AbstractAt two stations in a Norwegian land‐locked fjord, one more and one less influenced by freshwater run‐off, studies were made of the primary production, chlorophyllaconcentrations, and the quantity and composition of phytoplankton. Samples were taken for analyses of phosphate, nitrate, and silicate, and temperature and salinity were measured. Two kinds of bio‐assays were performed: combinations of phosphate, nitrate, and silicate were added to I, natural water incubatedin situ, and II, natural water incubated in the laboratory. Lower salinities and higher concentrations of all nutrients were found at the station most influenced by fresh‐water. Although the bio‐assays indicated that nitrogen was the main limiting nutrient, the biological parameters showed no consistent differences between the two stations. The similarity is tentatively explained by the growth of nanoplankton algae, partly depending on nutrients other than those