Large concentrations of picoplankton in tropical lakes: a flow cytometry study in Lake Kivu (eastern Africa)

SARMENTO, H.; UNREIN, F.; ISUMBISHO, M.; STENUITE, S.; GASOL, J. M.; DESCY, J-P.

FRESHWATER BIOLOGY (PRINT)

Wiley Blackwell

Año: 2008 vol. 53 p. 756 - 771

0046-5070

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 numberswere always high and spatial variations were modest. This represents an importantdifference from temperate and high latitude lakes that show high fluctuations in cellabundance over an annual cycle.2. Three populations of picocyanobacteria were identified: one corresponded to single-cells(identified as Synechococcus by epifluorescence microscopy, molecular methods andpigment content), and the two other that most probably correspond to two and four celledcolonies of the same taxon. The proportion of these two subpopulations was greater understratified conditions, with stronger nutrient limitation.3. High PPP concentrations (c. 105 cell mL)1) relative to HB (c. 106 cell mL)1) were alwaysfound. Lake Kivu supports relatively less bacteria than phytoplankton biomass thantemperate 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 ofSynechococcus-like cyanobacteria in large, oligotrophic, tropical lakes is very high.5. Photosynthetic picoplankton cell abundances in the oligotrophic tropical lakes Kivu andTanganyika are comparable to those of eutrophic temperate lakes. This apparentlycontradicts the view that PPP abundance increases with increasing eutrophy. More data onPPP in tropical lakes are needed to explore further this particular pattern.