Dissolved primary production and heterotrophic prokaryote reassimilation in a large oligotrophic tropical lake (Lake Kivu, Eastern Africa)

Conference (2012, July 11)

Microbial Ecology of Lake Kivu

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 ▲]

Zooplankton of Lake Kivu

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 ▲]

Phytoplankton of the River Loire, France: a biodiversity and modelling study

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 ▲]

The recent introduction of Lamprichthys tanganicanus in Lake Kivu (Eastern Africa): a threat for the pelagic fishery?

Poster (2011, June 13)

Diet overlap between the newly introduced Lamprichthys tanganicanus and the Tanganyika sardine in Lake Kivu, Eastern Africa

in Hydrobiologia (2011), 675(1),

Diffusive methane emissions to the atmosphere from Lake Kivu (Eastern Africa)

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 ▲]

A paleolimnological study of Lake Kivu (East Africa)

Conference (2010, May 13)

The ecology of Lake Kivu: a puzzle solved?

Conference (2010, January 14)

Spatio-temporal dynamics of phytoplankton and primary production in Lake Tanganyika using a MODIS based bio-optical time series

in Remote Sensing of Environment (2010)

Lake Tanganyika, the second largest freshwater ecosystem in Africa, is characterised by a significant heterogeneity in phytoplankton concentration linked to its particular hydrodynamics. To gather a ... [more ▼]

Lake Tanganyika, the second largest freshwater ecosystem in Africa, is characterised by a significant heterogeneity in phytoplankton concentration linked to its particular hydrodynamics. To gather a proper understanding of primary production, it is necessary to consider spatial and temporal dynamics throughout the lake. In the present work, daily MODIS-AQUA satellite measurements were used to estimate chlorophyll-a concentrations and the diffuse attenuation coefficient (K490) for surface waters. The spatial regionalisation of Lake Tanganyika, based on Empirical Orthogonal Functions of the chlorophyll-a dataset (July 2002–November 2005), allowed for the separation of the lake in 11 spatially coherent and co-varying regions, with 2 delocalised coastal regions. Temporal patterns of chlorophyll-a showed significant differences between regions. Estimation of the daily primary production in each region indicates that the dry season is more productive than the wet season in all regions with few exceptions. Whole-lake daily primary productivity calculated on an annual basis (2003) was 646±142 mgC m−2 day−1. Comparing our estimation to previous studies, photosynthetic production in Lake Tanganyika appears to be presently lower (about 15 %), which is consistent with other studies which used phytoplankton biovolume and changes of δ13C in the lake sediments. The decrease in lake productivity in recent decades may be associated to changes in climate conditions. [less ▲]