Abstract: Herbivory and bacterivory by phagotrophic protists were estimated in the southern basin of the oligotrophic Lake Tanganyika at different seasons (in the rainy season in FebruaryâMarch 2007 and in the dry season in JulyâAugust 2006 and September 2007), using two independent methods: the selective inhibitor technique for assessing community grazing on picocyanobacteria (PCya) and fluorescently labelled bacteria (FLB) and Synechococcus (FLA) to estimate bacterivory and herbivory by phagotrophic nanoflagellates (NF) and ciliates. Protistan grazing impact on both heterotrophic bacteria and PCya was mainly due to NF, which contributed up to 96% of the microbial grazing. There was a clear selection of FLA by protists. PCya represented the main carbon source for both flagellates and ciliates in the mixolimnion, accounting for an average of 83% of the total carbon obtained from the ingestion of picoplanktonic organisms. Protists were the main consumers of particulate primary production (46â74% depending on season). Significant seasonal variation of grazing rates (0.011â0.041 hâ1) was found, chiefly following variation of PCya production and biomass. Assuming a growth efficiency of 0.4, total protozoan production varied seasonally (189â313 g C mâ2 dayâ1) and was roughly half of particulate phytoplankton production. This study provides evidence that NF and PCya were tightly coupled in Lake Tanganyika and that herbivory by protists may be one of the reasons why this great lake has high productivity. Our results bring support to the idea that microbial herbivory is a major process in oligotrophic freshwater systems.
Abstract: In aquatic environments, small eukaryotes (mainly algae and protozoa of 1 to 5 µm in size) are a key link in the carbon transfer to higher trophic levels, e.g. through primary production and grazing of picoplankton. However, the diversity of these microorganisms remains poorly investigated in freshwater habitats, and is still unknown in tropical aquatic systems. In this study, we investigated the small-eukaryote diversity in the oligotrophic Lake Tanganyika, one of the African Great Lakes, at different depths in the water column using denaturing gradient gel electrophoresis (DGGE) and gene clone libraries based on 18S rRNA genes. Each sample produced complex DGGE fingerprints clearly discriminating the epilimnion from the metalimnion. Analysis, using genetic libraries, confirmed the high level of small-eukaryote diversity in Lake Tanganyika. Organisms from 5 taxonomic groups (Stramenopiles, Alveolata, Cryptophyta, Kinetoplastea and Choanoflagellida) were dominant among the species detected. Some sequences were nearly identical to those recovered in temperate freshwaters in North America and Europe, suggesting a high dispersal ability in some small-eukaryote lineages. However, 49% of sequences were <95% similar to any sequence in GenBank. This may result from undersampling of freshwater systems, but also raises the possibility that perennially warm tropical waters harbour particular assemblages of planktonic small eukaryotes.
Abstract: In keeping with the theme of this volume, the present article commemorates the 50 years of Hutchinsonâs (Am Nat 93:145â159, 1959) famous publication on the âvery general question of animal diversityâ, which obviously leads to the more important question regarding the driving forces of biodiversity and their limitation in various habitats. The study of phytoplankton in large lakes is a challenging task which requires the use of a wide variety of techniques to capture the range of spatial and temporal variations. The analysis of marker pigments may provide an adequate tool for phytoplankton surveys in large water bodies, thanks to automated analysis for processing numerous individual samples, and by achieving sufficient taxonomic resolution for ecological studies. Chlorophylls and carotenoids were analysed by HPLC in water column samples of Lake Tanganyika from 2002 through 2006, at two study sites, off Kigoma (north basin) and off Mpulungu (south basin). Using the CHEMTAX software for calculating contributions of the main algal groups to chlorophyll a, variations of phytoplankton composition and biomass were determined. We also investigated selected samples according to standard taxonomic techniques for elucidating the dominant species composition. Most of the phytoplankton biomass was located in the 0â40 m layer, with maxima at 0 or 20 m, and more rarely at 40 m. Deep chlorophyll maxima (DCM) and surface âbloomsâ were occasionally observed. The phytoplankton assemblage was essentially dominated by chlorophytes and cyanobacteria, with diatoms developing mainly in the dry season. The dominant cyanobacteria were very small unicells (mostly Synechococcus), which were much more abundant in the southern basin, whereas green algae dominated on average at the northern site. A canonical correspondence analysis (CCA) including the main limnological variables, dissolved nutrients and zooplankton abundance was run to explore environmentâphytoplankton relations. The CCA points to physical factors, site and season as key determinants of the phytoplankton assemblage, but also indicates a significant role, depending on the studied site, of calanoid copepods and of nauplii stages. Our data suggest that the factors allowing coexistence of several phytoplankton taxa in the pelagic zone of Lake Tanganyika are likely differential vertical distribution in the water column, which allows spatial partitioning of light and nutrients, and temporal variability (occurring at time scales preventing long-term dominance by a single taxon), along with effects of predation by grazers.
