The autotrophic picoplankton (0.2-2 micrometers) and the auto- and heterotrophic nanoplankton (2-20 micrometers) were studied in the Skagerrak, North East Atlantic Ocean. Autotrophic picoplankton were shown to be important contributors to primary production, nutrient dynamics and hytoplankton biomass in the Skagerrak, especially under oligotrophic conditions. Cyanobacteria of SYNECHOCOCCUS-type were most abundant during summer when cell
numbers reached 250 x 106 cells per liter, while eukaryotes, with maxima around 30 million cells per liter, also occurred in spring. Calculations using cell numbers of SYNECHOCOCCUS and total chlorophyll a concentrations indicate that SYNECHOCOCCUS comprises 20-30 % of total phytoplankton biomass in summer.

The composition of eukaryotic picoplankton in the Skagerrak remains largely unknown. MICROMONAS PUSILLA Manton et Parke, Prasinophyceae was the only identifiable species in fluorescence microscope constituting < 5% of total eukaryotic picoplankton abundance. The < 3 micrometer size fraction contributed on average 37 % to total chlorophyll a concentration. In a study of phosphate uptake, the < 3 micrometer size fraction dominated during the whole year except during the diatom-dominated spring bloom. Fluorescence microscopy is a convenient method for quantifying autotrophic picoplankton. The Prymnesiophyceae was the most abundant autotrophic nanoplankton group in the Skagerrak. Scanning electron microscopy was used to further identify nanoplankton. It was most succesful for the Cryptophyceae. Cell numbers of heterotrophic nanoplankton were on average ca 30 % of the autotrophic nanoplankton whose average abundance was ca 1 million cells per liter. The traditional method to quantify phytoplankton by use of sedimentation chambers (the Utermoehl method) works well for nanoplankton, although it is more difficult to discriminate between auto- and heterotrophs when compared to fluorescence microscopy. The Utermoehl method is unsuitable for quantification of autotrophic picoplankton. Blooms of pico- and nanoplankton seldom occur, probably because their grazers may respond quickly to increased growth. Subsurface chlorophyll maxima are common in the Skagerrak. Pico- and nanoplankton do not occur in higher numbers in these layers compared to the surface layer. HPLC analysis of pigments specific to certain algal groups was used to quantify the importance of different algal groups to total phytoplankton biomass. However, microscopical techniques are still necessary to identify species. A ultraplankter isolated from the Skagerrak, NANNOCHLOROPSIS GRANULATA sp. nov., Eustigmatophyceae is described and the 18S rRNA gene sequence is reported and compared to sequences from N. OCULATA (Droop) Hibberd and N. SALINA Hibberd. Also the pigment composition and the ultrastructure are compared.