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See detailProduction of dissolved organic matter by phytoplankton and its uptake by heterotrophic prokaryotes in large tropical lakes
Morana, Cédric; Sarmento, Hugo; Descy, Jean-Pierre et al

in Limnology & Oceanography (2014), 59(4), 1364-1375

In pelagic ecosystems, phytoplankton extracellular release can extensively subsidize the heterotrophic prokaryotic carbon demand. Time-course experiments were carried out to quantify primary production ... [more ▼]

In pelagic ecosystems, phytoplankton extracellular release can extensively subsidize the heterotrophic prokaryotic carbon demand. Time-course experiments were carried out to quantify primary production, phytoplankton excretion, and the microbial uptake of freshly released dissolved organic carbon (DOC) derived from phytoplankton extracellular release (DOCp) in four large tropical lakes distributed along a productivity gradient: Kivu, Edward, Albert, and Victoria. The contributions of the major heterotrophic bacterial groups to the uptake of DOCp was also analyzed in Lake Kivu, using microautoradiography coupled to catalyzed reporter deposition fluorescent in situ hybridization. The percentage of extracellular release (PER) varied across the productivity gradient, with higher values at low productivity. Furthermore, PER was significantly related to high light and low phosphate concentrations in the mixed layer and was comparatively higher in oligotrophic tropical lakes than in their temperate counterparts. Both observations suggest that environmental factors play a key role in the control of phytoplankton excretion. Standing stocks of DOCp were small and generally contributed less than 1% to the total DOC because it was rapidly assimilated by prokaryotes. In other words, there was a tight coupling between the production and the heterotrophic consumption of DOCp. None of the major phylogenetic bacterial groups that were investigated differed in their ability to take up DOCp, in contrast with earlier results reported for standard labeled single-molecule substrates (leucine, glucose, adenosine triphosphate). It supports the idea that the metabolic ability to use DOCp is widespread among heterotrophic prokaryotes. Overall, these results highlight the importance of carbon transfer between phytoplankton and bacterioplankton in large African lakes. [less ▲]

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See detailUnderstanding the performance of the FLake model over two African Great Lakes
Thiery, Wim; Martynov, Andrey; Darchambeau, François ULg et al

in Geoscientific Model Development [=GMD] (2014), 7

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See detailPrimary production in a tropical large lake: The role of phytoplankton composition
Darchambeau, François ULg; Sarmento, Hugo; Descy, Jean-Pierre

in Science of the Total Environment (2014), 473-474

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See detailImpact of the invasive Asian clam, Corbicula spp., on a large river ecosytem
Pigneur, Lise-Marie; Falisse, Elodie; Roland, Kathleen et al

in Freshwater Biology (2014), 59(3), 573-583

During the past recent decades, Asiatic clams (Corbicula spp.) have spread spectacularly in several large European rivers. In the River Meuse, a transnational lowland river, an important chlorophyll a ... [more ▼]

During the past recent decades, Asiatic clams (Corbicula spp.) have spread spectacularly in several large European rivers. In the River Meuse, a transnational lowland river, an important chlorophyll a decline has been recorded since the mid-2000s, which seems to be related to the invasion by these exotic bivalves. This study aimed at verifying that hypothesis, using clam density data from field surveys, water quality monitoring data, and a simulation model. Estimated Corbicula densities were between 50 and 900 individuals m-2, depending on the site. Using a maximum filtration rate per clam body mass of 0.086 m3 g C-1 day-1 at 20°C derived from the literature, we ran simulations with a non-stationary model to estimate the impact of the exotic bivalve on the river plankton and water quality. In the stretches where the invasive clams developed best, comparison with a clam-free scenario allowed estimating a 70 % phytoplankton biomass loss due to their filtration, leading to a 61 % loss of annual primary productivity. Model simulations also showed that zooplankton may have suffered as much as 75 % loss in terms of biomass. The simulations also point to substantial effects of the invasive Corbicula on the river oxygen budget and on nutrient cycling. We expect that, in the heavily regulated sectors of the river, the loss of planktonic production due to the invasive filter-feeders will negatively affect other suspension feeders and alter ecosystem function and productivity. [less ▲]

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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 detailLake Kivu: Past and Present
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)

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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 detailPhytoplankton of the River Loire, France: a biodiversity and modelling study
Descy, Jean-Pierre; Leitao, Maria; Everbecq, Etienne ULg et al

in Journal of Plankton Research (2011)

Most studies of phytoplankton dynamics in lowland rivers have been conducted in regulated rivers. This study deals the phytoplankton of the River Loire, a relatively unregulated river in Western Europe ... [more ▼]

Most studies of phytoplankton dynamics in lowland rivers have been conducted in regulated rivers. This study deals the phytoplankton of the River Loire, a relatively unregulated river in Western Europe, and aims at investigating the factors controlling phytoplankton composition and dynamics in this river. Phytoplankton biodiversity was assessed by pooling data from monthly sampling for 8 years at different sites. A correspondence analysis showed temporal and longitudinal gradients, with a dominance of small centric diatoms and green algae for most of the year, in the middle and lower river sectors. Phytoplankton dynamics were further explored using the POTAMON simulation model, run for the year 2005. The simulations, in agreement with the observations, confirmed the virtual absence of a Stephanodiscus spring peak, and reproduced well the development of small centric diatoms and green algae. Production and loss rates calculated by the model helped us to explain the dynamics of the three main phytoplankton categories, which attained high net production rates due to the low river depth (∼1 m at low discharge), but were subject to high sedimentation losses. Model calculations also showed that P limitation was likely, particularly for green algae, with a reduction of growth rate of up to 35%. In addition, the simulations showed a significant impact of the invasive Asian clam, Corbicula spp., on phytoplankton biomass. This study shows that the factors determining phytoplankton diversity and dynamics in this unregulated river are basically the same as those identified in other lowland rivers, but that key factors are habitat diversity and variation of water level in the river channel. [less ▲]

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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 detailProlonging the lifetime and activity of silica immobilized Cyanidium Caldarium
Rooke, Joanna; Vandoorne, Bertrand; Léonard, Alexandre ULg et al

in Journal of Colloid & Interface Science (2011), 356

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

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

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

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

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See detailCongo River 2010
Darchambeau, François ULg; Borges, Alberto ULg; Wabakanghanzi, José Nlandu et al

Poster (2010, October 04)

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