SEASONALITY AND COMPOSITION OF PICOPLANKTONIC AND NANOPLANKTONIC CYANOBACTERIA AND PROTISTS IN THE SKAGERRAK

Pico- and nanoplanktonic cyanobacteria and protists were studied at on e coastal and two offshore localities in the Skagerrak during 1991-92. Fluorescence microscopy was used for enumerating organisms and scanni ng electron microscopy (SEM) for further identification. A test of met hods showed that for eukaryotic picoplankton higher numbers were obser ved after prefiltering through a 3 mum filter. For nanoplankton the pr imulin staining method yielded lower counts compared to the sedimentat ion chamber technique. Phycoerythrin-rich cyanobacteria of Synechococc us-type were the commonest autotrophic picoplankton in summer with abu ndances of up to 250 x 10(6) cells L-1. Maximum abundance of eukaryoti c autotrophic picoplankton was 14 x 10(6) cells L-1. Several eukaryoti c autotrophic picoplankton taxa were observed but only Micromonas pusi lla (Butcher) Manton et Parke could be identified by means of fluoresc ence microscopy. The maximum abundance of nanoplankton at the coastal station occurred in April with 14 x 10(6) cells L-1 for autotrophs and 2 x 10(6) cells L-1 for heterotrophs. Average abundance based on 53 s amples from March 1991 to March 1992 was 0.78 x 10(6) cells L-1 for au totrophs and 0.38 x 10(6) cells L-1 for heterotrophs. Autotrophic nano plankton groups include Cryptophyceae, Chrysophyceae, Diatomophyceae, Dictyochophyceae, Dinophyceae, Prasinophyceae and Prymnesiophyceae. Al though many autotrophic organisms could be identified to species level several organisms could not be identified to class level. Common unid entified organisms are described. Most heterotrophic nanoplankton coul d not be identified. The commonest organism resembled the genus Spumel la. Other biflagellate heterotrophs resembled the bicosoecid genera Ca feteria and Pseudobodo. Heterotrophic nanoplankton also included choan oflagellates, dinoflagellates, Leucocryptos spp. and Telonema subtile Griessmann.