Cyanobacterial picoplankton from the Central Baltic Sea: cell size classification by image-analyzed fluorescence microscopy

Enumeration and measurements of cell size in Baltic picoplanktic cyanobacteria were per- formed by video recording of autofluorescent images of unicells with a diameter of <3 urn. Statistical analysis was used in order to assess the contribution of the smallest photosynthetic fraction to the biomass and size distribution in the photic zone of the Baltic proper during summer of 1995. The ver- tical distribution of picocyanobacteria varied between 6.7 and 1.8 x 10 5 cells mh1 from the surface to 20 m depth, with an average value of 5.6 and 3.8 X 105 cells ml"1 in July and August, respectively. Three size classes were distinguished on the basis of morphometric analysis, as well as four types of cell aggregates frequently occurring in the samples. Variations were detected between the 2 months in picocyanobact erial populations present in the mixed layer, mainly concerning decrease in cell number, and increase in dimensions in August. Ecological and taxonomic implications arising from the data are discussed.