Seasonality and Composition of Pico- and Nanoplanktonic Cyanobacteria and Protists in the Skagerrak

Abstract

Pico- and nanoplanktonic cyanobacteria and protists were studied at one coastal and two offshore localities in the Skagerrak during 1991-92. Fluorescence microscopy was used for enumerating organisms and scanning electron microscopy (SEM) for further identification. A test of methods showed that for eukaryotic picoplankton higher numbers were observed after prefiltering through a 3 ~m filter. For nanoplankton the primulin staining method yielded lower counts compared to the sedimentation chamber technique. Phycoerythrin-rich cyanobacteria of Syne­ chococcus-type were the commonest autotrophic picoplankton in summer with abundances of up to 250 X 10 6 cells L -I. Maximum abundance of eukaryotic autotrophic picoplankton was 14 X I 0 6 cells L -t. Several eukaryotic autotrophic picoplankton taxa were observed but only Micromonas pusil/a (Butcher) Manton et Parke could be identified by means of fluorescence microscopy. The maximum abundance of nanoplankton at the coastal station occurred in April with 14 X I 06 cells L -t for autotrophs and 2 X I 06 cells L -t for heterotrophs. Average abundance