When algal blooms (such as red tide) occur, early detection of increased algal growth in intake water offers desalination plant operators advanced warning of severe fouling and provides the necessary time for preventative measures. Monitoring basic water quality measurements such as turbidity, silt density index (SDI), dissolved oxygen, and bulk fluorescence can approximate algal activity; however, these measurements lack the resolution needed to accurately track growth throughout all phases of a bloom event. This study investigates the use of a laser scanning cytometer (LSC) that detects algal cells over a wide range of concentrations (50-150,000 cells/ml), and characterizes cell shape, size, and distribution parameters through fluorescence signal imaging. Water samples are filtered to collect algae on a 0.2-mu m membrane surface. Sample volume is variable so cells can be concentrated. The membrane surface is scanned with a 635-nm diode laser and the signal is processed to generate a cell count and distribution image. LSC performance was comparable to fluorescence microscopy and flow cytometry over a range of concentrations that may be encountered during a bloom event. The applicability of LSC was demonstrated using laboratory-grown cultures and seawater samples taken during a bloom event off of the coast of Long Beach, CA. (C) 2011 Elsevier B.V. All rights reserved.