Abstract: Photosynthetic picoplankton (PPP), particularly picocyanobacteria (PCya), are known to be a major component of phytoplankton in oligotrophic waters. We investigated the dynamics of PPP and heterotrophic bacteria (HBact) in Lake Tanganyika during the dry and rainy seasons of 2004 to 2007, in the two basins of this large lake. Flow cytometry analyses showed that PPP was mainly composed by PCya of the phycoerythrin-rich Synechococcus type, with maximal abundances (2.3 x 104â8.2 x 105 cells mLâ1) found in the mixolimnion (10â20 m depth). PPP biomass integrated over the water column depth ranged between 0.41 and 3.09 g C mâ2, with maximal values in the south basin during the dry season (2.28 ± 0.62 g C mâ2 on average). The contribution of PPP to total phytoplankton biomass ranged from 41 to 99%, with highest values in the south basin in the dry season. Cellular measurements by image analysis of epifluorescence microscopy images showed a significant increase of the cell volume of the PCya during this period. Flow cytometry also allowed enumeration of photosynthetic picoeukaryotes. Assuming a conversion factor of 530 fg C cellâ1, they contributed on average to 6% to PPP biomass, except during the dry season in the south basin, where their contribution increased to up to 20% of PPP biomass. Integrated over a 100-m water column depth, PCya biomass was on average 1.4 times higher than HBact biomass. This study establishes reference values for the biomass contribution of this part of the microbial food web, covering for the first time, spatial (different sites), vertical (profiles at different depths) and seasonal variations on a multi-year basis. The results strengthen the view of a major role of PPP in the pelagic food web of large tropical Lake Tanganyika.
Abstract: Abundance and production (BP) of heterotrophic bacteria (HBact) were measured in Lake Tanganyika, East Africa, during seasonal sampling series between 2002 and 2007, in both north and south basins of the lake. The major objective of the study was to assess whether BP can supplement phytoplankton production (PP) in the pelagic waters, and whether BP and PP are related in this large lake. Bacteria were enumerated in the 0â100 m surface layer by epifluorescence microscopy and flow cytometry; BP was quantified using 3H-thymidine incorporation, usually in three mixolimnion layers (0â40 m, 40â60 m and 60â100 m). Flow cytometry allowed distinguishing three subpopulations: LNA (low nucleic acid content bacteria), HNA (high nucleic acid content bacteria) and Synechococcus-like picocyanobacteria (PCya). HBact abundance was comprised between 1.2 x 10^5 â 4.8 x 10^6 cells/ml, and was maximal in the 0-40 m layer (i.e. roughly, the euphotic layer). Using a single conversion factor of 15 fg C/cell, estimated from biovolume measurements, average HBact biomass (integrated over a 100-m water column depth) was 1.89 ± 1.05 g C/m^2. The proportion of HNA was on average 67 % of total bacterial abundance, and tended to increase with depth. The range of BP integrated over the 0â100 m layer was 93 â 735 mg C/(m^2 d^1), and overlapped with the range of phytoplankton particulate production (150 â 1687 mg C/(m^2 d^1)) measured in the same period of time at the same sites. Depth-integrated BP was significantly correlated to PP and chlorophyll a, and BP in the euphotic layer was on average 25 % of PP. These results suggest that HBact contribute substantially to the production of organic carbon in Lake Tanganyika, and that BP may be sustained by phytoplankton-derived organic carbon in the pelagic waters.
Abstract: 1. We used flow cytometry to characterize freshwater photosynthetic picoplankton (PPP) and heterotrophic bacteria (HB) in Lake Kivu, one of the East-African great lakes. Throughout three cruises run in different seasons, covering the four major basins, phycoerythrin-rich cells dominated the PPP. Heterotrophic bacteria and PPP cell numbers were always high and spatial variations were modest. This represents an important difference from temperate and high latitude lakes that show high fluctuations in cell abundance over an annual cycle.
2. Three populations of picocyanobacteria were identified: one corresponded to single-cells (identified as Synechococcus by epifluorescence microscopy, molecular methods and pigment content), and the two other that most probably correspond to two and four celled colonies of the same taxon. The proportion of these two subpopulations was greater under stratified conditions, with stronger nutrient limitation.
3. High PPP concentrations (c. 105 cell mLâ1) relative to HB (c. 106 cell mLâ1) were always found. Lake Kivu supports relatively less bacteria than phytoplankton biomass than temperate systems, probably as a consequence of factors such as temperature, oligotrophy, nutrient limitation and trophic structure.
4. A review of PPP concentration across aquatic systems suggests that the abundance of Synechococcus-like cyanobacteria in large, oligotrophic, tropical lakes is very high.
5. Photosynthetic picoplankton cell abundances in the oligotrophic tropical lakes Kivu and Tanganyika are comparable to those of eutrophic temperate lakes. This apparently contradicts the view that PPP abundance increases with increasing eutrophy. More data on PPP in tropical lakes are needed to explore further this particular pattern.
