20 pages, 8 figures, 1 tableThe picoplankton (PP) is responsible for major ecosystem functions in most aquatic environments. However, not much is known about the factors that regulate its total abundance and community structure. Using flow cytometry (FC) to detect particles based on their pigments and nucleic-acid content, we described and quantified the photosynthetic picoplankton (PPP) and the heterotrophic bacterioplankton (HB) populations (or groups) composing PP in 32 water bodies located along a trophic and latitudinal gradient in the Argentinean Patagonia to determine flow-cytometrically defined community structures. We set out to identify the environmental variables regulating total PP abundance, group structure and cytometric diversity. We identified a total of 28 different cytometric populations within the HB, 14 of phycoerythrin (PE)-rich picocyanobacteria (Pcy), 8 of phycocyanin (PC)-rich Pcy, and 41 of picoeukaryotes (Peuk) in the different water bodies, with average 3.9 HB and 4.6 PPP groups per water body. We found a strong influence of environmental factors and a less marked effect of latitude on PP structure. HB and PPP abundances decreased towards higher latitudes but their cytometric diversity did not, whereas HB, PC-rich Pcy and Peuk abundances together with PPP diversities increased with higher values of chlorophyll a (chl a). The relative contribution of PE-rich Pcy to total Pcy decreased with chl a, whereas the relative contribution of PC-rich Pcy and the number of PC-rich cytometric populations increased with chl a values. Peuk prevailed over Pcy with increasing trophic status and light attenuation, whereas HB prevailed over PPP with increasing trophic statusThis work was funded by a grant of the Argentinean Funds for Technical and scientific Investigation (FONCYT, PICT 32732) and the Program ‘Luis Santaló’ of the National Research Council of Spain and the National Council of Scientific and Technical Research of Argentina (CSIC-CONICET, PROBA 2007AR0018). We thank Dr. Guillermo Tell, Dr. Rodrigo Sinistro, Dr. M. Laura Sánchez and Mr. Adrián Rua for their collaboration during the field campaigns, and Dr. Ramon Massana (ICM) for general supportPeer reviewe
