The procedure specified can be applied for phytoplankton in natural surface waters and for testing algal growth in bio-assays. Using appropriate sampling it can also be applied to phytobenthic communities. The principle is collection of algae by filtration, extraction of algal pigments, spectrometric determination of the chlorophyll-a concentration in the extract, evaluation of the chlorophyll-a and phaeopigment concentration from the difference of absorbance at 665 nm prior to and after acidification of the extract.