La composition, l'abondance et la biomasse de la communauté du picoplancton autotrophe (PPA, 0,2-2 m) ont été examinées dans un lac situé en Pologne et récemment considéré comme méromictique, le lac Czarne. Les échantillons d'eau ont été prélevés tous les mètres dans la colonne d'eau, en mars et en juillet 1998. Pendant ces deux dates, le PPA se caractérisait par des changements significatifs de densité dans la colonne d'eau. Au printemps, l'abondance la plus forte a été observée à 9 m (2,1·105 cellules·mL-1) tandis qu'en été elle est observée à 5 m (3,1·105 cellules·mL-1). À toutes les profondeurs, le PPA était dominé par des Cyanobactéries. Au printemps, dominent les cellules libres de picocyanobactéries, contrairement à l'été où les cellules coloniales étaient prépondérantes. La contribution des cellules eucaryotes était faible en termes d'abondance (, The composition, density and biomass of autotrophic picoplankton communities (PPA, 0.2-2 µm) were investigated in the meromictic and mesotrophic Lake Czarne (area 19.6 ha, water volume 2.2·106 m3, max. depth 28 m, mean depth 11.2 m). It is located within the Drawa National Park (NW Poland), in extensive lowlands, at an altitude of 55 m. Water samples were taken in the deepest part of the pelagic zone, in 1-m intervals from surface to bottom, in late March and July 1998. The organisms were analysed by means of an epifluorescence microscope after collection by filtration on black Nuclepore filters of 0.2 µm pore sizePelagic autotrophic picoplankton (PPA) taxa occurred in all water samples and the community was composed of cyanobacteria and eukaryotes (i.e. small chlorophytes). The cyanobacteria (Pcy) included Synechocystis, Aphanothece and Aphanocapsa species. Among eukaryotes (PAE), Choricystis minor (Skuja) Fott and Pseudodictyosphaerium jurisii (Hindák) Hindák were identified. These two chlorophytes were characterized by a larger mean cell size than Pcy. The most abundant species was the eukaryotic P. jurisii in spring and the prokaryotic Aphanothece bachmannii Komárková-Legnerová et Cronberg in summer (both species formed colonies). All PPA taxa in Lake Czarne are common, found in various lake types.The total abundance of PPA varied within three orders of magnitude: from 1·103 cells·mL-1 to 3·105 cells·mL-1. During both sampling sessions, the PPA communities were characterized by a substantial variation in abundance within the water column and mean values were 8.5·104 cells·mL-1 in spring and 5.4·104 cells·mL-1 in summer. In March, at a depth of 0 11 m, cell numbers were never lower than 105 cells·mL-1. The highest density was observed at a depth of 9 m in March (2.1·105 cells·mL-1) but at 5 m in July (3.1·105 cells·mL-1). In the deeper, anoxic water layers, PPA abundance was lower and less variable with about 103 cells·mL-1 in March and about 104 cells·mL-1 in July. The presence of PPA deep in the pelagic zone (in the aphotic and anoxic zones) during both sampling sessions may result from sedimentation, although its importance in the case of the small cells of PPA was probably lower than in the case of the larger organisms. In the study lake, because of the stability of the lower layers of water (i.e. lack of mixing with the overlying layers), that layer was not a source enriching the upper layers with PPA.At all depths Pcy were the most numerous components of the PPA community. This confirms the numerous reports about the dominance of Pcy within PPA. Single-celled picoplanktonic cyanobacteria (S-Pcy) dominated in spring, accounting for 82 98% (mean for the water column: approximately 90%) of the total Pcy abundance, whereas cells forming colonies (C-Pcy) prevailed in summer, reaching over 89% (mean for the water column: about 30%) of the total Pcy abundance. PAE were much less numerous or even absent in some water samples. In March, PAE abundance varied between 1.1·102 cells·mL-1 and 1.5·104 cells·mL-1 (mean for the water column: 4.6·103 cells·mL-1), and was the highest in the middle part of the water column. The contribution of PAE to the total PPA abundance ranged widely but did not exceed 20%. In July, PAE were less numerous and their presence was detected only in the bottom layer of water (9 m). Their contribution to the total PPA abundance did not exceed 10% (mean about 3%). Hence those results confirm that PEA prefer colder and nutrient-rich waters.The total biomass of PPA varied in March from 0.2 µg C·L-1 to 31 µg C·L-1 (mean value for the water column: 12 µg C·L-1) and in July from 0.8 µg C·L-1 to 49 µg C·L-1 (mean: 8 µg C·L-1). Thus the mean PPA biomass was 1.5 times higher in March than in July. During both sampling sessions PPA biomass was higher in the upper and middle layers of water than in the monimolimnion. During the first sampling session, values >10 µg C·L-1 were recorded to the depth of 14 m, while during the second session biomass clearly concentrated in the upper part of the water column, to a depth of 7 m. Within PPA, usually Pcy dominated in terms of biomass. In March their contribution to the total PPA biomass varied from 35% to 88% (mean: 67%) and in July from 65% to 100% (mean: 88%). PEA biomass was higher in March than in July and mean values were respectively 3.0 µg C·L-1 and 0.3 µg C·L-1, accounting for 33% and 12% of the total PPA biomass. In July, PAE biomass was everywhere lower than Pcy biomass. In contrast, in March, PAE biomass exceeded Pcy biomass at some depths in the lower part of the water column. The contribution of PAE to the total PPA biomass was higher than to the total PPA abundance, especially in March, when PAE were the most numerous. Generally, the contribution of eukaryotic cells was smaller in terms of abundance and much higher in terms of biomass, because of the larger mean size of cells.