large litter accumulations, notably on shallow water sand patches. Such accumulation host many organisms which participate to the degradation of this material. In a first step to understand the dynamics of these accumulations and of their associated biota, we have characterised their physico-chemical heterogeneity at different seasons. We measured the dissolved oxygen, nutrients and sulphide concentrations in interstitial waters from litter accumulations varying regarding their phytodetritus composition, fragmentation level and thickness. Results show that oxygen conditions were highly variable depending on litter thickness but also on local hydrodynamics. Anoxic conditions and presence of sulphide were sometimes measured, particularly in very thick litter or in degraded litter at the end of summer. Colonies of sulphur-oxidising bacteria were observed. Litter accumulations were also often enriched in ammonium and, sometimes, in dissolved phosphorus. It is not clear whether this results from the litter degradation within the accumulation or whether this is a consequence of a barrier effect between sediment and water column. Nevertheless, this makes litter accumulations particularly attractive for micro-phytobenthic producers. Litter accumulations appear as key habitats both to understand the dead-face of seagrass dynamics and its consequence for C cycle in coastal areas and to study the consequence of hypoxia on biodiversity in a natural context. [less ▲]

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See detailVariability of carbon dioxide and methane in the epilimnion of Lake Kivu
Borges, Alberto ULg; Bouillon, S.; Abril, G. et al

in Descy, J.-P.; Darchambeau, François; Schmid, M. (Eds.) Lake Kivu: Limnology and biogeochemistry of a tropical great lake (2012)

We report a dataset of the partial pressure of CO2 (pCO2) and me-thane concentrations (CH4) in the surface waters of Lake Kivu ob-tained during four cruises covering the two main seasons (rainy and dry ... [more ▼]

We report a dataset of the partial pressure of CO2 (pCO2) and me-thane concentrations (CH4) in the surface waters of Lake Kivu ob-tained during four cruises covering the two main seasons (rainy and dry). Spatial gradients of surface pCO2 and CH4 concentrations were modest in the main basin. In Kabuno Bay, pCO2 and CH4 concentra-tions in surface waters were higher, owing to the stronger influence of subaquatic springs from depth. Seasonal variations of pCO2 and CH4 in the main basin of Lake Kivu were strongly driven by deepen-ing of the epilimnion and the resulting entrainment of water charac-terized by higher pCO2 and CH4 concentrations. Physical and chem-ical vertical patterns in Kabuno Bay were seasonally stable, owing to a stronger stratification and smaller surface area inducing fetch limi-tation of wind driven turbulence. A global and regional cross-system comparison of pCO2 and CH4 concentrations in surface waters of lakes highlights the peculiarity of Kabuno Bay in terms of pCO2 values in surface waters. In terms of surface CH4 concentrations, both Kabuno Bay and the main basin of Lake Kivu are at the lower end of values in lakes globally, despite the huge amounts of CH4 and CO2 in the deeper layers of the lake. [less ▲]

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See detailFirst assessment of the biogeochemistry of the Congo River and its tributaries
Darchambeau, François ULg; Borges, Alberto ULg; Wabakanghanzi, José Nlandu et al

Scientific conference (2011, November 29)

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See detailVariability of methane in the epilimnion of Lake Kivu
Borges, Alberto ULg; Abril, Gwenael; Delille, Bruno ULg et al

Poster (2011, July 11)

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See detailThe recent introduction of Lamprichthys tanganicanus in Lake Kivu (Eastern Africa): a threat for the pelagic fishery?
Masilya, Pascal M; Isumbisho, Mwapu; Kaningini, M et al

Poster (2011, June 13)

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See detailBiogeochemistry of the Congo river: Preliminary data
Darchambeau, François ULg; Borges, Alberto ULg; Wabakanghanzi, José Nlandu et al

Conference (2011, June 03)

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See detailVariability of methane in the epilimnion of Lake Kivu
Borges, Alberto ULg; Abril, G.; Delille, Bruno ULg et al

Poster (2011, April 08)

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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, April 08)

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See detailPotential impacts of methane extraction on lake ecosystem
Darchambeau, François ULg

Conference (2011, February 08)

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