The use of Pulse Amplitude Modulated fluorometer (PAM) has in the last tenyears been widely used in higher plant photo-physiology. In marine biology thePAM technique has basically been used the last six years on culture work onphytoplankton. Since the introduction of the DIVING-PAM in 1996, it hastypically been used for fluorescence yield measurements from macroalgae andcorals containing zooxanthellae (algal symbionts). We report the use of thisinstrument in harmful phytoplankton bloom research. Optically thick layers of toxic dinoflagellates such as Alexandrium tamarense grown in cultures flaskswith different nitrate : phosphorus ratios, have been used for directfluorescence yield measurements (operation quantum yield for stable chargeseparation at photosystem (PS) II, IIe). We also show examples of the use ofthis instrument during mesocosm experiments. The use of IIe in combination withabsorbed quanta, and the corresponding fraction of absorbed quanta directed toPS II gives information of pigment-group present and photo-physiological state.Oxygenic photosynthesis from PS II can be estimated based on optical andbio-optical measurements, i.e. the combined information of irradiance (400-700nm), spectral irradiance, temperature, IIe, Chl a-specific absorptioncoefficients (400-700 nm) and scaled fluorescence excitation spectra (400-700nm) to estimate mg O2 produced mg Chl a-1 h-1. This bio-optical information maybe the future tools for in situ monitoring of harmful algal blooms and as anaid to estimate photosynthetic performance and growth rates of phytoplankton.