Anne-Laure Tarbe

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 10^4–8.2 x 10^5 cells mL–1) found in the mixolimnion (10–20 m depth). PPP biomass integrated over the water column depth ranged between 0.41 and 3.09 g C m–2, with maximal values in the south basin during the dry season (2.28 ± 0.62 g C m–2 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 cell–1, 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 0–100 m surface layer by epifluorescence microscopy and flow cytometry; BP was quantified using 3H-thymidine incorporation, usually in three mixolimnion layers (0–40 m, 40–60 m and 60–100 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 – 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 0–100 m layer was 93 – 735 mg C/(m^2 d^1), and overlapped with the range of phytoplankton particulate production (150 – 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. 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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