This European Standard gives guidance for sampling, preservation, storage, quantification and qualitative analysis of phytoplankton from marine waters. Guidance for quantification is limited to the use of light microscopy with phase-contrast and epifluorescence. This European Standard specifies: - the development of the sampling programme; - requirements for sampling equipment; - procedures for sampling and treatment of samples in the field; - methods for quantification; - qualitative analysis. This European Standard describes minimum requirements for environmental monitoring.