Abstract
Seasonal dynamics of bacteria, autotrophic picoplankton and small nanoplankton (<5µm) were studied by direct counts using an epifluorescence microscope. Samples were collected from the euphotic zone in the inner Oslofjord from April to September and in the Skagerrak from May to August during 1993. The first exponential growth period of picocyanobacteria was observed during May and June in the Skagerrak and 2–3 weeks later in Oslofjorden. Eukaryotic picoplankton isolated by serial dilution cultures were dominated by the prasinophyte Micromonas pusilla (Butcher) Manton & Parke with cell numbers > 104 cells ml-1 from April to May. Biomass of phytoplankton (<5µm) and heterotrophic bacteria were 25–56µg C litre-1 in Oslofjorden, with increasing biomass of small nanoplankton towards the inner fjord. In the Skagerrak biomasses were 17-52!J.g C litre, with increasing biomass of picocyanobacteria from May to August. The importance of competition for nutrients between autotrophic picoplankton and heterotrophic bacteria is discussed.