Abstract: 1. This study focused on phytoplankton production in Lake Tanganyika. We provide new estimates of daily and annual primary production, as well as growth rates of phytoplankton, and we compare them with values published in former studies.
2. Chlorophyll-a (chl-a) in the mixed layer ranged from 5 to 120 mg chl-a mâ2 and varied significantly between rainy and dry seasons. Particulate organic carbon concentrations were significantly higher in the south basin (with 196 and 166 mg C mâ3 in the dry and the rainy season, respectively) than in the north basin (112 and 109 mg C mâ3, respectively).
3. Carbon : phosphorus (C : P) ratios varied according to season. Phosphorus limitation seemed to occur more frequently than nitrogen limitation, especially during the rainy season. Severe P deficiencies were rare.
4. Measured particulate daily primary production ranged from 110 to 1410 mg C mâ2 dayâ1; seasonal contrasts were well marked in the north basin, but less in the south basin, where primary production peaks occurred also in the rainy season. Estimates of annual primary production, based on daily primary production calculated from chl-a and water transparency, gave values lower than those reported in previous studies. Picophytoplankton accounted on average for 56% of total particulate production in the south basin during the wet season of 2003.
5. Phytoplankton growth rates, calculated from primary production, ranged from 0.055 to 0.282 dayâ1; these are lower than previously published values for Lake Tanganyika.
Abstract: 1. A 2-year (2002â2003) survey of chlorophyll and carotenoid pigments is reported for two off-shore stations of Lake Tanganyika, Kigoma (Tanzania) and Mpulungu (Zambia), and from three cruises between those sites. Chlorophyll a concentrations were low (0.3â3.4 mg mâ3) and average chlorophyll a integrated through the 100 m water column were similar for both stations and years (36.4â41.3 mg mâ2). Most pigments were located in the 0â60 m layer and decreased sharply downward. Chlorophyll a degradation products (phaeophytins and phaeophorbides) were detected at 100 m depth, whereas carotenoids became undetectable. Temporal and seasonal variation of the vertical distribution of pigments was high.
2. The biomass of phytoplankton groups was calculated from marker pigment concentrations over the 0â100 m water column using the CHEMTAX software. On average for the study period, chlorophytes dominated in the northern station, followed by cyanobacteria T1 (type 1, or Synechococcus pigment type), whereas cyanobacteria T1 dominated in the south. Cyanobacteria T2 (type 2, containing echinenone), presumably corresponding to filamentous taxa, were detected in the rainy season. Diatoms (and chrysophytes) developed better in the dry season conditions, with a deep mixed layer and increased nutrient availability. Very large variation in the vertical distribution of algal groups was observed.
3. Our observations on phytoplankton composition are broadly consistent with those from previous studies. Our pigment data provide evidence for the lake-wide importance of picocyanobacteria and high interannual variation and spatial heterogeneity of phytoplankton in Lake Tanganyika, which may render difficult assessment of long-term changes in phytoplankton driven by climate change.
Abstract:
Progress in science always come along with the advent of new technologies. Underestimated for a long time, the crucial role of the microorganisms in aquatic ecosystems has only been recognized in the 1980s, when new techniques such as epifluorescence microscopy and flow cytometry revealed the extremely high abundances of these microscopic organisms. The picoplankton, defined as the planktonic organisms smaller than two micrometers, is now widely studied in the oceans and temperate lakes, where it contributes to the main part of the planktonic production and biomass, especially in the oligotrophic areas.
Located in East Africa, Lake Tanganyika is the second deepest lake in the world. With an area similar to that of Belgium, it is also known for its highly productive fisheries. Its nutrients poor and warm waters are a favourable environment for the picoplankton. Actually, picoplankton has a competitive advantage compared to bigger phytoplankton : its high surface-to-volume ratio allows a more rapid assimilation of the nutrients. However, excepted some recent studies suggesting that more than 50 % of the phytoplankton biomass in Lake Tanganyika is picoplankton (mainly composed of picocyanobacteria 'Synechococcus'), few data on the picoplankton of great tropical lakes are available.
On the basis of a multidisciplinary approach, this work was aim to characterize the picoplankton of the Lake Tanganyika, in order to identify its composition, and then to evaluate its distribution along the lake, by covering the spatial variability (north and south basins) and the temporal variability (wet and dry seasons). By combining flow cytometry and epifluorescence microscopy, abundance and biomass of these microorganisms were estimated. Among photosynthetic picoplankton, picocyanobacteria diversity was assessed by using two molecular techniques, DGGE and clone libraries of 16S rRNA. Phytoplanktonic primary production estimates brought new insights in the context of the Global Change. On the other hand, abundance, biomass and production of heterotrophic bacteria were also studied. Bacterial production was then compared to phytoplanktonic primary production.
The results of the present study bring substantial evidence to the major role of the picoplankton, especially the picocyanobacteria, in the ecology of one of the biggest freshwater reservoirs in the world, the Lake Tanganyika.
