Our technological goal was to test the new technology of DNA microarrays in the field of biodiversity assessments. The efficiency of DNA microarrays was compared against the "classical" methods for measuring and monitoring the biodiversity. We use cyanobacteria from freshwater lakes as model organisms belonging to a well defined phylum of much ecological relevance. The combination of the information about cyanobacterial biodiversity together with the data describing the water quality of the lakes may reveal useful information to design management actions to control the unwanted mass developments of cyanobacteria. Such blooms prevent the use of water for drinking and for recreation and can be toxic to animals and humans. The possibility to automate the microarray technology in water quality monitoring will be an interesting alternative to time-consuming microscopic enumerations.