Biomass and trophic fate of picoplankton in lake Tanganyika

STENUITE, S., A-L. TARBE, H. SARMENTO, F. UNREIN, A. WILMOTTE, J. M. GASOL & J-P DESCY

Montreal (Canadá)

Congreso; 30th Congress of the International Association of Theoretical and Applied Limnology (SIL 2007); 2007

International Association of Theoretical and Applied Limnology

Recent studies combining flow cytometry and epifluorescence microscopy have shown that picoplnakton, comprising photoynthetic picoplankton (PPP) and heterotrophic bacteria (HB), may be the largest plankton compartment in Lake Tanganyika. In this study, conducted in the different seasons of 2004-2006, PPP biomass in the upper 100 m was ranged between 2.5 and 3.4 g C m-2 on average, while HB biomass ranged between 0.52 and 1.32 g C m-2, with highest values observed during the dry season. Biomass ratios between HB and PPP showed clear seasonal patterns, with dominance of picocyanobacterial biomass in the upper layers throughout the year, but strengthened dominance of HB below the euphotic zone. Using clone libraries and DGGE based on the 16S rRNA gene, we found 5 different Synechococcus sequence types in 2 stations of Lake Tanganyika. Fractionated 14C incubations indicated that PPP accounted for a major part of particulate primary production. Grazing experiments with different techniques were used to evaluate grazing of picoplankton by microzooplankton. Heterotrophic nanoflagellates grazing impact on picoplankton was higher than of ciliates, and the grazing impact of protozoans was much higher on PPP than on HB. Our data confirm the hypothesis of a major role of picoplankton, especially, picocyanobacteria, in the pelagic food web of Lake Tanganyika.