A 2-year (2002)(2003) 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 (0.3-3.4 mg m )3 ) and average chlorophyll a integrated through the 100 m water column were similar for both stations and years (36.4-41.3 mg m )2 ). Most pigments were located in the 0-60 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 0-100 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.

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. 10 5 cell mL )1 ) relative to HB (c. 10 6 cell mL )1 ) 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.

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 m )2 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 m )3 in the dry and the rainy season, respectively) than in the north basin (112 and 109 mg C m )3 , 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 m )2 day )1 ; 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 day )1 ; these are lower than previously published values for Lake Tanganyika.

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In keeping with the theme of this volume, the present article commemorates the 50 years of Hutchinson's (Am Nat 93:145-159, 1959) famous publication on the 'very general question of animal diversity', 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 0-40 m layer, with maxima at 0 or 20 m, and more rarely at 40 m. Deep chlorophyll maxima (DCM) and surface 'blooms' 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 environment-phytoplankton 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.

1. Abundance and bacterial production (BP) of heterotrophic bacteria (HBact) were measured in the north and south basins of Lake Tanganyika, East Africa, during seasonal sampling series between 2002 and 2007. The major objective of the study was to assess whether BP can supplement phytoplankton particulate primary production (particulate PP) in the pelagic waters, and whether BP and particulate PP are related in this large lake. HBact were enumerated in the 0-100 m surface layer by epifluorescence microscopy and flow cytometry; BP was quantified using 3 H-thymidine incorporation, usually in three mixolimnion layers (0-40, 40-60 and 60-100 m). 2. Flow cytometry allowed three subpopulations to be distinguished: low nucleic acid content bacteria (LNA), high nucleic acid content bacteria (HNA) and Synechococcus-like picocyanobacteria (PCya). The proportion of HNA was on average 67% of total bacterial abundance, and tended to increase with depth. HBact abundance was between 1.2 · 10 5 and 4.8 · 10 6 cells mL )1 , 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 )1 , estimated from biovolume measurements, average HBact biomass (integrated over a 100-m water column depth) was 1.89 ± 1.05 g C m )2 . 3. Significant differences in BP appeared between seasons, especially in the south basin. The range of BP integrated over the 0-100 m layer was 93-735 mg C m )2 day )1 , and overlapped with the range of particulate PP (150-1687 mg C m )2 day )1 ) measured in the same period of time at the same sites. 4. Depth-integrated BP was significantly correlated to particulate PP and chlorophyll-a, and BP in the euphotic layer was on average 25% of PP. 5. These results suggest that HBact contribute substantially to the particulate organic carbon available to consumers in Lake Tanganyika, and that BP may be sustained by phytoplankton-derived organic carbon in the pelagic waters.