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.

Vertical and latitudinal differences in bacterial community composition (BCC) in Lake Tanganyika were studied during the dry season of 2002 by means of denaturing gradient gel electrophoresis analysis of PCR-amplified 16S RNA fragments. Dominant bands were sequenced and identified as members of the Cyanobacteria, Actinobacteria, Nitrospirae, green nonsulfur bacteria, and Firmicutes divisions and the Gammaand Deltaproteobacteria subdivisions. The BCC in the lake displayed both vertical and latitudinal variation. Vertical changes in BCC were related to the thermal water column stratification, which influences oxygen and nutrient concentrations. Latitudinal variation was related to upwelling of deep water and increased primary production in the south of the lake. The number of bands per sample increased with bacterial production in the epilimnion of the lake, suggesting a positive diversity-productivity relationship.Since the first application of molecular tools to the study of the ecology of aquatic bacteria, bacterial community composition (BCC) has been studied in a wide variety of aquatic ecosystems, ranging from shallow to deep lakes to coastal seas and oceans. Molecular studies of BCC in lakes and rivers have revealed a consistent set of typical freshwater bacteria. Zwart et al. (51) discerned 34 putative phylogenetic clusters which occur in a wide range of freshwater environments. The dominant divisions include the Proteobacteria, Bacteroidetes, Verrucomicrobia, Cyanobacteria, Actinobacteria, and green nonsulfur bacteria. Urbach et al. (46), however, detected unusual bacterial communities in the ultraoligotrophic Crater Lake, in which there was a greater dominance of Gammaproteobacteria. So far, few studies have focused on the large, ancient lakes of the world or on tropical lakes. To our knowledge, no information is available yet on BCC in tropical lakes. Of the large lakes, data are available only for Lake Baikal (1,3,6,40). Lake Tanganyika is the third largest lake by volume and the deepest lake after Lake Baikal. Studies on bacteria in Lake Tanganyika have been limited to monitoring data on bacterial abundance and production (15, 39) or taxonomic studies dealing with specific bacterial groups (9, 10). As this lake is well known for its rich endemic fauna (4), studies on the BCC may be worthwhile.Lake Tanganyika is a meromictic, permanently temperaturestratified lake with strong vertical oxygen concentration gradients. This lake is anoxic below a depth of 100 to 200 m, and it contains the largest volume of anoxic freshwater in the world. The permanent temperature and oxygen gradients may affect the BCC in the lake. In stratified lakes and seas, vertical zonation of BCC has been found along temperature and/or oxygen concentration gradients (21). Several studies have demonstrated a relationship between BCC and oxygen concentration (21) or temperature (42). During the dry season, Lake Tanganyika also displays pronounced latitudinal differences in the depth of the thermocline. The differences in thermocline d...

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.

1. This study focused on heterotrophic microorganisms in the two main basins (north and south) of Lake Tanganyika during dry and wet seasons in 2002. Bacteria (81% cocci) were abundant (2.28-5.30 · 10 6 cells mL )1 ). During the dry season, in the south basin, bacterial biomass reached a maximum of 2.27 g C m )2 and phytoplankton biomass was 3.75 g C m )2 (integrated over a water column of 100 m). 2. Protozoan abundance was constituted of 99% of heterotrophic nanoflagellates (HNF). Communities of flagellates and bacteria consisted of very small but numerous cells. Flagellates were often the main planktonic compartment, with a biomass of 3.42-4.43 g C m )2 . Flagellate biomass was in the same range and often higher than the total autotrophic biomass (1.60-4.72 g C m )2 ). 3. Total autotrophic carbon was partly sustained by the endosymbiotic zoochlorellae Strombidium. These ciliates were present only in the euphotic zone and usually contributed most of the biomass of ciliates. 4. Total heterotrophic ciliate biomass ranged between 0.35 and 0.44 g C m )2 . In 2002, heterotrophic microorganisms consisting of bacteria, flagellates and ciliates represented a large fraction of plankton. These results support the hypothesis that the microbial food web contributes to the high productivity of Lake Tanganyika. 5. As the sole source of carbon in the pelagic zone of this large lake is phytoplankton production, planktonic heterotrophs ultimately depend on autochthonous organic carbon, most probably dissolved organic carbon (DOC) from algal excretion.

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