The application of flow cytometry to the monitoring of phytoplankton is demonstrated. A comparison is made with conventional approaches to phytoplankton monitoring: light microscopy for the determination of species abundance, and chlorophyll a determination and in situ chlorophyll a measurement by fluorescence for the determination of the biomass. Flow cytometric measurements correlate well with these conventional types of measurements, as has been shown by comparing a full year of monitoring data obtained at a fixed monitoring location 10 km off the Dutch coast. Flow cytometry bridges the gap between labour-intensive, but highly informative, microscopic observations and simple biomass measurements with less information content: via flow cytometry optical data are obtained at high speed for individual particles, which can be translated into biomass information. On the basis of the flow cytometric measurements, rough discrimination of phytoplankton species groups is possible, particularly for the abundant species. Of crucial importance is careful calibration of the flow cytometer, to ensure quantitative and comparable measurements over a long period of time. Calibration and quality assurance aspects are covered in detail.

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